HELP! Diagnosis Needed.

[rumrumm]

I have a problem with my power brakes and I am at my wits end trying to figure it out so I could use some help diagnosing it. The car is my '32 which has a Corvette dual master cylinder and a singe power brake booster. When the car is shut off, the pedal seems fine--springs back like it should. As soon as I start the car and push on the brake pedal, the brakes grab but they will not release (both front and rear) when I let up on the pedal. I have replaced the booster thinking the diaphragm inside was bad, played with the length of the rod connecting the pedal to the booster. I checked the rod and it is perpendicular to the booster. The Wilwood residual valves are installed correctly. So far, nothing has helped. The vacuum hose is connected to manifold vacuum off the back of the Edelbrock carb. What could be wrong here?

[Fat Cat]

I have a problem with my power brakes and I am at my wits end trying to figure it out so I could use some help diagnosing it. The car is my '32 which has a Corvette dual master cylinder and a singe power brake booster. When the car is shut off, the pedal seems fine--springs back like it should. As soon as I start the car and push on the brake pedal, the brakes grab but they will not release (both front and rear) when I let up on the pedal. I have replaced the booster thinking the diaphragm inside was bad, played with the length of the rod connecting the pedal to the booster. I checked the rod and it is perpendicular to the booster. The Wilwood residual valves are installed correctly. So far, nothing has helped. The vacuum hose is connected to manifold vacuum off the back of the Edelbrock carb. What could be wrong here?

Have you checked the master cylinder to booster clearance? There needs to be 1 /16" clearance between the piston in the cylinder and the booster pushrod.

[40]

I agree with Frank,check the rod again....Just had the same problem with my 40 when I replaced the master cylinder...Without that 1/32 to 1/16 clearance you'll build residual pressure creating a full time braking situation.If you unbolt your master cylinder and it pushes away from the booster...you don't have the proper clearance.Good luck and let us know how it works out.See Ya at the Nat's!

[tomslik]

does the pedal stay down when you let off the brakes?
if so, you need a return spring on the pedal.

[rumrumm]

The pedal springs right back before I start the car. Thanks for the suggestions. My mechanic is coming over after work tonight and we are going to try to figure this out. I'll let you all know what the nature of the problem was.

[BFS57]

Hello;
Just went through break hell on my '57 chevy. I will tell you what I did.
First, it is of utmost importance that the rod pushing on the booster from the pedal be absolutely straight! If it's off, even by a RCH , the system binds!.
I found out that the rod end coming out of the booster, pushing on the Master, has adjustment too! At first, I ended up placing a couple of 1/16" washers between the master and booster, but my problem was pedal/rod input being off center.
I thought I had it fine and then I would drive and the brakes were pressing ever so slightly until the fluid heated up and then, the car was going nowhere!! I ended up in the "bad" part of town at nite waiting for my wife to come rescue me! (which didn't sit good with her).
Remember, any adjustment how ever minute makes a big difference when it comes to this booster/master combo! Thats what I found out!

Bruce

[HOTRODSRJ]

I'll post my answer here for what it's worth as well.

I think it's the vacuum booster one way check valve.  This valve is built into the connection port to all boosters allowing one way vacuum when the pedal is depressed. If the valve is faulty or installed wrong it can "capture" vacuum in the diaphram/plenum and not release the brakes until the engine is stopped.  Check the valve for proper operation.  You should be able to blow thru the booster end unfettered.

But, there is another internal valve inside the booster that could be the culprit as well. This is not accessible and could be a fault too.

[HOTRODSRJ]

See my comments here.. http://www.chevytalk.org/threads/showflat.php?Cat=0&Number=1272707&page=0&sb=&o=&vc=1&PHPSESSID=#Post1272707

[rumrumm]

Thanks, Steve. I will  be checking it out this evening.

Lynn