Upside down bike and the brake lever go slack

[steve_sordy]

I turned my bike upside down yesterday to take a look at something on the front brake. After about five mins I turned the bike the right way up again. The front brake lever was slack. I pumped it a few times and it was back to normal. There doesn't seem to be any air in the system - I had bled the brake the week before and got no air bubbles out then. Curious, I turned the bike upside down again for a few mins. Same thing. Before going "e" and using a tow bar rack, I used to transport my bikes in the back of the car. Turning upside down was part of the process; front wheel removal. So twice every trip my bike was turned upside down, sometimes more than twice! I can't recall ever having this lever going slack issue before. It is not causing me a problem, but I'm curious. Has anyone any insight into this?

 

[Gary]

Yes. it's due to air in the system.
Solution: Just don't ride your bike upside down

BTW that's an awful lot of typing to do while not specifying what actual brand/model brake it is

 

[towzer]

My gfs avid brakes can do that, firm at lever, then bike upside down and then get full travel at lever till pumped a few times, very strange

doing a full simultaneous bleed, at both master cyclinder and calliper appears to have fixed it.

 

[steve_sordy]

Yes. it's due to air in the system.
Solution: Just don't ride your bike upside down

BTW that's an awful lot of typing to do while not specifying what actual brand/model brake it is

Air in the system was the only logical cause I could think of, but I could get no bubbles out when I bled it. I will try again, only harder this time!
Once the bike is upright and back to normal, the lever doesn't get harder when I pump it.
Brake is Shimano BR-M520, they are 4-pot.
Do some brand and models of brake have this and not others then?

Unlike the hero stunt riders doing 360's all the time, I try really hard not to ride my bike upside down. I fail occasionally.

 

[Gary]

Brake is Shimano BR-M520, they are 4-pot.
Do some brand and models of brake have this and not others then?

It's common with all open system hydraulic cycle brakes.
Shimanos are pretty well known for it. among other poor traits