Proportioning Valve Outlet Fitting

[Father&son56project]

A possibly dumb question here, but here it goes: I’m putting in a disc/disc brake system and I need to know which fitting to use at the outlet (to the rear brakes) of the PV4 universal GM style proportioning valve. I plan to run 3/16 line out to the rear. The product info from MBM Brakes for the valve tells me that the outlet is a 9/16-18 fitting, but says nothing about line size. I can order a “9/16-18 Inverted Flare Nut; Line Size: 3/16" like this from Rockauto:

https://www.rockauto.com/en/moreinfo...1000606&jsn=67

Or I can get the “Proportioning Valve Adapter Fitting Kit - Set of 3” from CPP like this:

https://classicperform.com/cp2091

Are the fittings from both places in fact the exact same thing (other than getting 2 unneeded ones in the kit)? Sorry if this sounds dumb, but all my searching has turned up conflicting info and I have zero knowledge where fittings are concerned. Any input would be greatly appreciated.

[Willie Makeit]

search for 9/16-18 (thread size) and then sort by tube size. Tube nuts are made in various configuration.

The link to rock auto seems to be the one you want.

[Father&son56project]

Thanks Willie Makeit! I (erroneously) assumed that the outlet port in the proportioning valve was meant for a specific line size. This frees me up to get one of those inexpensive 9/16-18 fittings and hopefully start making some progress in the shop.

[cadillac_al]

Why 3/16"? Most trucks use 1/4" to the rear drums. discs might need even more fluid.

[Father&son56project]

Al, I did a ton of searching online, and the info was all over the map regarding 3/16 vs 1/4 rear line sizes. In the end I picked 3/16 as I will be running an Explorer disc rear end. The engineers at Ford picked 3/16 line for the entire rear system, so I figured I'd go with what they picked.

[chevyguyase]

I agree with cadilac al, I use 1/4 inch from the master cylinder to the prop valve and from the prop valve to the t fitting at the rear differential and from the t fitting to the rear wheel cylinders I use 3/16. That 1/4 inch line to rear feeds two rear wheel cylinders.

[Willie Makeit]

3/16" is fine for disc systems

[cadillac_al]

I guess I would have to agree if ford uses 3/16 in their rear disc systems. I have never seen it in any vehicle that I have owned though.

[theastronaut]

Line size doesn't really matter, 3/16 isn't small enough to be restrictive. Newer trucks have 3/16 all around, even with rear drums.

[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.

[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.