Intermittant brake failure
 

Today I was at 1600m in the Rif Mountains, 35km from the nearest tarmac road and on a long downhill section when all of a sudden my back brake stopped working. I got off and scratched my head (that's about the limit of my technical skills). Couldn't see anything obvious, my pads were OK, the calipers were floating OK, there was fluid in the little reservoir thingy.

So I started off rather gingerly using engine braking and front brake only which was rather disconcerting. Then all of a sudden the brake started working again. An hour or so later it happened again. This time I noticed the fluid in the reservoir was quite high, and when I pumped the brake pedal it gradually fell back to a normal position. And then the brakes worked OK again.

Could it be old brake fluid or an air lock or something that is affected when the brakes are really working hard?

I had the bike serviced just before leaving but I don't have much confidence in the dealership, they failed to notice my clutch cable was almost frayed through (despite replacing the clutch at the time). And there was nothing on the bill for brake fluid despite asking for a full service.

All suggestions welcomed.
Tim

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Indeed: replacing the fluid would be the first thing to do. Looks like it's either old fluid (containing water) or not so old fluid with air trapped.

HJ

Hi Tim, yep I think you are on the right track, brake fluid is hygroscopic- absorbs water, when the brake heats up the water is turned to vapour, a steam bubble, which is no use as the effort from the master cylinder is not transferred to the caliper. when the system cools the steam returns to liquid and the brake returns.

Its easy to replace the old fluid with new Dot 4. fit a hose to the caliper bleeder other end into a clear bottle etc, open [8mm iirc] now make sure you keep the resovoir topped up pump away until the old is flushed out. important- the last pump hold down the pedal and close the bleeder. pump a few times to make sure you have a firm pedal. and you are good to go.

bernard

Tim, with symptoms as you describe, another worthwhile check is the master cylinder seals, especially if your brake pedal was moving a long way with no feel. Test by standing on the brake pedal (bike stationary! :) ) and keep your weight on it and see if it starts sinking down, if it does then suspect the seals are leaking fluid past them. Pumping the pedal sharply a few times (which flairs out the seals and reseats them) should return feel at the pedal. Stand on the pedal again and see if it drifts downward again, if so suspect worn seals, strip and replace.

I'm with Bernard. Funny enough I recently had the same problem,, but on a MTB, fluid was about 4 yrs old......


<added later>

The service schedule for fluid is every 2 years, so I wouldn't expect this to be included normally, especially if the bike is three years old, it should have been changed at 2yrs, so it's next normal replacement would be 4yrs. However, they should have asked.

Mine gets changed every year, it's a wet place I live in, so for the sake of a few quid it's worth it.

Change dealer!!!!