is there a reason why when pressing on the brake somewhat hard and then let off you can feel the engine rev just a little

What you are seeing may just be the transmission down shifting. The brake booster uses engine vacuum which may account for the slight temporary increase in engine speed.

this happens when sitting still in park

Then I would suspect it is just the result of using up some engine vacuum for the brake booster.

would this have anything to do with the gear selector not going from park to drive.i replaced the master cylinder and brake booster. after I bled the system I started the truck up and noticed I left brake booster vacuum off. I put it on and then I was not able to shift from park..any ideas on this?

There is a check valve between the vacuum hose and the brake booster this valve is supposed to close off when brakes are applied to keep air from going back into engine causing the problem that you are having. Valve maybe stuck open letting air back into the engine.

thanks i'll check it out

thanks firebird,but the new check valve I installed didn't fix my problem. any other ideas?

The only other thing I can think of is you are loosing to much vacuum when brakes are applied. Start vehicle and have someone apply brakes listen under hood close to brake booster making sure there is not a vacuum leak anywhere.

thanks i'll give that a try after holiday. happy holiday to you and family

I tried applying the brakes,could not hear a leak.i also had them pump the brakes look like the booster was breathing.i guess that's normal. anything else you can think of.

firebird338 I had someone apply the brakes,could not hear a leak.i also had them pump the brakes it made the booster look like it was breathing,it that normal. anything else you can think of would be most helpful.thanks in advance

Why was the reason you changed out the master cylinder and brake booster ? Did you have this problem before replacing these parts ?

Question does your brakes go half way or all most to the floor before stopping ? If so here is one more thing you can try then after that I really do not know. Check to see if the brake rod ( the rod that goes between the brake pedal and the master cylinder ). See if it has an adjustment on it. If so adjust out by doing this it will take less air from the booster when brakes are applied. If brakes are going close to floor before stopping when brakes are applied hard taking to much air from booster at one time and engine is revving up trying to make up the difference. By doing what I suggest above may help. Hope this information helps.

To check to see if you have a adjustable brake rod remove master cylinder and turn rod that pushes it in if it is adjustable it will turn line a screw and you will be able to see screw treads on rod.

firebird338 thanks again. I'll try it. bleeding the system again o my.is there any way to tell how far to let the rod out? I didn't have this problem before. when I applied the brakes I could hear air leaking,so I replaced booster and the master since I had to remove it

Do you still have your old brake booster ? If so measure the rod length from it and adjust the new one to the same length if not you will just have to keep adjusting until the brake pedal feels right.

firebird338 when you get a chance can you give me a call. 301 332 0954