Brake problem on 68 Charger RT

[grease monkey]

Hello from Luxembourg (Europe)

I have the following problem on my 68 Charger with all four drumbrakes. Ich changed the brake booster because the mecanism inside was rotten. So I replaced the booster and the valve on it with new one's and also changed the master cylinder.
I did a bench bleeding with the master cylinder and after installing all, I did the bleeding on all four brakes. Till there everything alright.
When doing the test on road I have the following problem, the car brakes at first very good with good assistance from the booster, but after a few seconds the boost is gone and the pedal becomes hard, and I can hardly stop the Charger. I did some tests with always the same result. Powerbrake at the beginning and then hard brake pedal and complete loss of brake power.
I checked the underpressure at idling and it was a 15hg which seems to be ok
Where can the problem be ? What shall I do ?

Best regards
Robert

[Back N Black]

Is the vacuum line to the booster Tee attached to other things like the PCV , the booster needs a separate line to the intake.

[grease monkey]

It has no T, it is separately attached to the intake

[tsmithae]

Has your engine been upgraded with a new cam?  Too much duration can cause a lack of vacuum as can improper timing/idle/AF ratio.  I think with a little more information we can get a better idea of what's going on. 

[grease monkey]

Engine has its standard cam. All worked fine before changing the booster and master cylinder. I think the booster is faulty, it somewhere looses it underpressure and the engine is not capable of putting enough boost in to compensate the loss of pressure and to have enough pressure on the tires.
I think best is to open the brake booster.

[Paul G]

Try this, start the car and let it idle. You should have good brake pedal feel every time you apply the brakes. Step on the pedal and listen for hissing sounds. If you hear any hissing the booster is leaking internally, which means it is bad.

Also try this. After idling for a while shut the car off, let it sit for a few minutes. Then apply the brake with the engine off. It should feel normal for a few depressions of the brake pedal, then it will have used up all the vacuum, (you call it under pressure) and become very hard to depress. The booster should retain vacuum when the engine is shut off.

The first time you depress the pedal, with the engine off, if it is hard to depress it is loosing vacuum, it has a leak, only place it can leak is internally in the booster, or the check valve or the check valve grommet.