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Front brake becomes soft after ride


luckydim
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Afer about 1 hour of wheelie training, the front brake lever becomes very soft (like it's free play increased twice). So you I have to press lever very hard to apply the break. The brake disc and pads are not hot at this time. If I left the bike for a few hours - front brake starts working in a regular way. 

Does anybody experience the same problem?

 

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A couple of questions, 

Does the lever firm up after a few pumps?

Does your master cylinder piston return all the way to the stop when the lever is let out?

Is the lever feel correct at the start of the ride?

This almost sounds like you have a warped or bent rotor pushing the pads back slightly. A slightly misaligned caliper might also cause this. The throw of the pistons in the caliper is really short and rely on the elasticity of the O-rings around the pistons to pull them back. Any deflection by the rotor will push them back further than normal causing the symptoms you describe.

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7 minutes ago, dan williams said:

 

 

Hi Dan, thanks for a fast reply!

1) it is strange, but lever becomes softer if I continue riding and pushing it,

2) I will check the piston position tomorrow,

3) the lever feel correct every time I start a ride.

 

 

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It is possible to damage the return spring in the master cylinder. A crash onto the lever will do it. The caliper won't return the master cylinder piston all the way by themselves so it's left to the spring in the master cylinder to chase the piston to the end of its stroke. If that spring is damaged the master cylinder can, for lack of a better term, "pack It in" with repeated actuations. If there is any fluid seepage anywhere in the system it just gets mushy feeling. Remove the rubber cover from your master cylinder and hold the lever in for a bit. You want to see if the lever slowly squishes in over time which tells you the system is losing fluid somewhere. After a few minutes let the lever go and see if the piston returns all the way to stop on the circlip that holds the whole thing together. It should return without hesitation. If it doesn't you'll need to rebuild the master cylinder which Is fairly easy but bleeding is time consuming and requires patience. 

Also eyeball the front brake rotor. Spin the wheel with it off the ground and look for deviation sighted from the caliper body. If it wobbles you're pretty much stuck getting a new rotor. Getting one of these thin little stainless steel rotors to be flat again is almost impossible. It's possible the rotor is flat but something got into the floating mounts that is forcing it to be crooked but I've never seen this. Just mentioning it to be thorough.

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Don't forget to check the condition of the hose, I have been arguing with  my front brake for mushy feel for quite some time.

I finally got very mad at it and pulled the entire system off the bike, that is when I noticed the pinched spot in my hose.

I theorize this flat spot will cause inconsistent pressure due to the flat spot trying to go round again when the brake lever is pulled in.  I am awaiting a new hose to replace it.  Hopefully this fixes my problem

 

Edited by zippy
Edited for clarification, if not clear, than poor editing
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Thanks Zippy, I've checked the hose - didn't find any spots there.

Hey Dan, I removed the master cylinder cover. Should I see the piston position through these small holes at the bottom of the cylinder (photo attached)?

IMG_20180305_211920.jpg

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I didn’t see anyone mention this (sorry if I missed it).

Make sure there it play between the lever ‘rod’ and the piston in the master cylinder. If the adjustment is too tight, the piston cannot come all the way back and it will do as you describe.

Also check the adjustment that controls how far OUT the lever sits- if it is too far toward the handlebar it will do the same thing.

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So I am assuming the brakes on your bike are Grimeca/AJP? I have had these issues as well. All  of the suggestions so far are good, but I have not seen anyone talk about the brake master cylinder piston. This turned out to be the problem with mine, and several other bikes I have worked on. Of course, it could also be in your caliper, but it's easier to start with an examination of your master cylinder piston first. 

You can lean the bike against your work bench on a slight angle, with the brake side out. Then take off your lever and then remove the c-clip that holds your piston in the cylinder bore. You should not lose too much brake fluid this way. Then take a magnifying glass (I have a 10x photo loupe) and look at the piston seals. With wear, the seals develop tiny tears, worn away spots... the are easily visible, and you will mainly see them on the inner cup seal. These micro-tears result in an imperfect seal, leading to the inconsistent braking. If the seals are damaged, replace the piston (the kit is sold as a piston and seals).

If there are no tears, you can try putting some silicone grease on the seals, as they may be sticking in the bore. I have had to do that on some bikes as well. Once you have the brake back together, right the bike and do a quick bleed, just to top up the reservoir and make sure there is no air in the system.

If none of these suggestions work, it may be time to split the caliper and change the seals in the caliper. This is not a bad job, but ruling out the master cylinder first will save a lot of time. 

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On ‎3‎/‎6‎/‎2018 at 12:16 PM, lotus54 said:

I didn’t see anyone mention this (sorry if I missed it).

Make sure there it play between the lever ‘rod’ and the piston in the master cylinder. If the adjustment is too tight, the piston cannot come all the way back and it will do as you describe.

Also check the adjustment that controls how far OUT the lever sits- if it is too far toward the handlebar it will do the same thing.

Actually yes and no. If the lever is adjusted too far in so the holes in the reservoir are never uncovered and the system has no leaks then the brake can permanently pressurize due to temperature rise of the hydraulic fluid which drags and causes more temperature rise and eventually locks the brake. By the same token if there is a leak in the system, and that can include fluid leaking past the piston in the master cylinder, and the holes in the reservoir are never uncovered then the system is never replenished and will go soft once it cools down. This is why I suggested start by making sure the piston in the master cylinder is returning all the way. If it is then the system is self adjusting and the cause is likely a mechanical displacement of the caliper pads. If the master cylinder is not returning fully then you lose the self adjusting ability of the hydraulics and have to go after the subtleties of the hydraulics starting with why is the piston in the master cylinder not returning and there can only be a few reasons. In order of likelihood; One being adjustment as you pointed out. The second being a damaged return spring above the master cylinder piston. The third being a damaged piston/bore in the master cylinder.

The hydraulics on a trials bike are pretty simple once you realize they are not a closed system at rest but once actuated (and the master cylinder piston sweeps past the ports into the reservoir) the systems are closed and behave accordingly. But the throws are so small that it doesn't take much to bung things up.

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