At idle the brake pedal slowly depresses to the floor. If I let my foot off and press again, it regains original pressure but then slowly depresses to the floor again. If I give it a little throttle above 1000 rpm, the brakes maintain hydraulic pressure. I can't find any sign of leaks, and I've bled it to be sure there's no air. Given the dependence on engine RPM, my guess is that the brake booster or master cylinder have gone bad. Any advice?

 

Try these tests:

 

hmm i have a similar problem but idk if it's a problem...

RonJ: If I am idling right after driving, the idle is a little high (~1.1k) but after a few seconds it drops to normal. I assume this to be normal. However, when the idle drops to normal, i feel the brake pedal go down a little bit... not a lot, just a tiny bit. I'm not sure if it's suppose to do this or not.

 

Trust, it doesn't sound like a major problem, but you may want to test the check valve.

 

i checked it and it's fine
thanks

i'm done with my short little threadjack

 

RonJ: Thanks for the info. It failed functional test #1 and passed the other tests, so it appears the booster is ok. Today while driving home I noticed that I'm now starting to lose substantial braking power, and it's getting a little dangerous. The master cylinder, lines, and calipers are all original. I've just ordered a new master cylinder. I'd rather not have to replace the calipers if possible--is there an easy way to test for faulty caliper or lines?

 

it probably wont need calipers. mos def sounds like a master cylinder to me. depending on how bad the master cylinder is, you probably wont really be able to check the calipers for piston movement.

 

I agree. The master cylinder is the prime suspect.

 

Thanks guys. Hopefully this will do the trick.

 

I replaced the master cylinder, and no more problem. Many thanks!