Bleeding brakes is supposed to be simple right?

I cannot figure out what I am doing wrong. The car is a 69 Olds Cutlass with a bored out 350. It has some modifications, but I am nearly sure the brakes are same as stock–drum in the rear and disk up front. The Mityvac vacuum pump I have for bleeding the brakes says to start with the wheel nearest to the Master Cylinder and work to the furthest; this is contrary to what I read online, but I figured I would try it by the book the first time. By the book being ten pumps on the vacuum after hooking up the nozzle and then bleeding while continuing to pump until there is no air in the fluid, finally shutting the bleed valve before fluid stops flowing, keeping the fluid level in the master cylinder topped up the whole time.
Following the procedure in the manual for the bleeding system everything went according to plan for the front brakes, but the rear brakes just let a tiny amount of fluid into the vacuum tube and the air bubbles are quite large and never stop. The amount of fluid is so small that I couldn't say that it ever starts flowing. So I kept adding to the master cylinder and sucking out fluid, but it looks like the amounts I am adding to the master sylinder are the same as the amounts I am pulling out. I have used a quart and a half of brake fluid to no avail. I think it may be an air leak in the cap on the rear part of the master cylinder; maybe it is not sealing properly. Does anyone agree or have any other ideas?

Bleed it the old fashion way. One person pumping the pedal and one person opening and closing the bleeder.

I got the same result from bth the front and rear brakes attempting it with the two person pedal method. Any other ideas? I put the wheels back on and it stops but the pedal is very spongy and doesn't always rebound.

Find the brake combo valve, you will see a button on the end of the valve, pull that out about 1/8 inch hold it out as you bleed the rear.

Two brake lines come off the Master cylinder. The first leads to a small metal cylinder on the brake booster(I think that is what it is called). A line comes off of that and meets up with the other line from the master cylinder to a small metal rectangular prism. It has two lines going out to the front calipers, one line to the rear brakes and and electrical line. There are not any buttons on it though, just the mounting screws. I couldn't find anything else, but I'll keep looking. Can you tell me about where it might be or what shape it is? Also, I read about residual valves and was wondering if lacking a ten pound inline residual valve would cause this problem. Thanks for the help so far.

There are usually no residual valves in this system, they are normally required where the m/c is below the floor board. Google or look up a brake combo/proportioning valve on the net, you will see this button on the end of the valve, it may have a rubber protector sleeve over it.