Brake booster failed so I ordered an aftermarket for my 73 Cuda. I've installed and verified the booster functions, bench bled the master and verified, bled system 3 times, and bench re-dressed the proportioning valve. Every time I re-assemble I get an extremely firm brake pedal and almost no brakes. I can't even lock them up on gravel.
I think its the proportioning valve but blowing through the valve primary and secondary MC reservoir inputs on the bench with air hose I have communication through to each brake output port.
Rear brake lock-up is the usual symptom...either that or spongy brakes from what I've read.
Not sure I understand the concept of re-centering the valve. I see the spring loaded piston inside the valve but nothing that can be re-centered???
Everything worked fine until the booster started leaking so I'm confident the brake cylinders/caliper pistons are good.
Could the prop valve exhibit a very very stiff pedal and no brakes?

 

Bad booster?

 

I don't think so. With engine running and vaccuum line attached I can manually push in the plunger that the brake pedal pushes and I can easily feel the booster assist. I also get good travel with the booster mounted and MC disconnected. I can visually see good stroke from the booster when the pedal is pressed.

 

Check the booster and see if it hold vacuum..... ???????

 

I would get a brake pressure tester with the necessary adapter fittings to start at the wheels and work your way back. When you find a large jump in pressure that is your restriction. Also I did an aftermarket brake booster for my Jeep and the length of the push rod for master cylinder had to be shortened. I think this looks to be pretty good but the smart guys on here can chime in.
https://www.ebay.com/itm/23372310619...waAurBEALw_wcB

 

Thanks guys.
A couple comeback comments/questions;;
1) Racerhog...when you say check to see if the booster holds vacuum, how would I do that? I can't hear any air escaping with the hose attached and engine running. It appears to be holding but if you have a specific method of checking it I'm all ears.
2) XAZA...I purchased the booster and MC as a set and they appear to be well matched. The booster push rod is just barely touching or about to touch the MC bore when mated so it does not need to be shortened. The only issue I had was the linkage between the brake pedal and the booster. I've had to work with it but I have visually verified booster push rod stroke when depressing the brakes so I'm confident that when I press the brakes that the booster and MC are activated.

If rebuilding the combination valve does not fix my problem I think it must be the MC. I bled it and when I blanked off the primary and secondary outputs it got solid real fast...( very little pedal motion). Not sure what else I can do to check the components out I guess a brand new modern combination valve is next.

 

To my comment: Check the check valve first and make sure it works properly.... You should be able to blow with flow one way and then suck and no flow that way...
After that is confirmed to be good.... Start the engine... Let it run for a moment.... Turn the engine off.... Unplug remove the check valve from the booster and you should hear vacuum sucking while you try to remove the valve.. If it does this, then the booster and the valve are doing its job.... If not, that tells us that your loosing all your vacuum assist.
Hope this makes sense....

 

I had a similar problem on my classic Charger after changing the booster.
Ultra hard pedal and almost non-existent braking, it was super scary to drive…
In my case, it was the pushrod between the booster and the master that was too long, which kept a constant pressure.
The valve prop can cause problems, but in general it gives more of an imbalance than a complete blockage.
Have you tried adjusting the length of the pushrod or checking the alignment with the master?
I also think that the valve prop can lock if centered incorrectly, but that would rather explain a partial loss, not a total one.
Keep us posted if you find the problem, it will help others!