Rear brake gets harder sometimes when riding

[KuRi]

Hi!

I know this is not directly Giant related but because my bike is a Giant Fathom E+2 Pro I will try to ask for help here

Recently I replaced the rear break with a mt520 double piston caliper. I replaced the Hose and the lever as well (deore xt m8000). I did the same with the front brake. The front brake is working perfectly.

The problem with the rear brake is that it is softer or harder depending on the moment. Sometimes it is pretty hard (almost no travel) and sometimes it works as it should (similar to front lever).

Do you know what could be the cause? I have tried 2 different levers and some bleedings with no improvement.

thanks!

 

[DrStupid]

My first guess would be air in the lines. I swapped lines, front to rear on my wifes bike, and had a similar experience. Though in my case, once the brakes heat up, they dragged a bit.

 

[Mikerb]

I agree. The rear brake is more difficult to bleed because of the extra length of hose and a lot of that hose is on the level or maybe even travelling up. There is also a greater risk of kinking or excessive bending depending how you have routed it. I suggest you do a physical check of the hose and alleviate any constriction of excessive bending found and then bleed again with the bike in a workstand tilted such that the lever piston is much higher than the caliper.

 

[KuRi]

I didn't know that excessive bending was something that could happen
I think the cable is ok but I will check it again. I bleeded the rear brake prior to the hose replacement a couple of times and never had this problem (except once that I overfilled the system and had something similar).

Thanks for the tips!

 

[DrStupid]

It doesn't excatly sound like air, based on the trouble I've had with air (spongy and
or draggy after heating up)..but it easy to bleed again.

One old dirt bike trick for mushy brakes from air, was to hold your brake lever on over night with a zip-tie. Not too tight, but about as tight as you would squeeze under heavy braking. Rumor was this would force the air up the line to the reservoir ?. Not sure about the theory behind it, but I know it worked like magic on more than one occasion.

 

[KuRi]

I have noticed that even not using the bicycle, the brake gets harder when the temp is hot and then gets softer at night... It is very disgusting because the bite point changes during a long ride on the mountain

I have checked all the cable and it moves freely along the insides of the bike so I don't think it is too bent, but who knows...

 

[DrStupid]

If its heat related, it would almost have to be air I think.

 

[Mikerb]

When you fitted the new caliper did you make sure the pistons were pushed fully back?

 

[KuRi]

Yes, I did it again once I discovered the problem.
But thanks for all the suggestions! more are welcomed!

 

[Jamze]

I have noticed that even not using the bicycle, the brake gets harder when the temp is hot and then gets softer at night...

That sounds like the SRAM problem they had a couple of years ago. The pistons in the levers were plastic, and if you left the bike somewhere hot it would swell and the levers would pull on but not release properly. You could either strip the lever down and file a bit of material off the piston, get an updated one from SRAM or a metal one from eBay.

Never heard of the same issue on Shimano though.

 

[Mikerb]

Dont think Shimano are 2 chamber like SRAM

 

[Alcove]

If you cut the line maybe it wasn't a clean cut?

Flip the bike upside down and see if that makes the lever feel change and if so that would be air in the line moving. I've found gravity bleeds work best with my Shimano's especially the rear due to length and bends. Good luck!

 

[KuRi]

That's next thing to try... cut a little bit more. I have some room to work because I left the cable a little bit longer just in case

 

[KuRi]

One additional question... I tried using a bl-t8000 lever to know if the problem was lever related (it looks like not) but then after a bleeding and when trying to use the lever to brake, a lot of oil started spitting from the 'cylinder'? (take a look of the attached picture).

Is this lever broken or what? Thanks!!!

 

[Alcove]

Did you pop that open? I don't believe that's serviceable and hopefully you have warranty on those brakes

 

[KuRi]

No I did not open it... I have already asked the shop for a warranty replacement, but it is an online shop and now I will be without the lever for a month or so. It is new and just installed it today and noticed the problem. The left one is working perfectly.

Thanks for your comment!

 

[DrStupid]

Can you put the old stuff back on while you wait? NO WAY would I be without an emtb for a month. I'd end up refinancing the house, or raiding my retirement to get more bikes.

