what size master cylinder bore

[theastronaut]

Originally Posted by LONGHAIR

The issue with proportioning valves is that the stock disc/drum unit has a residual valve. It is designed to keep some pressure on the rear lines, because the drums have return springs, which discs do not have/need.
Using the original will put that pressure into your calipers. It's only a couple of pounds, but it is there and can cause drag on your rotors.
Switching to a disc/disc is the way to go.
The original question about bore size is mostly about fluid volume for the same amount of stroke. Discs require more volume, but it may not be enough to require the change? Especially since the fronts do most of the stopping anyway.

The residual valve is in the master cylinder on stock drum brake trucks.

A proportioning valve is just a proportioning valve- it limits rear line pressure keeps the rear brakes from locking up. I am not aware of any stock proportioning valves that has a residual valve built in.

Disc/drum systems have a combination valve that is commonly but mistakenly called a proportioning valve. They have a hold off valve in the front port, that “holds off” the discs from applying until line pressure rises enough to overcome the shoe return spring tension, that way the discs and drums begin applying at the same time to even out shoe and pad wear.. The rear port has a proportioning valve. I am not aware of any aftermarket combo valves that have residual valves in the rear port for the drums.