I just got the dart together and put a brake booster from another dart on it. Now the brake pedal does not want to come back up after i use the brakes and it makes the brakes stick. But if i unhook the vaccum line the pedal does come back up, its just stiffer.
Is it possible i have to much vaccum and its holding the pedal down? I have it hooked to the back of the holley 4 barrel carb.
I have never heard of this problem before.
I unbolted the master.cylinder and started motor and the brake pedal returns by itself everytime i push it down, when i turn the motor off and push the pedal down this way it stays down? Now i'm confused. I dont see how the master cylinder could keep the pedal from returning since the rod that pushes on it is not really attached it just pushes.

Any ideas?

Not from too much vacuum. Not sure if too little pushrod clearance could cause your symptoms but check it just to be sure as it might be the cause & it's easy to adjust (want ~.060" clearance) but the prob is either the check valve in the booster or the booster itself

Is there an easy to check the clearance? When its all together i dont see any way to measure it , but i do see where the adjustment is.

I measure the depth of the piston recess in the "thimble" & subtract the amount the round nub protrudes from the booster flange from that dimention but someone posted about having a helper work the pedal & you looking in the M/C & seeing how much travel you have before the compensating port in the floor is closed by the piston cup. Might need clear fluid or to drain most of the fluid then adding clear fluid (have not done it so just thinking out loud). At rest the piston needs to move a hair before it starts building psi but I'd think that'd be more of a caliper drag issue rather than the booster being triggered but to k.i.s.s. I'd check it & I'm assuming the booster worked good on the donor car it was on.

I didnt get to check the booster when it was on the donor car so i dont know for sure it worked then. I tried it with the master cylinder unbolted from the booster and the motor running and the pedal returns after its pushed down.

I tried putting 1/16" washers between the booster and master cylinder and it didnt make a differance. Something else i noticed is the the brake pedal does not stick down if the car is not moving, but if i move it even a little while i push on the pedal it sticks.

I almost want to just put the old manual master cylinder back on and just forget the booster before i lose my mind trying to fix it.
I think i'm going to pull the booster off and open it up . May be i can see something inside if its messed up.

There is a valve in the booster that can stick and cause this.
My 66 New Yorker did the same thing. As soon as you touched the brake pedal, the booster would pull the peddle away from your foot and hold the brakes on. Made for some rather ungraceful traffic stops.

I put a return spring on the brake peddle that "fixed" the problem until I converted to discs and a dual diaphragm booster.

Kevin

The master cylinder is new.
I tore the booster apart thinking that i will probably get a new one any way so i cant hurt it. It was a little dirty inside but the rubber looks ok. I am taking the valve apart and cleaning it now, i think it may be the problem. It might not be letting the one side of the booster get vaccum after taking your foot off the pedal and that would keep the pedal down. I spent a couple hours reading up on brake boosters and how they work. I didnt know there was a valve inside them with a spring and seals connected to the rod. I figure i may as well learn now. There was some gunk in the valve and hopefully that was the problem.
I will post later when i get to try it out.

I took the booster apart and cleaned everything and tried it out. It didnt make any difference.
But i did notice that the reason the pedal sticks down only when moving is because when the clutch pedal is pushed in it puts pressure on the bushing that the brake pedal pivots on and that bushing looks like its rusted tight and the bushing is actually turning in the pedal housing and will not let the brake pedal bushing rotate on it.
Looks like i need to pull the steering column out to pull the brake housing out

Brakez work fine now that i pulled out the pedal assembly and cleaned it up. The brake pedal bushing was so tight i had to beat it out with a hammer, but the plastic bushings were still ok. It just rusted inside and would not rotate ,it was turning the whole bushing that it rides on instead .
I wire wheeled everything and greased it up good and reassembled it and it fixed it. At least after i lost all the little bearings in the clutch pedal bushing and had to run around with a magnet finding them all.
At least i got to see what the inside of a brake booster looks like now and i know its clean inside. I'm glad i didnt go buy a new booster first, it wouldnt have fixed anything.
Thanks for the help everyone.