The front brake on my 2010 250 evo stopped working after the bike sat for a few months. They worked fine when the bike was parked and now there is little/no pressure. There seems to be no fluid leak. If I pump the lever several times, I get a little pressure and the brake works somewhat for a short time.

I'm thinking the problem is at the master cylinder and need a rebuild kit for it. Is this the first thing I should try?

Try bleeding it first. My Gas Gas master cylinder seals failed, common problem recently mostly due to poor finishing of the cylinder, the symptoms were that the brake would work fine if the lever was pulled quickly with force, but if it was pulled gently if would creep back to the bars with little brake effect.

It does sound like your master cylinder is letting the fluid escape back past the seals under pressure but seal kits are relatively cheap and easily obtainable. Meantime you have nothing to lose by dismantling / re-assembling the master cylinder to examine the condition of the seals and then flush / bleed the system with fresh fluid...something that should be done annually anyway!

Good luck, brake problems can be a total pain and I should know, I've just had 3 bikes with master cylinder / calliper seal problems in the space of a few weeks.

 

Mike.

First look at the condition of your pads. It`s a lot easier to fix with new pads. Almost 50% time people have worn out brakes. Loss of pressure can be a lot of things. As long as you have no leaks, a m/c rebuild kit would be the next thing to try. Good luck.

Same thing happened to me. It's the ' cup seal ' on the piston in the mastercylinder that has swollen.. They will swell enough ( 0.5mm) to block the small hole in the mastercylinder causing your problem

Get a rebuild kit

Edited January 5, 201610 yr by lockie28

Sounds like my issue. Brake lever holds firm for 5-6 seconds then creeps to the bar. No leaks good pads and flushed and bled. Noticed when squeezing the lever hard with cover off it shoots fluid out of the res. Piston kit for both ends on route. Fingers crossed.