Quote:
Originally Posted by BigDukeSix
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at about 400 miles or so, my rear brake began giving me issues - would only work if I pumped it. I stripped a screw when attempting to bleed the brakes and wound up having to replace the master cylinder.
Once that was replaced, all was good for another 100 miles or so and now the problem is back - but not every time I ride. I'm betting that I still have the original problem but it was just temporarily solved when I replaced the master cylinder.  ... |
Not much can cause the problem that you are describing. Since the master cylinder is new, I think that you can eliminate it as a suspect. Things to check would be the rotor for warpage, which can cause knock back of the piston into the caliper, which would necessitate pumping the pedal like you describe, to get any rear braking. The caliper could be binding, so check the forward caliper slider pin, make sure that it is lubricated with silicone grease, AKA plumbers grease. The slider pin should slide in and out of the rubber bushing effortlessly; and the caliper itself should rotate upwards clockwise easily when you remove the rear pin. If you haven't already, I would replace the brake bleeder with a Speed Bleeder, and then re-bleed the brakes. If I recall correctly, the bleeder is a M8 x 1.25 - these are the ones that I used:
https://www.amazon.com/dp/B000WJ2SYK...ustomerReviews
They make bleeding the brakes pretty much foolproof, and air in the line can cause your symptom. Water in the brake fluid can do it as well, if your brake fluid has been sitting around awhile in a container that was not tightly sealed. Hard braking can cause the water to boil, flashing into steam, and causing you to have to pump the pedal. If yours is suspect, get some new DOT 4 brake fluid. One more thing to check would be for any soft spots in the rear brake line. To spot these, look and feel for any bulges in the line when you apply the rear brake.