What is up with my brakes on my 1982 Toyota Odyssey Mini motor home.

I have a 1982 Toyota 1 ton Odyssey mini motor home, I have changed the master cylinders twice, replaced front calipers, replaced both rear drum wheel cylinders, replaced disc pads on front twice, changed fluid 2-3 times, (fluid flows smooth, no dirt observed ). Vehicle sat for years, proportioning valve looks like a half ton, however the truck is a one ton chassis. When I apply brakes front skids and the rear seems not to do much at all, I have adjusted the proportion valve all the way in, and all the way out, very little difference either way observed. What do you think could be the problem? Could this vehical have a metering valve, if so where would it be located ( as we have found absolutely no book, and no info even from the dealer on this truck), do you think it might still be the master? Or do you think it would be the proportioning vaulve? Anything would help.