variation in brake pressure

Hello all,

I have a 2000 Impala. I bought it used with just over 40,000 miles on it. So far it has been a pretty good car. It's mainly used as a commuter car.

The problem I'm having with it, which I hope someone can provide some help, is as follows:

My car is typically parked in my garage. I put the vehicle in reverse to pull out, almost immediately I hear "clunk" noise (it could be a separate problem of course). I pull into the street and stop. At this time the brake pedal feels fine regarding pressure. When I shift the car into Drive is when the pressure changes. After I drive a few feet and step on the brake pedal, it sinks to the floor. If I pump the brake pedal two times, the pressure returns to normal. The pressure never changes again until I do the same process (reverse to drive). Additionally, if I step on the brake pedal and keep pressure on it, it does slowly sink an inch or so.

After some investigation I determined that this must be a master cylinder problem. So I replaced it and bleed the brake lines.

This Impala has 4 wheel disc brakes, but not Anti-lock brakes. A very uncommon option, which is indicated by the price of the replacement part: $150 for a refurbished master cylinder versus $300 for a brand new one from GM.

Well, I'm back at square one. Immediately after doing the work mentioned above the car acted no different. I am going to replace it again with another one, in case it was a bad master cylinder. Fortunately, the auto parts store is willing to work with me on this since it has a warranty. But it seems unlikely that I would get a refurbished part with the exact same problem as the one it was replacing.

Does anyone have any thoughts on what this problem could be?

It has to be the master cylinder, doesn't it????

Thanks

D

I had the exact same problem that you just described. But mine went out rather quickly...sent me through a stop light because i couldn't stop. Towed to the dealer and they put a new master cylinder on it. Make sure you get the one that isn't for ABS. After they replaced it, it was fine haven't had a problem since. Costed like 350 and a few pennies to get it fixed...

mike

Thank you for the reply. I appreciate it.

I replaced the master cylinder again. This time I noticed a new pattern.

Did yours do the EXACT same thing regarding going from reverse to drive? Did you hear a noise when you first start to go in reverse like a clunk in the front end of the car?

After having the third master cylinder in this car I still have the same problem, but this time I seem to notice that it only does it going from Park to Reverse to Drive. If I switch between reverse and drive it doesn't do it, but if I put it in Park and then Reverse the pattern repeats itself.

So now I'm wondering if there is a sensor that is malfuntioning in the transmission or something. This is perplexing.

Thanks for your input.

-D

Check the vacuum line on the brake booster it might be worn or cracked and when you put it into rev it moves and looses its vacuum. As per the noise i would look at the motor mounts that are worn and letting the motor move to much and that would cause your possible vacuum booster problem