Can the valve by the MC on a 69 be replaced with an aftermarket proportioning valve and still let the system work correctly or does this piece need to be rebuilt or replaced(I cannot compress the button on the back), I know Ground Up sells them, but I don't want to spend $145 if I don't have to. Originality does not matter that much to me. And exactly what is this piece called, I've heard it called all three, but which is correct?
Thanks
Mike

The valve under the master cylinder on a '69 is a hold-off valve. It allows some pressure to be applied to the rear brakes to overcome the spring tension in the drums before pressure is routed to the front discs. This means even timing of brake application between the front and rear brakes.

Without the valve, rear drum action would be delayed and you will probably have little to no rear brake action in light stops.

Troy

Generally, these things are referred to as metering valves. They act to delay the onset of the front brakes, which in theory will match the front braking with the rear, because the rear shoes are farther away from the drums than the front pads are from the discs. At least that's the theory, anyway. You can't put an aftermarket proportioning valve in its place because such a valve must go in the rear brake line, not the front.

I think MasterPower brakes offers a rebuild service, the last heard they wanted $70- 80 bucks. I can't remember exactly.

A couple of years ago I was having problems with mine. It started leaking out the back where the button is. And since I didn't want to pay for a rebuild, I just took it out completely. I plumbed the front brake line directly to the MC instead of passing through the metering valve.

I did notice a slight difference in braking, in that I think I can feel when the back brakes start working....it's just a moment or two after the fronts start working. It's not very noticeable to me and is certainly not in any way dangerous.

If I could find a new metering valve that wasn't super expensive, I'd get one. But I don't see that happening anytime soon, so I believe I'll leave it the way it is for now.