power brake conversion help with booster

hello all im new here seeking help with my power brake issues. i have a 58 buick century which i replaced the treadle vac brake system with a 90 chevy truck vac booster and a drum/ drum 68 lesaber dual master cylinder.. my problem lies with the vac booster.. when i start the car the brake pedal sucks down about a half inch and the brakes are still very hard but stop well.. i have to keep my foot under the pedal holding it up for the brakes to release... i unbolted the master cylinder from the booster and started the car and the booster pin extends all by itsleft while the car is running... i broke the old check valve going in the booster so i picked up one from the parts store in the help section. is my booster bad? am i doing something wrong? is the check valve not doing its job?

 

also this only happens when the enigne is running... if i shut the engine off the vac booster releases

 

Sounds like incompatability between mc and booster. Is it a power mc? The cup in the rear where the booster push rod inserts may be too shallow causing the booster to push on the brakes when it is not supposed to.
I found a pretty good brake article written by a guy named Dean Oshiro. Try contacting him: dean@hotrodsusa.com

 

its a power drum/drum master... my issue is with the master unbolted the booster rod extends by itself with the engine running and goes back in if you shut the engine off.. im thinking its not supose to do that

 

any other advice?

 

What Chuck says.This is pretty common with mixed booster/master setup.
I converted hundreds of drum/drum Mustangs.Here ya go:

You have to shorten the pin that comes out the booster a bit.You want
only a very slight preload here.
At the moment that is way too much.

When you have the pin short enough,test drive the car and slam the brakes hard a couple of times.Hot brakefluid expands.So see if you can still push the car easy(out of gear) with the hot brakes.
Thats it..

Ollie

 

If your check valve is inline, Is it possoble that it's in backwards?

 

For sure your booster is bad

 

Here is a link for your booster test.
http://www.hotrodsusa.com/store/tech.html
Click on brake booster test page

 

I think you need to check the push rod going from the pedal arm into the booster. It sounds like the brakes are being immediately applied on engine startup and it seems to me that could result from the booster being "activated" in the same manner as when you apply foot pressure. If the pedal push rod is too long for your booster I would think you would get the results you're experiencing.

The other suggestions about being sure there is correct push rod to piston clearance on the master cylinder is valid too, but I think that is a seperate issue from what is causing your booster to apply brakes on it's own.

Ray

 

Hnstray has it . Disconect the pushrod from pedal to booster,start engine if the rod/pin onthe front of the booster does not move shorten the pedal to booster pushrod. If with the pushrod disconected the pin still moves THEN you gota booster prob.