1968 Dart Brake and Booster issues?

I had a front disc brake conversion done on my '68 Dart and the brakes seemed OK for a while, the guy who did the swap pulled my Distribution Block and plumbed in an aftermarket adjustable Proportioning Valve (which started leaking almost immediately). I recently bought a new Disc/Drum correct master cylinder so I can go back to the Dist Block setup and get my dummy switch working again. The new mechanic installed it for me and plumbed the old Dist Block back in (I'd love to see a plumbing diagram for the original Dist Block if anyone has one???). The swap went fine and the front seemed to work well but the rears just won't grab to lock up when idling in gear on the hoist? There seems to be a little sigh/hiss/wheeze from the brake master after it first is depressed. So that leads to 2 areas for questions:

1) Is there some sort of plumbing issue with the Distribution Block, and can I get a plumbing map to make sure we have the lines in correctly? The mechanic can get a hard pedal when bleeding but once the car is running and the brakes are pressed the rear tires keep spinning. Is there an internal lockout of some sort in the distribution valve that is designed to isolate half the brake circuit if there is an imbalance? Where do I get a 1968 A body proportioning valve to get rid of my Distribution valve? Am I chasing my tail?

2) What is the correct Brake Booster for my application? Does Summit Racing have a part number for one which is correct for my vehicle (1968 Dodge Dart convertible had a 273 before the engine swap)? I'm in Canada so Summit seems to be the only Mopar shop up here. Could it be a check valve issue on the booster? A collapsed line on the rear? Rear pistons too large?