First Shimano Brake Bleed

[Oppa]

2023 Orbea Rise M10. So just recently clocked 1400 miles after purchasing my Orbea 9/23. Brakes were feeling squishy. I watched the Park video about 1000 times and said self, you can do that, after all you did it to your car drum brakes back in the day. Didn’t want to drop $150 on the Park bleeder kit (sorry Park) picked up a kit on Amazon for under $30. It’s decent with brass fittings. Also picked up metal pads for $50. So fast forward…. Front first. Guy in the Park video made it look like no mineral oil drips, well that part didn’t go so well and drips how about water fall so DYIers be prepared. Also lesson learned on doing the front, strap your fork, if it flips as mine did, that little reservoir you screw in at the handle doesn’t react well when the fork spins. 🫗. Also try to loosen the bleeder at the caliper slightly before starting. Cleaned everything with alcohol, put the new pads in and wow what a difference. Did the back, went better but still a bit messy. Burned the brakes in and honestly better than new. Aside from the mess I made it was worth the effort and saved some bucks.

 

[mooby77]

I use the following method the majority of the time , easy and always have good results , just clean well and clean your pads,

 

[Oppa]

I use the following method the majority of the time , easy and always have good results , just clean well and clean your pads,

Thanks will give it a roll

 

[Stihldog]

The only time I do a full bleed is when I install a new and complete hydraulic brake system or when I need to replace the fluid (mineral or Dot). When my brake levers get squishy, or the lever(s) almost touches the bar/grips, I check the brake pads and rotors for wear. Other than that, I do a mini-bleed at the master cylinder (brake levers).

Air bubbles will collect in the master cylinder over time and use in most hydraulic brake systems. There is usually a small screw in the master cylinder that keeps the hydraulic system closed and prevents hydraulic fluid loss. That small screw needs to be removed to start a mini-bleed.

Insert the funnel or cup specific to your model of master cylinder (Usually a kit is available with a variety of screw adapters) and screw it into the threaded hole finger tight. Add a small amount of brake fluid (1/4 full) to the funnel and remove the plastic plunger.

Now pump the levers and watch air bubbles rise into the funnel. Tap the hoses, tap and slightly rotate the master cylinder. Continue doing this until no more air bubbles rise into the funnel/cup.

After this is completed, re-insert the plunger, remove the funnel, replace the small master cylinder screw. (Save the remaining fluid in the funnel/cup).

Secure the clamp for your brake lever position, pump the levers, go for a ride. If that doesn’t feel good, or stop you properly, do a full bleed and inspect everything in your brake system.

Use a brake fluid specific to your brake system. (Dot, blue mineral oil, red mineral oil, green mineral oil).

 

[Oppa]

The only time I do a full bleed is when I install a new and complete hydraulic brake system or when I need to replace the fluid (mineral or Dot). When my brake levers get squishy, or the lever(s) almost touches the bar/grips, I check the brake pads and rotors for wear. Other than that, I do a mini-bleed at the master cylinder (brake levers).

Air bubbles will collect in the master cylinder over time and use in most hydraulic brake systems. There is usually a small screw in the master cylinder that keeps the hydraulic system closed and prevents hydraulic fluid loss. That small screw needs to be removed to start a mini-bleed.

Insert the funnel or cup specific to your model of master cylinder (Usually a kit is available with a variety of screw adapters) and screw it into the threaded hole finger tight. Add a small amount of brake fluid (1/4 full) to the funnel and remove the plastic plunger.

Now pump the levers and watch air bubbles rise into the funnel. Tap the hoses, tap and slightly rotate the master cylinder. Continue doing this until no more air bubbles rise into the funnel/cup.

After this is completed, re-insert the plunger, remove the funnel, replace the small master cylinder screw. (Save the remaining fluid in the funnel/cup).

Secure the clamp for your brake lever position, pump the levers, go for a ride. If that doesn’t feel good, or stop you properly, do a full bleed and inspect everything in your brake system.

Use a brake fluid specific to your brake system. (Dot, blue mineral oil, red mineral oil, green mineral oil).

That’s pretty much the procedure I followed. It was just a bit messier than I expected. Maybe I won’t have to 2 or 3 beers before hand . Just kidding on the beer part but after it’s done all bets are off.

 

[Hattori-Hanzo]

Gravity bleed is my preference also for my xt's. Seem to get a better lever feel and no faffing around with syringes.

 

[Oppa]

Maybe a dumb question but how does that work? I assume you fill the reservoir open the bleeder at the caliper and pump the handle ?

