Odd failure on front disc brake

[m2468]

Hi,
I bought a new bike for my son. He reported I don’t think the front brakes are quite working right.

I dutifully inspected them and found the following:
1. If I brake harshly (eg quickly and with force as if emergency stopping) then the brake lever moves a little and stays in position until released (as expected) and the brakes work as expected. For me this rules out air in the system as the brakes would feel spongy and not be working tremendously well.
2. If I brake gently as if gently coasting to a halt over a few hundred metres then something alarming happens. As you are gently applying constant pressure to the brake lever it slowly moves back until it touches the handlebar when you then run out of travel and hence brakes. A quick release of the brakes recharges the system and you can begin the whole process again.
3. There is no visible evidence of brake fluid leaking at the bars, the hoses or the callipers.

The bike was returned to a store owned by the seller who bled the system 4 times, did not find air in the system and confirmed the fault was still present. This did not surprise me as under hard braking the lever is firm so I did not expect to find air in the system,

The bike was returned to the factory where they have bled the system and are reporting system OK.

My thoughts.
If there was air in the system it would affect braking whether gentle or hard. I do not believe air in the system can explain harsh braking being OK and gentle braking meaning the brake lever moves to the handlebar.

I cannot think of a condition where brake fluid itself can cause the issue, it does not compress more under gentle load than hard load!

My thoughts are there is a seal somewhere that is damaged or mis formed. Under hard pressure the pressure of the fluid is splaying the seal sufficiently to create a seal and the brakes feel normal. Under low pressure the seal is not splayed and the brake fluid can creep past the seal. When the brake is released there is a return path for the brake fluid to recharge the system.

So what I am asking is do the above conclusions sound sensible and has anyone come across this type of failure before?

Braking system is Shimano 105.

Thanks all,

Mark

 

[raleighnut]

Hi,
I bought a new bike for my son. He reported I don’t think the front brakes are quite working right.

I dutifully inspected them and found the following:
1. If I brake harshly (eg quickly and with force as if emergency stopping) then the brake lever moves a little and stays in position until released (as expected) and the brakes work as expected. For me this rules out air in the system as the brakes would feel spongy and not be working tremendously well.
2. If I brake gently as if gently coasting to a halt over a few hundred metres then something alarming happens. As you are gently applying constant pressure to the brake lever it slowly moves back until it touches the handlebar when you then run out of travel and hence brakes. A quick release of the brakes recharges the system and you can begin the whole process again.
3. There is no visible evidence of brake fluid leaking at the bars, the hoses or the callipers.

The bike was returned to a store owned by the seller who bled the system 4 times, did not find air in the system and confirmed the fault was still present. This did not surprise me as under hard braking the lever is firm so I did not expect to find air in the system,

The bike was returned to the factory where they have bled the system and are reporting system OK.

My thoughts.
If there was air in the system it would affect braking whether gentle or hard. I do not believe air in the system can explain harsh braking being OK and gentle braking meaning the brake lever moves to the handlebar.

I cannot think of a condition where brake fluid itself can cause the issue, it does not compress more under gentle load than hard load!

My thoughts are there is a seal somewhere that is damaged or mis formed. Under hard pressure the pressure of the fluid is splaying the seal sufficiently to create a seal and the brakes feel normal. Under low pressure the seal is not splayed and the brake fluid can creep past the seal. When the brake is released there is a return path for the brake fluid to recharge the system.

So what I am asking is do the above conclusions sound sensible and has anyone come across this type of failure before?

Braking system is Shimano 105.

Thanks all,

Mark

Sounds like a faulty master cylinder to me, with such a 'safety critical' fault I'd be returning it for a refund or an alternative bike

 

[Drago]

It definitely wasn't air.

 

[prestonhere]

Hi,
I bought a new bike for my son. He reported I don’t think the front brakes are quite working right.

I dutifully inspected them and found the following:
1. If I brake harshly (eg quickly and with force as if emergency stopping) then the brake lever moves a little and stays in position until released (as expected) and the brakes work as expected. For me this rules out air in the system as the brakes would feel spongy and not be working tremendously well.
2. If I brake gently as if gently coasting to a halt over a few hundred metres then something alarming happens. As you are gently applying constant pressure to the brake lever it slowly moves back until it touches the handlebar when you then run out of travel and hence brakes. A quick release of the brakes recharges the system and you can begin the whole process again.
3. There is no visible evidence of brake fluid leaking at the bars, the hoses or the callipers.

The bike was returned to a store owned by the seller who bled the system 4 times, did not find air in the system and confirmed the fault was still present. This did not surprise me as under hard braking the lever is firm so I did not expect to find air in the system,

The bike was returned to the factory where they have bled the system and are reporting system OK.

My thoughts.
If there was air in the system it would affect braking whether gentle or hard. I do not believe air in the system can explain harsh braking being OK and gentle braking meaning the brake lever moves to the handlebar.

I cannot think of a condition where brake fluid itself can cause the issue, it does not compress more under gentle load than hard load!

My thoughts are there is a seal somewhere that is damaged or mis formed. Under hard pressure the pressure of the fluid is splaying the seal sufficiently to create a seal and the brakes feel normal. Under low pressure the seal is not splayed and the brake fluid can creep past the seal. When the brake is released there is a return path for the brake fluid to recharge the system.

So what I am asking is do the above conclusions sound sensible and has anyone come across this type of failure before?

Braking system is Shimano 105.

Thanks all,

Mark

Well on a car it would be Master cylinder failure or flexible hose expanding easy checked By fitting a bleed nipple just to cylinder if you can replicate fault then it's master cylinder if not it's hose or caliper again easy checked

 

[CXRAndy]

Its either air in the system or the lever seals are failing in some way.

I prefer to either apply pressure or vacuum to bleed and not use gravity bleeding.

 

[chriswoody]

I was leaving this thread to more learned souls than me, I have no real idea about the vagaries of Shimano brakes. Then this morning on the train I was reading about the new Shimano XTR groupset when I came across this which describes your situation to a T:

"If you’re not familiar with wandering bite point, it can result from a specific combination of conditions and actions that cause a brake lever to suddenly pull almost to the bar. It can happen to any brake, but it’s common with Shimano. Some power still gets to the caliper, but not much. To understand a primary cause for this, we need to understand caliper seals. They move the pads by flexing in and out against the pistons kinda like the springs around the edge of a trampoline. As heat builds up, the seals’ rebound speed increases. Simultaneously, fluid viscosity drops, and it can be pushed into the brake-lever reservoir too quickly. Then, if you immediately pull the brake again, there may be be less fluid in the system and the bite point can “wander” in. It’s the opposite problem that I had with the SRAM Maven brakes, whose bite point would wander out because the seals couldn’t push the fluid up fast enough. The solution is balance"

Link to full XTR Review

Shimano seems to be more than aware of the issue and have redesigned the new XTR brakes to eliminate the problem. All of which doesn't help you, but at least it explains the issue.