Vito - Thanks for the input on the proportioning valve. You're absolutely right It's not truly a proportioning valve that "distributes" fluid. I realized that just after reading your post and did some research trying to figure out exactly what this thing does. Basically I concluded the valve is basically a fail safe that closes off the front or rear brakes in the event of a blowout. The significant pressure difference causes a plunger in the valve to move either up or down and block off flow from the front or rear master cylinder feed. When the plunger moves it also activates a switch in the valve which causes the brake warning light to come on. Pretty neat setup actually.
I would love to install my old bleed screws but they were returned with the old calipers so I could get my core charge back. I may buy some new ones from Summit.
My next step will be to verify the plunger in the valve isn't off center. I can do this pretty easily with my ohmmeter. If that checks out I'm going to try bleeding the brakes one last time.
Bob - I know exactly what you mean about pressure bleeders making a huge mess with these old master cylinders. They're much easier/neater to use with new systems that have screw top reservoirs. I've decided to try something a bit different. I'm going drill a hole in my master cylinder cover and gasket and attach an air fitting to it. I'll seal it up real good and attach my air hose to it (thinking of using my trigger operated tire filler). I'll then fill the master cylinder about 3/4 full with fluid, put the cover on, hold it down with the bails, and put about 10 pounds of pressure in the system. I know that, when the master cylinder reservoir is full, I can fill the bottle on my vacuum bleeder twice. So, I'll simply attach a hose to the bleed screw, crack it open, and let it drain into my bleeder bottle until it's full. I'll then close the bleed screw, depressurize, top of the master cylinder, and repeat. This should minimize excess fluid running all over - at least that's the idea. The new cover and gasket run about $17 so it's cheap enough to replace them. Much cheaper than $100 for a pressure bleeder system.
If that doesn't work I'm going to have to do a system tear down like Vito did to check things out (I need to reread your posts on teh subject Vito, there was some good info on there). To my mind it just shouldn't be this hard to bleed the brakes. It makes me think something isn't right.
The system sat for a long time without fluid in it which worries me. I'm really not excited at the thought of pulling the calipers, master cylinder and proportioning valve but it's pretty important the brakes work right - gotta slow down from those burnouts somehow!
I'll keep you guys posted. I have some time today but have a family get together this afternoon. I also have tomorrow off and may get out in the garage then too. Fingers crossed that I don't need to tear the entire system down.