That is NOT a 1978 Toyota proportioning valve. Looks GM to me. It may work as well as an original but I wouldn't know.took the proportioning valve off a few years ago because I was having a lot of trouble getting the fittings to match up. The brakes ran just fine without it for a few years. I asked the guy doing my brakes if we should put it back on and he said it would be fine without it, just be ready for your back end to shift.
This is the valve I took off. But again the brakes worked great without it. I am down for reinstalling it if you tell me it might help.View attachment 2828121
On a 78 disc/drum set-up only the rear line passes through the prop valve. This is to prevent rear wheel lock-up. There's no other way of doing it as, with a tandem MC, the rear piston is the same (or could be larger) diameter as the front and receives the same stroke - i.e. - volume. The valve meters it down.