 

[RustyIron]

Cleaned everything with alcohol, put the new pads in and wow what a difference.

Good on ya, man. Ain't nothin' better than a machine that's running perfectly... except maybe a machine that's running perfectly because you personally maintain it to run perfectly.

 

[Stihldog]

…It was just a bit messier than I expected.

OH! …it can get messy!
Especially when you forget to remove the funnel plunger/stopper during a full bleed with a syringe at the calliper. It’s amazing how fast a full syringe can empty when you exert too much pressure and a fluid hose pops off .

Me, the bike and the floor needed some cleanup after that surprise .

 

[Hattori-Hanzo]

Maybe a dumb question but how does that work? I assume you fill the reservoir open the bleeder at the caliper and pump the handle ?

This linked video shows the process.


You just need a Shimano bleed funnel, get the MC high up and the calipers low, remove the bleed nipples and let fluid drain through the system from the funnel.

 

[Oppa]

This linked video shows the process.


You just need a Shimano bleed funnel, get the MC high up and the calipers low, remove the bleed nipples and let fluid drain through the system from the funnel.

Thanks peeps

 

[Oppa]

This linked video shows the process.


You just need a Shimano bleed funnel, get the MC high up and the calipers low, remove the bleed nipples and let fluid drain through the system from the funnel.

Thanks again for this pointing me to this process for the break bleed. Tried it on another bike and it was much better and easier than fumbling around with the syringes.

 

[Rod B.]

2023 Orbea Rise M10. So just recently clocked 1400 miles after purchasing my Orbea 9/23. Brakes were feeling squishy. I watched the Park video about 1000 times and said self, you can do that, after all you did it to your car drum brakes back in the day. Didn’t want to drop $150 on the Park bleeder kit (sorry Park) picked up a kit on Amazon for under $30. It’s decent with brass fittings. Also picked up metal pads for $50. So fast forward…. Front first. Guy in the Park video made it look like no mineral oil drips, well that part didn’t go so well and drips how about water fall so DYIers be prepared. Also lesson learned on doing the front, strap your fork, if it flips as mine did, that little reservoir you screw in at the handle doesn’t react well when the fork spins. 🫗. Also try to loosen the bleeder at the caliper slightly before starting. Cleaned everything with alcohol, put the new pads in and wow what a difference. Did the back, went better but still a bit messy. Burned the brakes in and honestly better than new. Aside from the mess I made it was worth the effort and saved some bucks.

Oppa,

We've all been there when it comes to performing a first time Shimano brake bleed and it being messy. Oh the joy of having the bleed hose pop off and spray the garage with mineral oil. Fortunately, the bleed process does get easier the more times you perform a bleed service, whether SRAM or Shimano. Practice makes perfect. I congratulate you on performing the brake service as opposed to taking your bike to a bike shop to get the brake service done. Awesome job!

Here's a truth learned the hard way. The better the brake bleed kit, the less messy the bleed process. My first few bleed kits where Amazon specials. The kits were garbage and the messiness of the bleed process was reflected in the low cost of the bleed kits. I later switched to a Shimano bleed kit which worked "Okay" but not much better. I finally got tired of cleaning up mineral oil and bought Park Tool's mineral oil bleed kit and have never had an issue since. The Park Tool's plungers and hose assembly are a bit more robust in design than those on Amazon, or what's offered by Shimano.

As you've noted, a new set of pads and a fresh bleed work wonders when it comes to brake lever action. As brake pads and or brake rotors fall outside of wear tolerances, it takes more brake fluid to spread the brake caliper pistons out and engage the brake pads against the brake rotors. However, brake levers only have a finite amount of hydraulic action, i.e. fluid movement per lever stroke. Worn pads and or brake rotors will cause a squishy brake lever feel and or, require multiple pumps of the brake lever to engage the brakes. A quick bleed using just the brake bleed cup at the brake lever can help with lever squishiness when brake pads are at half life. Frequently, brake rotors are overlooked as a wear item. All brake rotors have a minimum allowable thickness.

If your interested, I wrote a three part technical series on Shimano Brakes for the eMTB Orbea Forum. Part Three in the series talks about brake rotors and how to measure your brake rotors for wear.

