Yes, you will need to make something up to block the vacuum when you remove the hose.

Step 1) start the car with the vacuum hose connected and apply brakes
Step 2) while car is running remove vacuum hose from the PB booster and block off than apply brakes.

Tell us what is happening with the brake pedal during each of those steps.

 

Ok #1 Same as always goes to floor, no resistance!
#2 Some resistance but goes to floor.

 

So this is where you’re at today;
1) New calipers that are not leaking
2) 3 Master Cylinders
3) 3 PB Boosters
4) New distribution block (Proportioning Valve)
5) Gravity, vacuum and pressure bled system

Wild cards
1) Blocked current m/c off and have a hard pedal, this tells me the problem is downstream from the Master Cylinder.
2) When engine is started pedal goes to floor, this tells me you have a bad PB Booster.
3) Tried blocking front/rear lines at the m/c but got no pedal either way while engine running, this contradicts wild card #1.

I just went out to my 1973 Vette with PB and without starting the car I have a good pedal and brakes. There comes a time when you have to throw logic out the window and start back at ground zero. If this were my car I would do the following today in this order until I had brakes.
1) Power bled the brakes at all caliper bleed valve ports. I use a Power Bleeder from Motive Products at www.motiveproducts.com PN 0105 - Early American Rectangular Power Bleeder.
2) Verify I had no visible leakage or wet spots along the entire brake line system to include the calipers (remove one caliper at a time if required to see lip seal area). Most lip seal leakage is obvious to the naked eye without removing caliper.
3) Replace the Master Cylinder, bench bleed the MC and repeat #1 above
4) Replace Proportioning Valve and repeat #1 above
4) Replace the PB Booster after throwing wrenches out the garage door.

I honestly will be surprised if you don’t have brakes after completing step #1 & #2 given the amount of parts and steps you have already done on this car. This is a very basic brake system and not overly complicated to work on. Are you doing all this work yourself or relying on local mechanics? Wish I was local I would come over and fix those brakes.

Don’t let this get you down, lets beat this monkey into the ground.

From the GM Service Manual

Symptom – Excessive brake pedal travel

Cause
1) Leaking brake line or connection
2) Leaking piston seal
3) Leaking Master Cylinder
4) Air in brake system (most probable cause)
5) Uneven brake pad wear
6) Caliper attachments loose or missing
7) Faulty Proportioning Valve

 

OK will do. I also can't stop the car with the e-brake. Leads me to believe some issues with rear calipers. I would remove the rear calipers and inspect but wouldn't know if they are bad.
I have done a lot of the work but didn't install calipers or the booster. Have been present when some of the mechanic's work was done. None of the mechanics want to re visit my problem.
They want to start all over with all new parts. I've lost track of $ spent but it's around $1300.00 so far. Your input is greatly appreciated.

 

The e-brake (Parking Brake) has nothing to do with the brake calipers or braking system , not an issue to worry about at this point. You don't need to remove the calipers at this point just jack up the car and get it on jack stands (all four wheels off the ground would be best and re-do the brake bleeding process. Here's the deal, if the pedal is going to the floor due to a leak you WILL see brake fluid somewhere be it floor, frame, calipers etc. I think were chasing air right now or eleminating this as a cause.

If you can't get all four wheels off the ground (and wheels off the car) you can do this one at a time.

 

Oh and for the "None of the mechanics want to re visit my problem. They want to start all over with all new parts". Find a new mechanic.

 

I would not spend another dime on your problem until I followed Richard's advice and contacted Dub at Corvettes Etc.

 

I can't imagine how frustrating this must be.

If you watched the mechanics do the work, did any of them use a pressure bleeder to get the air out? That, or a too-short actuator rod are all I can think of.

Did you check rotor runout? And did you do a continuity test on the switch on your combination proportioning valve?

 

Discovered I was not using my pressure bleeder properly. Thanks 68ssconv. re bled using 15-18 lbs. of pressure. Both rears good. Good flow no bubbles. I have no flow to front calipers. Removed rr rubber hose and no flow from hard lines. Suspect faulty comb valve install from last mechanic. i have a tool on order to bleed fronts . Think it's a good plan?

 

Do these have a reset on the proportioning valve? I had a similar problem on my 77 Chevy pickup and had to reset the valve. Not sure if the corvettes did and wen I went to disc disc on the truck I used a diffrent prop valve.

mare your hard lines original??

 

Re-bleed the whole system (front and rear) after you replace the proportioning valve (strange it's bad or stuck). Glad you are making progress.
What tool do you need for the front if you already bled the rears?

 

[QUOTE=Mr D.;1603037552]Re-bleed the whole system (front and rear) after you replace the proportioning valve (strange it's bad or stuck). Glad you are making progress.
What tool do you need for the front if you already bled the rears?[/QUOT

He needs the proportioning valve bleeder tool.

 

Had to go look that tool up



Having owned and wrenched on several 65-77 Corvettes over the last 35 years I have never used nor do I own that tool and I have bled a lot of brakes. I just did run through my GM, Chilton, Haynes and Motor manuals for brake bleeding and proportioning valve replacement and none of them call out this tool. Not saying you don't need it or it doesn't have a function just that I have never used one.

 

I have also heard of resetting proportioning valve. Heard about it but never done it. I hate when the easiest things on a car to do goes south. Good luck

 

when I get the tool and install it where the switch goes, is there anything else I do except bleed the fronts? Does the tool reset the valve?

 

In your case the porportioning valve has shifted preventing brake fluid from passing through the valve to the front brakes. I don't think that tool is going to fix that, you use the tool to prevent that when bleeding the brakes. See below YT video on the subject.

 

I assume the brake warning lamp is illuminated.
To reset the proportioning valve, start the car and stand on the brake pedal hard, I mean hard, and the valve will re-center.
You may need to pump it a couple times prior to standing on the pedal, to get enough stroke/ fluid pressure.

This should work.

Good luck
Jeff

 

Agree with Jeff, the GM manual says press down on the brake pedal hard to reset the proportioning valve.

 

Mr D. That video is great. Right now I have no flow going to fronts. took line off from M?C going to valve and no flow. If I step on brake pedal it will force fluid from the valve with the line off.
Didn't quite understand resetting by bleeding opposite ends. Getting that valve off, while lying on your back to reset the slide is gonna be a bear.

 

Did I understand that correctly that you disconnected the front brake circuit line that comes from the Master Cylinder at the proportioning valve than pressed the brake pedal and nothing came out of the line? If that is the case than you are not passing brake fluid through the proportioning valve front brake circuit because you are not pushing brake fluid from the Master Cylinder front brake circuit.