My 1947 ford pickup had a single jam jar master cylinder and booster from a 65 mustang, I assumed. During the winter I picked up a 1989 mustang master and booster to replace the leaking master. I removed the leaking single unit and measured the rod stick out from the existing booster and it matched the rod stick out from the new booster. The inside of the master depth for the rod to enter was also the same so I went ahead and just bolted the new master to the old booster changed the front lines from 1/4" to 3/16 to match the new masters size and added adapters to change the iso bubble flares to standard and bench bleed the master. The original setup was a mess of lines and tees to all wheels. This new master has 2 front outlets which go to the front calipers. When I bled the new front lines to ensure they had fluid in them before attempting the rears, they bled OK. I added a new 1/4 line with ISO bubble flare at the master to a tee for the brake light sender and then another 1/4" line to meet up with an existing joint at the firewall. The remainder of this line is also 1/4". The problem I have is I do not get a good flow to the rear bleeders, and after about 20-30 attempts at bleeding only have gotten about 1/2" of fluid out of it in a jar. Sometimes, I only get drips. I went back and removed the rear line from the master and there is fluid there. I removed the last line on the tee where the brake light sender is and get fluid there.
Does it take a long time to get the fluid to flow to the rear after a major replacement. If I leave the bleeder open it does drip slowly. Should I check at the rear tee where it splits off on the rear end for proper flow. Could there be an air blockage somewhere. I have not installed an adjustable proportioning valve as yet, so there is nothing but brake lines now. The other issue I have is that I have never really tested the brakes as they were b4 when I picked up the truck, so it may have been an existing condition. It has front disk and rear drum brakes. I am stumped.

 

First thing i would do is take one or the rear bleed screws right out. See if you get flow then.
If still no flow loosen the pipe nut on the M/C side of the rear flex hose, if the flow is good there tighten it and loosen the pipe nut on the other side of the hose. The flow should be the same there, unless the hose has colapsed internally.
Does it have a load sensing valve mounted above the rear axle and in the brake line? If so check the flow on both sides of it.
Finally you could check all hard line for kinks or crimps.
Good luck.