Part # 1: Shimano Brake System Service

This will be a three part technical article discussing Shimano braking system components, how to service your Shimano brakes and brake system modification on your Rise. Part # 1 will discuss basic maintenance on your Shimano brakes. Part # 2 will discuss brake modifications to your Orbea...

www.emtbforums.com

PART # 2: SHIMANO BRAKE SYSTEM – Orbea Rise Brake System Modification

This technical article is the second installment in a two part series on Shimano brake systems. Refer to “Part # 1: Shimano Brake System Service”, for information on bleeding and servicing Shimano brakes and brake pad compounds...

www.emtbforums.com

Part # 3: Shimano Brake System Service

After submitting Part # 1 and Part # 2 of the tech series on Shimano brakes, I realized I had not included anything on checking your Shimano brake rotor for minimum thickness or signs of uneven rotor wear. You can review Part # 1 and Part # 2 in the series here...

www.emtbforums.com

Be safe,
Rod

 

[Stihldog]

I can confirm that a pressurized mineral oil hose coming loose and spraying fluid all over the floor, bike and yourself …is not fun…it’s not!

 

[John Beedham]

... first time is usually a bit messy ... for a few things...

 

[Oppa]

Oppa,

We've all been there when it comes to performing a first time Shimano brake bleed and it being messy. Oh the joy of having the bleed hose pop off and spray the garage with mineral oil. Fortunately, the bleed process does get easier the more times you perform a bleed service, whether SRAM or Shimano. Practice makes perfect. I congratulate you on performing the brake service as opposed to taking your bike to a bike shop to get the brake service done. Awesome job!

Here's a truth learned the hard way. The better the brake bleed kit, the less messy the bleed process. My first few bleed kits where Amazon specials. The kits were garbage and the messiness of the bleed process was reflected in the low cost of the bleed kits. I later switched to a Shimano bleed kit which worked "Okay" but not much better. I finally got tired of cleaning up mineral oil and bought Park Tool's mineral oil bleed kit and have never had an issue since. The Park Tool's plungers and hose assembly are a bit more robust in design than those on Amazon, or what's offered by Shimano.

As you've noted, a new set of pads and a fresh bleed work wonders when it comes to brake lever action. As brake pads and or brake rotors fall outside of wear tolerances, it takes more brake fluid to spread the brake caliper pistons out and engage the brake pads against the brake rotors. However, brake levers only have a finite amount of hydraulic action, i.e. fluid movement per lever stroke. Worn pads and or brake rotors will cause a squishy brake lever feel and or, require multiple pumps of the brake lever to engage the brakes. A quick bleed using just the brake bleed cup at the brake lever can help with lever squishiness when brake pads are at half life. Frequently, brake rotors are overlooked as a wear item. All brake rotors have a minimum allowable thickness.

If your interested, I wrote a three part technical series on Shimano Brakes for the eMTB Orbea Forum. Part Three in the series talks about brake rotors and how to measure your brake rotors for wear.

Part # 1: Shimano Brake System Service

This will be a three part technical article discussing Shimano braking system components, how to service your Shimano brakes and brake system modification on your Rise. Part # 1 will discuss basic maintenance on your Shimano brakes. Part # 2 will discuss brake modifications to your Orbea...

www.emtbforums.com

PART # 2: SHIMANO BRAKE SYSTEM – Orbea Rise Brake System Modification

This technical article is the second installment in a two part series on Shimano brake systems. Refer to “Part # 1: Shimano Brake System Service”, for information on bleeding and servicing Shimano brakes and brake pad compounds...

www.emtbforums.com

Part # 3: Shimano Brake System Service

After submitting Part # 1 and Part # 2 of the tech series on Shimano brakes, I realized I had not included anything on checking your Shimano brake rotor for minimum thickness or signs of uneven rotor wear. You can review Part # 1 and Part # 2 in the series here...

www.emtbforums.com

Be safe,
Rod

Hi Rod thanks so much for the info much appreciated. One of our peeps suggested a gravity bleed which seemed a bit easier. Curious as to your thoughts on this method.

 

[Rod B.]

Hi Rod thanks so much for the info much appreciated. One of our peeps suggested a gravity bleed which seemed a bit easier. Curious as to your thoughts on this method.

Which method to use depends on what you are trying to achieve. I use both methods.

A gravity bleed is useful if you are attempting a quick bleed, I.e. you remove the bleed screw on the brake lever and install the bleed cup. You fill the cup about half way and pump your brake lever. You’ll typically see a few bubbles come up out of the bleed screw port and into the cup. You keep pumping your brake lever until no more bubbles rise up into the cup.

The term Gravity Bleed comes from the fact that air in a brake system will with enough time rise to the highest point in the system, i.e. brake lever. A quick bled will remove the residual air.

