On my 72 Monte, I've been through 2 masters, the rear hose, and 2 wheel cyl's. Fronts bleed fine. Rears after sitting give 2 good spurts then NADA. Master shoots a good strong jet out the rear port. Button on the combi? Stuck combi? It's the only thing I haven't replaced, but I've never had a problem with one of them. Help please, I'm at me wit's end!

Sounds to me like you've sourced your problem. I think it's stuck, and after it sits a while it leaks into the rear of the system, and those are the two shots of fluid that you get.

Have you tried unbolting the rear supply line from the combination valve and checking flow out of the valve when you step on the brake? (Step GENTLY so as not to shoot fluid fifty feet!)

If you do get good fluid flow out of the combination valve, then you've got an obstruction somewhere in your rear line. How about the distribution block that splits the fluid over the rear end? Maybe that has blockage? Could have a bunch of rust or something in it.