Gs250t

[bensutton]

My GS is ready to go to its MOT but my front brake master cylinder keeps locking up a couple of hours after I clean and oil it. I haven't had a chance to fill it with brake fluid as it locks up so quickly and the liquid won't drain down the system. I can't see anything wrong with it. The rubber seal on the piston looks OK and the barrel of the cylinder looks clean. Maybe the rubber changes over time and becomes too sticky for the spring to move it. I'm confused.

Anybody else had this? ...all the parts are there and its a shame not to use it.

Buying another of ebay is risky as about 50% of the used parts i've bought off there end up not working.

 

[mike_of_bbg]

What, exactly, are you oiling on your master cylinder?

When it's stuck, is the caliper engaged (i.e., is the wheel hard to turn)? There's a little tiny relief passage in the master cylinder which can become clogged, preventing pressure from being released from the lines. I'd have this when my bike would sit outside all day at work, and the brake system would heat up. The fluid would expand and with no relief the brakes would become stuck. Cleaning out the hole fixes the issue, but also indicates that you probably need to flush out the brake system (at the very least).