My 70 stock ls5 ss chevelle sat in a barn for 18 years. I replaced master clyinder, rear wheel clyinders ,shoes, rear lines at axle ,rubber line at rear,front roters, calipers, pads, rubber hoses at calipers.Every thing bleed well, rear,calipers ,master clyinder. My problem is in garage with front jacked up when you hand spin tires and hit brakes, tires stop imedialty but when driving say on a dirt road only rear brakes lock up.
Talked to many people (motor heads) and no real concrete answers.
Question #1 beside master clyinder is a porpional valve? in reminders you of a wiskey glass in size what does it do and PLEASE be specific.
Question #2 on frame under master clyinder is another block that has about 5 lines and 1 wire coming off it, what does this do.
Question #3 at rear end before rubber hose is yet another block what does it do. PLEASE be specific with your helpful answers.

Many Many Thanks Tim

#1. should be a proportioning valve to limit pressure to the rear brakes. It could be in need of replacement.
#2 is the distribution block which contains a switch to light up the brake light on the instrument panel if either front or rear circuit loses pressure.
#3 is just the tee fitting that connects the hose to both right and left rear steel lines . No special technology here.

If everything is in good working order, then I would suspect the proportioning valve. These can be replaced with an adjustable aftermarket valve if you like. But I would also recommend re-checking the rears for contamination, like from a leaky axle seal; this can also give premature rear wheel lockup.

#3 is not a tee to each axle but bolts to FRAME then to rubber hose then to the tee you spoke of.

Take a look at this thread on TC. I think we are working on the same problem
http://www.chevelles.com/forums/showthread.php?t=181653