NOTE: some brake calipers can trap air pockets in the caliper’s internal ports. This is especially so on four piston calipers where a piston is sticking and the cavity behind the sticking piston is trapping air. In instances like this, a gravity bleed will not rid you of air pockets trapped in a caliper with sticking pistons.

SRAM calipers have phenolic pistons as opposed to Shimano which uses a ceramic piston. Both have pro and con. After about three to four months of hard use, SRAM phenolic pistons will score and begin to stick. If you’ve ever fully bled a SRAM system and despite doing so multiple times, the brake lever is still squishy, it’s because there is a sticking caliper piston which is trapping air. In this instance it’s best to clean and free up the sticking piston. Better yet, spend $20 and buy a piston service kit and replace the pistons and seals. Shimano does not offer a piston service kit for its four piston calipers. You have to buy a new caliper. Like I said, Pro and con…..

You would want to do a quick gravity bled when you are adding additional fluid to accommodate for worn pads, i.e. you’re squeezing the brake lever and it’s traveling almost to the handlebar. Adding additional fluid will firm up the lever and remove air from the brake lever reservoir. There are also times where I’ve disconnected a brake hose from the lever to shorten it or re-route the brake hose differently. As long as you don’t drain a lot of fluid from the hose or lever you can usually get away with a quick gravity bleed.

You will usually want to do an annual, full on bleed to flush out the old fluid which will darken with time due to rubber seal debris and dirt debris which makes it past the piston seals. Yes, I know…..there are those who never swap out there brake fluid. There are also those who blow out piston seals at a critical section of trail and crash due to loss of a brake. John if you’re reading this, yes, I’m throwing you under the bus.. Be nice to your bike and do a full flush occasionally.

Lastly, I’ll do a full brake bleed if I have to service the brake lever seals or brake caliper pistons and seals. A gravity bled is usually not effective in this instance. There’s usually too much air in the system to get by with a gravity bled.

I hope this helps, if you have further questions, let me know.

Rod

 

[Hattori-Hanzo]

Which method to use depends on what you are trying to achieve. I use both methods.

A gravity bleed is useful if you are attempting a quick bleed, I.e. you remove the bleed screw on the brake lever and install the bleed cup. You fill the cup about half way and pump your brake lever. You’ll typically see a few bubbles come up out of the bleed screw port and into the cup. You keep pumping your brake lever until no more bubbles rise up into the cup.

The term Gravity Bleed comes from the fact that air in a brake system will with enough time rise to the highest point in the system, i.e. brake lever. A quick bled will remove the residual air.

NOTE: some brake calipers can trap air pockets in the caliper’s internal ports. This is especially so on four piston calipers where a piston is sticking and the cavity behind the sticking piston is trapping air. In instances like this, a gravity bleed will not rid you of air pockets trapped in a caliper with sticking pistons.

SRAM calipers have phenolic pistons as opposed to Shimano which uses a ceramic piston. Both have pro and con. After about three to four months of hard use, SRAM phenolic pistons will score and begin to stick. If you’ve ever fully bled a SRAM system and despite doing so multiple times, the brake lever is still squishy, it’s because there is a sticking caliper piston which is trapping air. In this instance it’s best to clean and free up the sticking piston. Better yet, spend $20 and buy a piston service kit and replace the pistons and seals. Shimano does not offer a piston service kit for its four piston calipers. You have to buy a new caliper. Like I said, Pro and con…..

You would want to do a quick gravity bled when you are adding additional fluid to accommodate for worn pads, i.e. you’re squeezing the brake lever and it’s traveling almost to the handlebar. Adding additional fluid will firm up the lever and remove air from the brake lever reservoir. There are also times where I’ve disconnected a brake hose from the lever to shorten it or re-route the brake hose differently. As long as you don’t drain a lot of fluid from the hose or lever you can usually get away with a quick gravity bleed.

You will usually want to do an annual, full on bleed to flush out the old fluid which will darken with time due to rubber seal debris and dirt debris which makes it past the piston seals. Yes, I know…..there are those who never swap out there brake fluid. There are also those who blow out piston seals at a critical section of trail and crash due to loss of a brake. John if you’re reading this, yes, I’m throwing you under the bus.. Be nice to your bike and do a full flush occasionally.

Lastly, I’ll do a full brake bleed if I have to service the brake lever seals or brake caliper pistons and seals. A gravity bled is usually not effective in this instance. There’s usually too much air in the system to get by with a gravity bled.

