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When restoring a classic GM vehicle, two common upgrades include replacing brake lines and swapping drum brakes for 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
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. Proportioning valves were usually only found on vehicles with front disc brakes.
Distribution Block- This block simply distributes the fluid to different lines in the system. 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 causes the rear drum brakes to engage just slightly before the front, which results in more level stopping.
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.
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.
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.
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PV05
1967-69 F Body |
PV06
67-69 GM A Body |
PV08
1967 Chevelle Only |
PV07
1970-72 GM A body |
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.
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Metering blocks mount to a bracket bolted to the master cylinder mounting stud on the driver's side. '67 GTO is an exception, mounting on the passenger's side stud. This block features a single inlet and outlet. The black button bust be depressed for vacuum bleeding. If you pressure bleed the system by pumping the brake pedal, you do not need to press this button.
- PV68 will function correctly on 1967-1969, but it is only a exact reproduction for 1969.
| PV68 |
1967-69 A / F / X Body |
| PV67C |
1967 GTO |
| PV70C |
1970 A / F / X Body |
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PV68
1967-69 F Body / 1967-69 X Body / 1967-69 A Body |
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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.
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AMB7001
1970 A Body |
CMB7001
1970 Chevelle Only |
FMB6701
1967-69 F Body |
FMB7001
1970 F Body |
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Starting in 1971, all the functions were built into a single block. This valve combines the distribution block, proportioning valve and metering block into one piece. This will be the only block on the front of these vehicles.
The wiring pigtail on the top is for a dash warning light. It is not required for proper function.
The black boot in the front of the valve simply covers the metering assembly. It is not required to press this button to bleed the brakes.
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PV71
1971-77 GM A / F / X Body
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The vast majority of disc brake conversion kits on the market today include one of two style valves. The most common is the 1971-1977 GM style valve shown below. This includes kits for The GM A / F / X body vehicles referenced above, as well as full size Chevy vehicles such as Bel-Air, Biscayne, Impala, Nomad, etc. Other kits have the customer use their factory distribution block along with an adjustable valve. It is important to know the type and location of valve if you want to purchase a brake line package.
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PV71
Front disc / Rear drum |
PV72
Front disc / Rear disc |
PV01
Adjustable Valve |
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