Brake failure that is intermittent?

[70Challenger440]

My question is about a 1970 RT with drum brakes all around. It has the power brake option and is all stock. The brake system was completely replaced with everything new a month ago by a garage used to working on classic cars. The  issue happens with normal driving around town, so nothing is being stressed.

After about 15 miles of driving the brakes all of a sudden felt as if they failed, but if you would press down extremely hard on the pedal then it will finally temporarily lock up all the tires. While trying to make it home like that VERY slowly it got so bad that they  would not even lock up under heavy leg pressure anymore. So I pulled over and got the car towed.  The odd thing is once the car got tested a couple hours later, the brakes were working perfectly fine. Also worth noting is the brake failure light does work on this car, and it did NOT light up during the failure incidents driving home. There are no brake fluid leaks. The stock engine runs fine while the brake failure occurs and nothing was recently changed on it.  With everything working perfectly again simply on it's own,  it becomes hard to diagnose. Could this be the power brake booster failing intermittently?

Linkback: https://forum.e-bodies.org/wheels-tires-brakes-suspension-and-steering/12/brake-failure-that-is-intermittent/29281/

[moreparts]

I'm waiting to see some answers for you from one 4 wheel drum guy to another.  That sounds horrifying.  I've been riding on 4 drums over 30 years and the worst I've had is a hard pull to one side.  Fingers crossed in getting that fixed asap.

[MoparLeo]

Does the brake pedal feel change when it happens?
 Meaning does the pedal drop ? Get soft ? Get hard?

[blown motor]

What do you mean by “everything was completely replaced”? What is “everything”?

A friend just had a similar thing on his 69 RR. Pedal to the floor on the first pump and then got “some” brakes on the second pump. Needed a new booster.

[70Challenger440]

.
Does the brake pedal feel change when it happens?
Meaning does the pedal drop ? Get soft ? Get hard?

>>>>> Pedal doesn't really drop much at all, but does get hard to press when the brake failure is occuring. Once forced it goes from the feeling of no brakes to all 4 wheels locking up. Eventually as said, (when I was trying to make it home like that at under 5 mph), the car began to no longer even lock up the tires when hard pressed and was becoming 'complete' brake failure.

.
What do you mean by “everything was completely replaced”? What is “everything”?

>>>>>Replaced: wheel cylinders, pads, master cylinder, flex hoses, proportioning valve, and they bought a kit with all the new pieces that go inside drums, new drums.
They decided not to replace: the hard brake lines which they said were perfect and the booster which they said 'almost never fails' and they felt was also good. When bringing them the car for the brake job,  I just said 'replace everything', but they did not replace those 2 items by their own judgement.

Note that the car sat for many years unused, but was always kept in a warm dry garage covered.

[bdschnei]

Brake booster check valve maybe?
Easy test to check brake boosters is to pump the brake pedal a few times with the car off to dump the vacuum and then apply light pressure on the brake pedal when starting the car. You should feel the pedal get pulled down as vacuum is built in the booster.

[70Challenger440]

So it seems, the general thought of everyone here is the brake booster is the culprit??? Any other suggestions in case it is not?

[Rich G.]

Has to be the booster or mc. Nothing else will cause a brake to be hard or soft. Make sure all you lines aren’t pinched anywhere. That would screw things up. We’re assuming everything you had installed was done right.

[HP_Cuda]

Here is a thought for you. If it's an intermittent problem then you possibly have a vacuum leak for the break booster.

[Cudajason]

Brake booster check valve maybe?
Easy test to check brake boosters is to pump the brake pedal a few times with the car off to dump the vacuum and then apply light pressure on the brake pedal when starting the car. You should feel the pedal get pulled down as vacuum is built in the booster.

That was my thought as well.  But if the booster was not changed, I would start there.  I am sure they fail all the time after 50 years.

Did they blow out all the old hard lines or just put the new parts up front and say good.  I wonder if there could be some gunk stuck in the lines that plug the proportioning valve.

Dont drum brakes use a residual pressure valve?  Coudl that be the problem, say installed backwards it is holdign pressure?

[MoparCarGuy]

I was under the belief that the Power Brake option required disc brakes on the E-Body. In other words, no Power Drums were offered on the E-Bodies.
Disc Brake option B41 required Power Brake option B51 and vice versa. No exceptions to this.
Anybody know different?
Was there a possibility of Power Drums with the Trailer Towing Package or other unusual combination?

[RUNCHARGER]

In an early E-body you could add PB to a drum brake car, at least in 70 and 71. To get disc brakes though, you needed the PB option as well. Not sure 72 and later.

[70Challenger440]

Has to be the booster or mc. Nothing else will cause a brake to be hard or soft. Make sure all you lines aren’t pinched anywhere. That would screw things up. We’re assuming everything you had installed was done right.

Being the Master Cylinder is just changed and the brake booster is not recently changed, and in addition a hard brake pedal that does not cause the brake failure light to illuminate, I will first assume the Booster as the culprit.