Proportioning Valve Outlet Fitting

[theastronaut]

Originally Posted by HO455

As far as I know back in the day GM (And others) used 1/4 lines on the rear brakes as a way to delay the operation of the rear brakes slightly so the fronts would always apply first. Newer vehicles have more sophisticated proportioning valves or ABS systems.

My sister's original 51 GMC has drums all around and 3/16" lines. You were expected to adjust the brake drums individually for proper braking.

So this being said means you can use 3/16" every where but be aware you may need some way of adjusting the braking balance between front and rear brakes.

The hold off valve performs the task of engaging the front and rear brakes at approximately the same time on disc/drum vehicles, not the brake line size. The rear wheel cylinders have to overcome the pressure of the shoe return springs; I've read that it takes roughly 40 psi of line pressure before the shoes actually start moving. The front calipers have nothing restricting their movement (no return springs) so they would start applying immediately if it weren't for the hold off valve. The hold off valve restricts fluid to the calipers until pressure in the line builds to around 40 psi, so the front and rear apply at pretty much the same time.


4 wheel drums used staggered wheel cylinder bore sizes to set the front to rear bias. On my ‘66 the front used 1.25” wheel cylinders, rear was 1.0”. The same drums and shoes were used front and rear, only difference in braking power was the wheel cylinder bore diameter. For a stock '66 with a 1" master cylinder bore and a 6.25:1 pedal ratio, 100 pounds of input force on the brake pedal makes 791 psi out of the master cylinder. The 1.25" bore wheel cylinder turns this into 973 psi out of the wheel cylinder onto the shoe, and 625 psi out of the smaller rear wheel cylinder. So you get 64% front, 36% rear braking force via the wheel cylinder size being staggered.