When i first start the car: if the car is not reving at least 3000 rpm the brakes go all the way to the floor

While driving: brakes work fine on the interstate, but around 2-3000 rpm range *somtimes* the pedal goes to the floor.

After its been running awhile: even with the car sitting still idle at 1000 rpm the brakes *sometimes are fine*

Its like they work when they want to,could this be from a poorly done valve job and the motor is not pulling a good vacuum? I would say its the booster or master cylinder going bad but this has only been happening since i put my motor in. Its been about a month/3000 miles. And its done it since the night i started the car with the new motor in. Any help before i spend $$$ on brake system? Thanks

 

i just had a booster problem that i fixed today... the one way vavle was on wrong... saved me 100+ bucks noticing that... the things people do..... but anyway... disconnect the vaccuum line from the booster, leave it connected to the intake manifold... then stick your finger over the other end with the one way vavle still hooked up.... the engine should almost stall (will if you dont let your finger off the tube), if you feel the vaccuum going through, you've solved your vaccuum question (dont put your lip up to the tube, and dont ask lol stupid testing ideas haha) but anyway, if you dont feel alot of vaccuum, disconnect the one way vavle, and check the intake manifold vaccuum tube again... if you feel it now, replace the one way vavle... if all of that still leaves you there... id replace the booster/master cylinder ... cuz if the master is old, it most likely will go soon after the new booster is put on, so why not knock it all in one shot... other things to follow with are bleed all of the brakes... look/listen for air, look for milky fluid or debris... and keep bleeding until you get back to clear fluid.... but all in all id start with the vaccuum line, then move to bleeding the lines out, and then worry about master/booster... start with the free or cheap stuff... cant hurt to do/replace anyway.

 

well it probably your master cylinder or check valve. If the brake boost went bad it would just be hard to stop you would have to press harder.

 

The only thing that is engine dependant in your breaking system is the booster, which can also leak internally. But the problem with that is, the pedal should not be going all the way to the floor. You may want to look at your booster, as well as a master cylinder or bleeding your brake system.