Oops!

 

[KuRi]

haha, yes, I have some spares to try (original mt501 lever). I was 5 weeks without the emtb when the motor died and I bought another temporary bike that I have already sold xD

 

[DrStupid]

haha, yes, I have some spares to try (original mt501 lever). I was 5 weeks without the emtb when the motor died and I bought another temporary bike that I have already sold xD

These things are like crack

 

[Pdoz]

Check your hub for side play - the giant hubs are a pita to get right, but I find when mine develop a bit of slop the rear brake has less travel when I'm hitting left berms hard. I've never been able to work out what's happening, but adjust those hubs before the freewheel pawls disintegrate!

 

[HORSPWR]

Are you able to bleed them like a Sram dropper post by pulling a vacuum with one syringe at the caliper and pushing oil in at the lever?
That method is awesome for getting all the air out!

 

[urastus]

It doesn't excatly sound like air, based on the trouble I've had with air (spongy and
or draggy after heating up)..but it easy to bleed again.

One old dirt bike trick for mushy brakes from air, was to hold your brake lever on over night with a zip-tie. Not too tight, but about as tight as you would squeeze under heavy braking. Rumor was this would force the air up the line to the reservoir ?. Not sure about the theory behind it, but I know it worked like magic on more than one occasion.

I saw this video recently - your mention of this technique is only the 2nd time I've heard of it. See from 5'23"

 

[DrStupid]

I saw this video recently - your mention of this technique is only the 2nd time I've heard of it. See from 5'23"

It has pretty much worked just like in that video for me, but I've never done it on my emtb's, just dirt bikes.

Certainly worth a try for only the cost of a zip-tie and nearly zero technical skills required.

 

[Tmeyer37]

It has pretty much worked just like in that video for me, but I've never done it on my emtb's, just dirt bikes.

Certainly worth a try for only the cost of a zip-tie and nearly zero technical skills required.

I did this trick on the hydraulic clutch on my 300 GasGas and it worked wonders.

 

[ziscwg]

I have noticed that even not using the bicycle, the brake gets harder when the temp is hot and then gets softer at night... It is very disgusting because the bite point changes during a long ride on the mountain

I have checked all the cable and it moves freely along the insides of the bike so I don't think it is too bent, but who knows...

This issue is with servo lever. My m8000 had this issue and Shimano USA warrantied them without even questioning it. Since then, I have had no changing bite point problems.

Just make sure you have your email receipt when you purchased them. It's a 2 yrs warranty in the US.

 

[urastus]

It has pretty much worked just like in that video for me, but I've never done it on my emtb's, just dirt bikes.

Certainly worth a try for only the cost of a zip-tie and nearly zero technical skills required.

In that video, immediately after that zip tie fix, he goes on to burp the brakes, which is logical.

 

[steve_sordy]

I noticed that my front brake started to feel a bit "wooden", the progression reduced dramatically. The lever still pulled and the bike still stopped, but I had to pull extra hard. Turned out the pistons on one side were jammed.

So maybe, you have intermittently jamming pistons?

 

[KuRi]

Hi! I got something by applying all tutorials at the same time xD

I tried another caliper and had the same results so:

I disconnected lever and caliper, cleaned everything, pushed pistons all the way in, reconnected, new pads, bleed, 8 hours of zip tie, bleed again... and some other things like bleeding while the caliper was out of the bike and at the bottom of the bike and bleeding with two sryinges

I have only made a 15 minutes ride but the brake was consistent all the time.

if the problem appears again I will start from scratch by replacing the cable, which I prefer not to do because I have to remove the battery lock mechanism, the motor cover, ...

Thanks all!

 

[KuRi]

Bad News... now at night it is softer again (bite point closer to the handlebar). Not too much... but it has lost a 10-15% of strength.

I am tired and out of ideas (the only one left is a new hose replacement, but I need a lot time and effort because it is somehow tricky to get the cable through the battery compartment), so I think I will try to live with this problem until I decide what to do

The front brake which I also cut and installed is very very consistent, always the bite point at the exact same distance... wth!

 

[Gary]

it's your caliper.
warranty it