Thread: 4 wheel drum brake ??? Need help asap!

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My friends mom asked me to look at the brakes on her 63 bonniville, car has original power 4 wheel drum brakes.
The problem

When u very lightly touch the brake pedal the brake lock up on all 4 wheels,
What would cause this????
I replaced the master cylinder, re bleed all four drums, no change.
Thanks 4 any help
( also master cylinder is the old single pot)

My first guess would be something in the power brake booster. Maybe disconnect the vacuum line to the booster and see if that makes any difference. If the MC is new and all 4 are locking up there isn't a lot left in the system. Is there a proportioning valve in that system that splits the fluid when it goes from the single line out of the MC? Is the MC brand new or rebuilt?

I used to own a 63 Bonneville, but it has been so long I forget what the system looks like. The more I think about it I'm betting bad booster.

Don

That why I'm thinking, no proportion valve, just brass tee's. When I dis connect the Booster, I couldn't even press the pedal down. But how can the booster be over assisting? I don't know what else to check/ or replace. Notalot the brake sys on car

Originally Posted by hottrodd57

That why I'm thinking, no proportion valve, just brass tee's. When I dis connect the Booster, I couldn't even press the pedal down. But how can the booster be over assisting? I don't know what else to check/ or replace. Notalot the brake sys on car

There is a valve inside the booster and that is failing \ has failed. I had one in my Mustang do that. Was a B_ _ ch to drive with that problem! I removed and sent it out for a rebuild to a reputable shop but it failed again soon after So I bought a new one. I don't know what else fits a Bonneville but I think any good parts store should be able to help.

I'll assume the system didn't always do this. What changed? Did someone "fix" somethig then the problems started? Has it sat for a long time unused? That kind of info would help with suggestions.

I am just getting background from the owner, the car has sat for the last 5 years, no work been done to car, apprarantly she stop driving it when the brake started to lock up as a stated earlier, she knew I have hotrod and asked if I would look at the car for her, she has had the car since 1964.it is all original, but for parts replaced over the years. The brake sys. Is stock,
I am not really a big help on drum brake sys. Like this,I always replace the entire brake sys, & up graded to dual res. Master cylinders, disc brakes, etc.

The brake system on that car is as basic as you can get.........just a MC, booster, and four wheel cylinders. If one was locking up I would suspect something in that wheel, but if all four do it as soon as you say, it has to be the booster. They do go bad, as 34_40 mentioned.

Don

34 40 nailed it ....ted seen it before

Repair or replacing the MC will probably cure the problem, but those brakes are very touchy regardless (I had a '64 Grand Prix, my parents had both a '66 GP and a Catalina). The OEM brakes are also not very effective after a couple of hard stops - that is you don't stop without prayers and lots of distance or a large tree in the way.

in the center of the diaphragm in the booster there is a spool valve there is vacuum on the back side of the diaphragm when you apply the brake pedal it moves that metered spool valve and allows vacuum to move to the apply side and pushes the apply rod to the m.c. now this spool valve is crimped into the diaphragm after old age or non activity for long periods of time the valve will separate from the diaphragm and allow vacuum to leak around the outside and apply too much brake pressure to be applied I have seen this happen before I hope this helps its easier to explain in person than when sit down and try and type it out especially with my limited typing skills bottom line you need a booster ........ted

After setting for that long---entire hydraulic system should be replaced---booster, master, wheel cylinders and lines( at least the rubber hoses and rusted sections of the hard lines)

Drum brakes are self actuating and the intial force against the drums causes the shoes to expand further so even rusty drums will grab ---maybe just some driving around will wear off the rust but I wouldn't bet on it---