I hope this helps, if you have further questions, let me know.

Rod

I maybe wrong, but it seems you are describing a "lever bleed" not a gravity bleed?

Quick summary,

With a gravity bleed, you install the funnel onto the MC and fully fill it, unscrew lever free stroke screw a few turns, remove calipers from fork/frame and hang them below the MC. (for the rear caliper, stand the bike up on it's back wheel so the MC is high up)
Remove pads and push the pistons almost fully into the caliper (which should remove any trapped air behind them) remove bleep nipple, remove funnel bung and let old fluid drain through the system until new clear fluid is seen. Rotating the calipers while the fluid is draining should help to remove any trapped air in the caliper.
Before the funnel is empty, refit the bung and refill with fresh fluid, repeat until you are satisfied the system has all fresh fluid.
Refit the bleep nipple, clean the calipers with brake cleaner, fit pads, fit calipers back to fork/frame and then perform a lever bleed as you described.

 

[Stihldog]

I maybe wrong, but it seems you are describing a "lever bleed" not a gravity bleed?

Quick summary,

With a gravity bleed, you install the funnel onto the MC and fully fill it, unscrew lever free stroke screw a few turns…

I thought the same. I believe a gravity bleed would be fine if you’re changing all the fluid in the brake system.
A lever bleed can be quickly done with a funnel and a few drops of fluid. Pump the lever to extract the bubbles.
A full bleed is necessary when a new brake system is installed ( lever, hose calliper), but I’d still do a lever bleed after that, or after the first ride.

 

[Rod B.]

I maybe wrong, but it seems you are describing a "lever bleed" not a gravity bleed?

Quick summary,

With a gravity bleed, you install the funnel onto the MC and fully fill it, unscrew lever free stroke screw a few turns, remove calipers from fork/frame and hang them below the MC. (for the rear caliper, stand the bike up on it's back wheel so the MC is high up)
Remove pads and push the pistons almost fully into the caliper (which should remove any trapped air behind them) remove bleep nipple, remove funnel bung and let old fluid drain through the system until new clear fluid is seen. Rotating the calipers while the fluid is draining should help to remove any trapped air in the caliper.
Before the funnel is empty, refit the bung and refill with fresh fluid, repeat until you are satisfied the system has all fresh fluid.
Refit the bleep nipple, clean the calipers with brake cleaner, fit pads, fit calipers back to fork/frame and then perform a lever bleed as you described.

Yes, you are both correct, my bad, I was thinking he meant a lever bleed.

 

[Stihldog]

Yes, you are both correct, my bad, I was thinking he meant a lever bleed.

Sometimes I call a “lever bleed” a “mini bleed “ which confuses even more than I already am.

 

[Rod B.]

Sometimes I call a “lever bleed” a “mini bleed “ which confuses even more than I already am.

Thank you, I don’t feel so lonely now. I thought I was the only confused person on this eMTB Forum. I’m also easily distracted by shiny objects that go round and around….

 

[Oppa]

Hey everyone thanks for all the info. For my first bleed I followed the Park tool video which was start at the caliper and push the fluid to the handle reservoir and attached cup then reverse that from the reservoirs, pump the lever and bleed at the caliper. Messy part is when I did a dumb ass move and flipped the fork with the cup filled with fluid, yikes. The gravity feed video a couple post up seemed to be a bit simpler. Fill the cup remove the caliper, open the bleed nipple, rotate caliper and let momma nature take its course and the have a beer( I added that last one ). I was just curious if one method is preferred. I kinda like the gravity feed.

 

[Stihldog]

98% of the time I preform a lever bleed(or mini-bleed) for Magura, Shimano, Sram DB-8, Tektro , and one Dot fluid brake system. I have a small supply of Red, Blue and Green mineral fluid for different brake systems. I also have a very small container of Dot fluid which is rarely used and will probably expire while it sits on my shelf. (I personally don’t like using Dot fluid brake systems.)

Anyways, I don’t know the viscosity of mineral oil(s) or its susceptibility to absorb micro air bubbles. But in each of the mineral oil brake systems they eventually need a bleed. Micro air bubbles rise to the master cylinder (lever) because of vibration, heat or small leaks. How this happens in a closed hydraulic brake system …well there could be several reasons (pick one).

The bottom line is that air eventually collects at the highest point …master cylinder. Sometimes the air can be trapped in the calliper (Magura systems) and a ride can free up those bubbles.

