QuestionHi Bill, Hope you can help. I,ve got a bike that when it sits unused for a month or so - the front brake is spongy. I bleed it, no evidence of air and no external leaks visible. After bleeding all ok until the following ride. Rear continually works fine. Bike 6 years old and only 7,000klm. Any clues. P.S. Now pretty much new fluid through system from so many bleeds as well. Thanks.
AnswerMark, I am not familiar with Yamaha brakes, but the whole scenario sounds a bit of a mystery, indeed. It could be that that the master cylinder piston cups relax somewhat after sitting unused and are becoming deformed or more brittle from sitting. I would go through the master cylinder and freshen things up. Often when bikes or cars sit, corrosion can build up in the nooks and crannys of the hydraulic system, requiring more than just a flush.
Brake fluid draws moisture from the air and absorbs it into the system where it falls to the lowest point, generally. That is why calipers often stick because of moisture induced corrosion getting into the seal grooves.
Make sure that the bleed holes in the bottom of the reservoir or top of the mastercylinder are clear, depending on design. One feeds the system and the other relieves the pressure when you release the lever.
For best performance, I would change out the brake lines, too, with braided steel units.
Bill Silver
