Well, its not the check valve, that checked out ok. The actuating rods for the booster and between the booster and master are all the stock mustang units, so I would assume they're correct. The master does bottom out just before the pedal hits the floor, so I think I have enough travel in the pedal. Just as a side note, the pedal is about the right level of firmness with the truck not running, not sure what that means??? I think I'm going to replace the front rubber lines, just in case I have a buldging/defective hose or something.

 

Wow.. Lots of help here.

If I understand the problem the reality is that the closed hydraulics system isn't solid. There's air or spongy stuff goin on someplace..... The Pedal should never go to the floor..... ever

 

i once had a 72 cutlass sx that did this . it would be fine and then all of a sudden wham , no pedal . it ended up after beating my head against the wall for awhile that the diaphragm in the booster would somehow leak every once in a while and cause my problem . it even sucked the fluid through a couple of times . replaced the booster and the m/c as a precaution and never had another problem . be worth checking out ....

 

Whoa, better back up. Right level of firmness when it is not running?
So, without the engine running, and no vacuum booster help, the brakes are fine. Pedal is rock hard at some point? Brakes are all locked up?
It's only when running the pedal gets soft and doesn't firm up?
That would be a booster problem. Or maybe, the wrong size. If the pedal is easy to push but gets harder at the bottom, without stopping the car well, the booster is too small and running out of vacuum.
If it is mush all the way, and not stopping well, replace the booster.

 

Well...the problem is finally fixed...and I'm a dumba$$. After a brainstorm session with some local rodders, I say "I wonder if I'm getting enough pedal travel?" I get a few "That wasn't the first thing you tried!?!". Well I lengthened the adjustment rod out a ways, and the first time out I mashed the brakes, and just about put myself through the windsheild!! So to anyone still following along, try the simple stuff first, even if you "sure" it can't possibly be that.

 

Good day Truckers. Hydrolics can be tricky, and it would be nice if some of the suppliers took a class or two, but, well you know. Old pickups have a few inherant brake problems that are seldom addressed. First off, the stock mustang metering block will make a great door stop in your garage, but has no place on your truck. Prop, 2 and 10 is the way to go. Master bore should be 1", no smaller.

Bigger bore = MORE fluid VOLUME, and LESS PRESSURE
Smaller bore = LESS fluid VOLUME, and MORE PRESSURE
This is something we need to remember., BUT there is one more thing. PEDAL RATIO. And, this is pretty important. I am willing to bet 3 cold ones, yes 3, that this is where the problem lies. The stock pedal has about a 9-1 ratio, had to, thats the only way to build enough pressure to stop the truck, but it takes a lot of pedal travel. Most of the aftermarket copied one another and worked as hard as they could to cut costs, thats why they're all about the same. 25 yrs ago, No Limit started building P/B kits that work. One thing we learned is that the stock pedal ratio is to high, and with power brakes and dual masters, this has two big problems, 1, no pedal feel with the booster, 2, not enough fluid travel to actuate the brakes. The solution is pretty simple, change the pedal ratio. So, soon (23 yrs ago) westarted to make 'ratio adapters' that bolt to the stock pedals and fix this. They are made to get the ratio down to about 5-1, and most truckers can then cut the upper section of the pedal shorter, making the truck more comfortable to drive. Sound like a sales pitch? what-ever, I'm trying to help. they're 9 whole dollars, and, if it doesn't solve your 'soft pedal, no stoppy' problem, I'll give you all your hard earned money back.
I'm still willing to wager 3 cold ones though.