brakes go to the floor when car starts.

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They feel great with the car not running. the pedal holds pressure really well. It does not bleed down. I have no leaks and i am not loosing fluid. As soon as you start the car it falls to the floor. The brakes work with the engine running but not well! if im driving 10 mph and i lock them up. The pedal goes rite to the floor and the car will brake with like 25% of what it should. I have a 11inch dia booster with a cast matser. I think its gota be the booster cause why would the master hold pressure with the car not running?
 
Does the brake light come on at all?
 
you never changed the master?
 
not since restoring the brake system. master was replaced two years ago because it was leaking down the booster.
 
replace the master
 
I was thinking that as well. I would like to replace the booster and master but i was just woundering about narowing it down. the car has always had a low pedal even after replacing the master two years ago. it didnt seem to do any good. If the master is bad why would it hold with the engine off?
 
check the vacuum check valve and just in case, gravity bleed the brakes.
i don't know what it is, but i've had a few times where cars gave me a bad pedal and gravity bleeding always helped. just pop the top off of the master and open the bleeders one at a time. when you open one, walk away. make sure the master stays full, but give each one a couple of minutes to bleed.
 
the vacume check valve is the black pastic container imbetween the brake booster and the hardline off the carb? what is the plastic fitting coming out of the brake booster face. it is so delicate if you touch it you can hear vacume pressure leak out.
 
usually the check valve is the fitting on the booster and there isn't anything in-between there and the carb. but yours might be different. either way, it should NOT be leaking air at all. definitely replace it.
 
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