[valiant67]

You can rule out the booster. The booster would make the pedal hard if it was faulty.
I would vote for some combination of air in the lines or the master cylinder. Or a restriction like deteriorated flex hoses or corroded caliper slides.

 

[valiant67]

I'm running into the same problem with my car now. The problem is intermittent. When the car is cold the pedal has to go down considerably before I begin stopping when I apply the brakes the first couple of times. As the car warms the pedals act normally unless I have to brake hard. If I brake hard, then the pedal will have normal travel for a few seconds and then "give up" and travel all the way down. The car will still stop though. Once I pump the brakes in this situation, the braking returns to normal. Brake booster is relatively new (circa 2006), and it does not make a hissing noise or anything that would suggest to me that its leaking. The master cylinder however is original (1984). I initially thought it was a problem with the vacuum, but my cousin is positive its the master cylinder. Anyone have thoughts?

You are describing a hydraulic problem. It's not the booster. All the booster does is make it easier to press the pedal. The pedal dropping is almost always a master cylinder. To know what a failed booster is like, press the brake pedal with the engine off. There is still good braking, you just have to push really hard.

 

[valiant67]

Is the "new" master cylinder really new or a reman one? I've heard horror stories on the reman ones being bad right out of the box.

 

[valiant67]

On most cars it is the fitting on the brake booster where the hose attaches.
But you are not having a vacuum problem. That would be a hard pedal, not a soft/sinking/no pressure applied pedal.

 

[valiant67]

You've got a problem somewhere in the hydraulic part of the system:
1) Bad master cylinder again.
2) Air in the system still. It may be in a high spot where it's hard to move. May need more pedal pressure if bleeding traditionally or using pressure or gravity bleeding. I assume the new master cylinders are properly bench bled, either on or off the van
3) Some off mechanical problem like a flex hose swelling outward, too much wheel bearing slop on a disc brake. rear drums way too loose, etc.

Here's the best test (from a Mopar Action article):
Remove one of the hard lines to the master cylinder and block that port off with a bolt. Did the pedal get firm? If so the problem is in that circuit, I don't know if these are one circuit front and rear or diagonal.
If the pedal is still bad, remove the other hard line and plug it. Did the pedal get firm? If so the problem is that circuit.
If the pedal is still not firm, it's the master cylinder.