power brakes work by themselves

[WZ JUNK]

My neighbor is building a very nice 39 Ford coupe.  He has just fired the motor and is checking everything before the first ride.  He has noticed that when he fires the motor, the brake pedal goes down slightly and it will actually apply enough pressure to hold the rear wheels from moving with the car on jack stands.  What is wrong?

John

[Charlie Chops 1940]

It's possessed? That's a new one on me.

[Fat Cat]

My first thought would be a problem interally on the booster

[Crosley.In.AZ]

internal problems..... I've seen these things slam the brakes on when barely touch the brake pedal.

[wayne petty]

check to see if the pedal stop is applying the brake a bit...

be sure to see how much manifold vaccum there is...

i also agree that there may be something wrong with the power booster unit...

some boosters use the return spring in the master to return the pedal to its stop... i have not had one apply quite like you describe..  but i have had to add return springs to the pedal to get everything working right....


wayne....

[rumrumm]

I had the same problem with my '32. Tell him to put a spring between the master cylinder and the pedal. The weight of the pedal was engaging the booster. When I did that, it solved the problem. I went to my local hardware store and purchased a couple fender washers and a spring--simple fix.

[t-vicky]

My 40 Olds did that when I first got it fired up.  Didn't lock up by its self but if you just looked at the petal, hang-on. Turned out it was the power booster. Vacuum  was not going the right way or something

[Pep]

check to see if the pedal stop is applying the brake a bit...

be sure to see how much manifold vaccum there is...

i also agree that there may be something wrong with the power booster unit...

some boosters use the return spring in the master to return the pedal to its stop... i have not had one apply quite like you describe..  but i have had to add return springs to the pedal to get everything working right....


wayne....

Yes....After just going through the same drama, even changing the booster, I found a return spring fixed it.

[unklian]

Could be because the pivot is too far one way,
so the weight of the pedal is resting on the booster.

[2rods]

Or the pushrod is a tad too long.

[bucketmouth]

2rods beat me too it but in the same vein the rod between the master cylinder and brake booster might require backing off.
The brake pedal will drop some when the engine is fired up.
Some other good suggestions there also.

[Crosley.In.AZ]

the return spring fix would be an easy check to see if that is the problem.

[WZ JUNK]

Thanks everyone.  We will give it a try and I will let you know what works.

John