Therefore a gravity feed bleed is good for changing old and dirty fluids, but it may not release trapped air in the calliper or the micro bubbles in the fluid. A bumpy ride is usually necessary after this type of bleed…then a lever bleed for a few bubbles (micro bubbles).

When my levers feel spongy I usually do a lever bleed …if everything else in the brake system is fine. Takes a few minutes.

 

[Mrj35]

I use the following method the majority of the time , easy and always have good results , just clean well and clean your pads,

That was the most helpful video for me too when I first tried it. gravity bleed is the way to go.

 

[RickBullotta]

That was the most helpful video for me too when I first tried it. gravity bleed is the way to go.

Except if you have air trapped in your caliper, in which case it might still leave a spongy feel after a gravity bleed.

 

[Mrj35]

Except if you have air trapped in your caliper, in which case it might still leave a spongy feel after a gravity bleed.

Yeah to be honest im not exactly sure how it works. My lever wasn't returning much at all before the bleed but after it now returns as it should. I just did the tapping thing on the caliper to get any air out while it was doing the gravity bleed.

 

[RickBullotta]

Yeah to be honest im not exactly sure how it works. My lever wasn't returning much at all before the bleed but after it now returns as it should. I just did the tapping thing on the caliper to get any air out while it was doing the gravity bleed.

The real test is the next ride. I've done bleeds that felt perfect on the work stand, but after 5-10 minutes of riding the mushy lever returned. almost every time it was due to bubbles hiding in the caliper (known challenges bleeding maguras)

 

[Mrj35]

The real test is the next ride. I've done bleeds that felt perfect on the work stand, but after 5-10 minutes of riding the mushy lever returned. almost every time it was due to bubbles hiding in the caliper (known challenges bleeding maguras)

Its also possible my levers are permanently mushy LOL because the're cheaper 4 piston shimano non servo brakes (mt420s). The bite point is quite vague. my issue was the lever would barely return so I had to almost flick it back with my finger, once I bled it though its been good ever since.

 

[Rod B.]

Hey everyone thanks for all the info. For my first bleed I followed the Park tool video which was start at the caliper and push the fluid to the handle reservoir and attached cup then reverse that from the reservoirs, pump the lever and bleed at the caliper. Messy part is when I did a dumb ass move and flipped the fork with the cup filled with fluid, yikes. The gravity feed video a couple post up seemed to be a bit simpler. Fill the cup remove the caliper, open the bleed nipple, rotate caliper and let momma nature take its course and the have a beer( I added that last one ). I was just curious if one method is preferred. I kinda like the gravity feed.

Oppa,

I sorry, I misunderstood what you were referring to when you mentioned performing a gravity bleed. I thought you were referring to a quick bleed using the bleed cup at the brake lever to dispel air. A quick bleed at the brake lever works in a similar manner as performing a gravity bleed at the caliper.

I speak only for myself personally. I do not use the gravity bleed procedure. While I've tried it in the past, I do not find it 100% effective and reliable at removing air from a brake system. Using a good quality bleed kit to remove all air from a brake system insures a well bled system. Don't get me wrong, a gravity bleed will absolutely work, however it may not remove all air from the system.

When I bleed a brake system, I want my brake levers to be "Harder than a Whore's Heart" with absolutely no "Air" play in the brake lever. A brake lever with no air in the system allows you to feather the brakes with finesse which is especially important on critical sections of trail.

Captain Ahab, Moab Utah

As I mentioned, a gravity bleed will work, provided your pistons are fully collapsed and not sticking in an open position that can potentially create an air pocket behind the piston and trap air. Brake calipers, especially four piston calipers, have fluid ports machined in the calipers which link up the piston bores and can trap air bubbles. Additionally, calipers may have internal machining/milling which if not done correctly will create an area which an air bubble can attach via surface tension and not release during a gravity bleed.

Lastly, I would like to add that while a gravity bleed will work and is especially effective if you are on a bike trip and do not have access to a bleed kit to affect an emergency brake repair. However, a gravity bleed does not always insure that you have removed 100% of all air from the brake system. A good quality brake bleed kit with heavy duty, good quality brake syringes will insure that all air has been removed from the brake system, i.e. SRAM, Shimano, TRP, Magura, Tectro, etc.

I took apart one of my spare Shimano XT calipers to show you Shimano's internal caliper machining and brake fluid transfer ports which can potentially trap air via surface tension while performing a gravity bleed:

I hope this helps forum users with their braking adventures....Embrace the messiness.....I crack myself up....

Be safe,
Rod