When restoring a classic GM vehicle, two common upgrades include replacing brake lines and replacing drum brakes with disc brakes.  For either project it is very important to understand which valves are being used and where they will be mounted.  This article will lay out the different factory valve configurations that were used on GM mid-size vehicles  from 1964-1977.  We will look at both drum and disc brake applications.

Getting the terminology right

Not every block on your car is a Proportioning Valve!!!

Proportioning Valve- A proportioning valve restricts the amount of pressure to the rear of a brake system under heavier braking conditions to prevent the rear brakes from locking up too soon. Proportioning valves were usually only found on vehicles with front disc brakes.

Distribution Block- This block distributes the fluid to different lines in the system and includes a failure warning switch that turns on a warning light in the event of a half system failure.  It does not proportion or meter the brake fluid coming from the master cylinder. 

Metering Block- A metering block is found on vehicles with front disc brakes and rear drum only.  It holds pressure off the front brakes up to a certain pressure. This delay in pressure to the front brakes allows pressure to overcome rear brake springs and start rear braking. This results in better front to rear brake balance at low pressure.

Combination Valve- Combination valves combine all three of the above blocks into one.  On GM disc brake cars from 67-70 all three of the above valves were on the car seperately.  From 71-77 the three valves were combined into one combo valve for better performance and fewer hydraulic connections.

The following guide will tell you what valves would be found on cars from the factory

1962-1966  A / F / X body

All of these vehicles came from the factory with drum brakes and a single port master cylinder.  When converting one of these vehicles to a dual port master cylinder, it is also necessary to to change the distribution block to a later model 5 port distribution block.  Otherwise you will have two lines coming out of the master cylinder and only one port to receive them in the block. We currently do not carry replacement 4 port distribution blocks for 1962-1966 vehicles.  

1967-1972  A / F / X body  with 4 wheel drum brakes

All of these vehicles with 4 wheel drum brakes use a distribution block.  The different options are shown below. This was the only block on a drum brake car.  

GM A / F / X body 1967-70 with factory front disc and rear drum

From 1967-1970, factory front disc brake cars shared the same distribution block as the drum brake cars.  GM used additional valves to improve overall system performance and meet federal guidelines under various road conditions. These metering blocks and proportioning valves are shown below.

1967-1970 GM A / F / X body proportioning valves

These valves mount on the frame inline in the brake line that runs from the distribution block to the hose at the center of the rear axle.  Not all cars with disc brakes had this style of valve.  

GM Disc Brake Conversion Valves

PV71 shown below is a factory replacement combination valve for 71-77 GM mid-sized vehicles, but is also used universally in many disc brake conversion kits. PV72 is the same valve, except with the metering function removed, which makes it more appropriate for four wheel disc configurations. PV01 is an adjustable valve that is used in many disc brake conversion kits in conjuction with a stock distribution block. It is important to know which valve your disc brake conversion kit includes and where it will be located in order for us to know what line set is correct for your vehicle.