If the distribution block is just a block, then nothing needs to be done.  The issue is when you get into combination valves that also affect system pressure, but I don't think our cars use such a valve; that's a GM thing.  I'm pretty sure our distribution blocks only contain a switch and a shuttle valve, the purpose of which is to seal off the front or rear in the event of a leak so that some braking is maintained, and light the warning light on the dash so you know something is wrong.  Front/rear bias was done with a valve in the main rear brake line, now served by your adjustable proportioning valve.  Front/rear pressures are built into the master cylinder. 

Mustangs prior to 1967 had a distribution block --essentially nothing more than a Tee to split the brake line from the single reservoir MC off to the front and rear brake circuits. One line rupture or component failure, where there's a loss of brake fluid/pressure in the system, means ALL brakes on the vehicle fail.

1967 was the first year for a tandem MC that split the front brake circuit, independently, from the rear brake circuit. A failure of the rear brakes does not disable the front brakes and vise-versa, although, braking ability will be greatly reduced in this scenario.  On a 4-wheel drum brake setup, the only brake valve in the system was a pressure differential valve. It had a pressure differential valve spool inside and a nylon electrical switch sticking out of the valve body assembly. --'67 had a 1-wire connection on the PDV switch. '68 and beyond had a 2-wire conection but, the two prongs on the '68-up switch were tied together and, electrically, are at the same potential.

As long as system pressures (front and rear) are equal, the spool remains centered and the brake warning light will remain off. If there was an imbalance in system pressures --if there's a rear brake line rupture or wheel cylinder/caliper failure, etc., for example, the higher system pressure of the front brake circuit will push on the end of the pressure differential valve spool and shift the spool over to the (lower) pressure failed side.

The PDV spool is hour glass-shaped in the center. The plunger on the switch resides in this detent when the spool is centered an thus the contacts of the switch are open so, the warning light does not illuminate. When there's a failure, a shift in system pressures moves the spool over and the switch plunger rides up a ramp on the valve spool. This pushes the plunger in which closes the switch contacts. This completes an electrical path to ground through the body of the valve and through the chassis. This in turn lights up the brake warning light on the dash.

Some information suggests the shift of the valve spool cuts off fluid flow to the failed side. Some suggests it just turns the warning light on to alert the driver that there's a problem with the brake system.

So long as the OEM brake valve is a pressure differential valve and not an OEM disc/drum pressure differential/proportioning valve, a manually-adjustable proportioning valve can be plumbed inline between the PDV and the rear brake circuit without any other changes.

If an OEM combination valve (designed for discs/drums) is used with a manually-adjustable proportioning valve, the OEM proportioning valve components will have to be removed (gutted) from the OEM brake valve body so that there aren't TWO proportioning valves battling each other for control over the rear brakes.

Internal workings of the pressure differential valve.

Last edited by ultrastang (12/30/2015 7:42 PM)

airdrew99 wrote:

Thanks for the excellent information ultrastang. I have family that is from Camden, Arkansas.
My last question is: if my pressure differential valve (on my original drum/drum setup) has shifted the plunger and turned on my warning light, will this affect any brake fluid flow or brake pressure to either side? It seems like every time we reset the plunger and bled the brakes, it would move and turn the light on again. 

That's funny since a lot of my family is from the Laurel, MS area.

There are two things that will shift the valve spool and turn the warning light on: a system/component failure where there's a loss of brake fluid or, bleeding the brakes out can trip it because you are creating a low pressure side when you open the bleeder and then apply pressure on the brake pedal.

There are two items that will make your life a LOT easier when it comes time to bleed the brakes out. One is to install Speed Bleeders in the calipers (or wheel cylinders on vehicles with drums) and the other is to get a valve lock tool to keep the PDV spool centered when bleeding out the brakes.

I've had spools that were shifted and could still get fluid through the system to bleed the brakes out. So, I guess it's up to interpretation as to what the valve does or doesn't do in that regard.

This item in the following link is a big time (and sanity) saver to have during brake bleeding. --this will not recenter a valve spool that's already off-center but, it will keep a centered valve spool from moving off center.

https://www.musclecarresearch.com/brake-valve-tool