In August of last year, I got an Auto Zone reman master brake cylinder and a new reservoir. It came with the car'* purchase price, sort of. 1337ssei'* dad and I did all new brake lines, the cylinder, synthetic brake fluid, and front and rear brakes. We even got brand new calipers, pistons, etc. The car has no remnants of the old brakes except perhaps for the brake pedal.

I first noticed a problem when I was driving on the highway at about 60(+) mph, and there was a sudden traffic jam up ahead - perhaps a quarter of a drag strip or so. I hit the brake pedal as I would usually, but nothing happened. I pressed down hard on it, and it just went straight down with about twice the resistance of the gas pedal. However, as soon as it bottomed out, the car started slowing down just so that I stopped nearly right at the bumper of the guy in front.

When my car was stopped, I put on the parking brake, put it into neutral, and started pumping the brakes while the engine was idling. The pedal just went straight down with a whooshing sound and came right back up just as fast the first few times, and then it started giving some resistance. It was time to move, so I released the parking brake, put the car into drive, and drove at about 20 mph with the line ahead. The brakes worked just fine after that, but still I parked it in my driveway until tomorrow for some work whorage during hopefully nice weather.

Well, actually, I did drive it a little after that. I wanted to see if it would happen again. This seems to happen when the engine isn't warmed up fully, but that could just be a weird coincidence. It happened again, but I know now that when the brake pedal bottoms out, my car slows down just as hard as with more than half pedal with new brakes. Pumping helps, and for the rest of that trip, the brake thing never happens again.

Is this a failing master cylinder? I plan to flush the fluid tomorrow and see if there'* a difference then. In that respect, I think it could be that the fluid has absorbed a lot of moisture over time for some strange reason. Is this sort of thing a common problem with this car, brake system, or anything of that nature?

Master cylinders are like that.. won't work once then will work for the rest of a day.

You've described a classic master cylinder being bad. If it was consistent or you saw fluid on at any of the wheels or a puddle on the ground, even had a low master cylinder level then it could be explained.

When it'* intermittent and no external physical issues are observed it'* frequently the master cylinder.

Did you also replace the rubber hoses from the metal lines to calipers?

I agree with the diagnosis. It happened the same way to me.

Can it be a vacuum problem with the brake booster line?

I figured i'd blame the cylinder as well, though, for now. The rubber lines were also replaced, and I think that was all that was leaking when I got the car, anyway. We still replaced all the lines for good measure, though.

If it were the vacuum supply, it would be like you pushed on the brakes when the engine isn't running. You know that feeling?

I've had only had brake booster trouble on one of my vehicles (99 Tahoe) & strangely enough that truck went through two of them. You described a "whooshing" sound that makes me think it might be a possibility. I would some times hear a vacuum leak when pushing on the brake pedal when the truck was stopped. It would do it in drive or park.

I do know that feeling really well, but would such a brake pedal state still slow down the car?

I missed your post, Ted. I hear th whooshing sound coming from the brake pedal area, so I don't know if it'* a vacuum leak. Also, I recently replaced all my vacuum lines and have had no leaks since, as far as I know. When I'm in P and the engine is off, I don't hear a vacuum leak.

Yes, you just loose your vacuum assist, so you gotta push really hard, but the pedal won't go to the floor, IIRC.

I still have a warranty on the cylinder, so I'll see if I can get that replaced today. I guess since that'* all it could be, I might as well. Thanks for helping me narrow it down.