I have restored a 1939 Chevy Business Coupe with a 1973 Camero front and rear suspension. I mounted a Hydroboost on the master cylinder so I could put the brake pedal through the floor and not have to mount a vaccume boost on the firewall. Problem: Fluid will only come out of bleeders when motor is running. I bought a little hand operated vaccume pump and still limited fluid comes out w/o motor running. Shouldn't the fluid come out unrestricted w/o motor running? Also........There is no slot on the rear backing plates for a brake ajusting tool? How do you adjust the rear brakes.........I have a major pull to the left rear Any help would be greatly appreciated

if the master is lower than the wheels cylinders and calipers, bleeding could be a problem. but the street rodders do it, so it can be done. maybe figure out a way to force new fluid in thru the bleeders and out the master cylinder- do it backwards.
might also need some residual pressure valves in there to keep a small amount of pressure in the lines so they won't need to be pumped 4 times when you are in the process of rear ending a semi truck.
as for the brake adjusters, some drums have slots on the front (wheel side) for adjusting purposes.

if the master is lower than the wheels cylinders and calipers, bleeding could be a problem. but the street rodders do it, so it can be done. maybe figure out a way to force new fluid in thru the bleeders and out the master cylinder- do it backwards.
might also need some residual pressure valves in there to keep a small amount of pressure in the lines so they won't need to be pumped 4 times when you are in the process of rear ending a semi truck.
as for the brake adjusters, some drums have slots on the front (wheel side) for adjusting purposes.

If the master is lower than the cylinders/calipers, you need residual pressure valves to prevent from draining back to the master and retracting the pistons. Drainback will make the pedal get soft for the first pump, and makes bleeding more difficult.

I've never seen drainback cause the problem he's describing though. I'm wonder if he's got a combination of problems.

1) Are you seeing this on the fronts, rears, or both?

2) What procedure are you using? Pump, the pedal, hold it down, and then someone else opens the bleeder screw and the closes it. If that is what you are doing and still not getting any fluid out of the bleeder with the engine off, I'd suspect either you have a bad hydroboost unit, or you are using a short-pushrod hydroboost with a long/deep master cylinder.

3) When you try with the MityVac, is it holding a vacuum? You may be leaking enough around the threads of the bleeder or the jar lid that you aren't pulling hard enough to suck anything out. Also, is there a hold-off valve plumbed into the system? If so, you'll need to hold in the button on the back while you're trying to bleed that part of the system.

I haven't worked on anything in the early 70's, but I've worked on both older and newer rear drums with the same results. The rear brakes should have a set of knock outs on the bottom side of the mounting plate. Knock it with a hammer and punch a few times and it will bend inward into the drum. I believe they came this way from the factory. I've had to knock a few out to do a some brake jobs.
If the knock out isn't in the mounting plate there will be a thinner area cast into the drum, just as someone else suggested. I've never worked on these personally so I wouldn't be of any help.
Good luck!