by Scott Korb



application: F6/F8 67-69 ALL,

                      X6/X8 68-70 ALL,

                      X6/X8 71-74 with drum brake 

location: under master cylinder

part #: 3904270

bracket #:  3909933 F6-F8 ALL 67-69

purpose: distribute front and rear lines, failure warning switch/ differential switch

type: shuttle valve

operation: spring loaded shuttle valve remains centered unless loss of circuit pressure allows a shift. pressure difference unknown, possibly between 150 -250 psi will shift the switch.  Once shifted, dash circuit is grounded, causing red warning light in dash to illuminate. Shuttle valve self centers when pressures equalize.

ports for Firebird:

front inlet 3/8x24, wrench size 3/8"

rear inlet 1/2"x20, wrench size 9/16"

rear outlet 7/16x24, wrench size 7/16"

front left outlet 1/2"x20, wrench size 9/16"

front right outlet 3/8"x24, wrench size 3/8"

notes: 1969 Camaro/Nova, and possibly some Firebirds swapped the line distribution using the same valve. Inlet used the 1/2" port on the front, left side went to right brake, right side to left brake, 3/8"x24 ports. 1967-8 Corvette used this same hydraulic block, with a different bracket.


application: F6-F8 front disc/rear drum

location: next to master cylinder

service part # 3905525 67-69

purpose: to delay front discs until rear drums overcome their return springs, allowing near simultaneous braking

operation: This valve meters the flow of fluid to the front brakes. The valve shuts off flow between approximately 2-50 psi

Type: metering, delay, "hold-off"

2 ports, 3/8"x24

notes: button toward firewall moves out as brakes are applied. Hold button in IF pressure bleeding.


          Valve, rear brake pressure regulator

application: 67-69 F8 disc brake with air conditioning

location: under driver, on outside of frame rail

service part # 3908326

type: pressure activated proportioning

purpose: to regulate rear brake pressure in proportion to front during hard stops, to compensate for both weight transfer, and drum brake energizing.

operation: This valve regulates rear brake pressure in proportion to the front. Proportioning amount approximately 45 psi out for every 100 psi in, once the valve closes at approximately 250 psi ("knee point"/slope ~250/.45)

2 ports, 7/16"x24

notes: requires 2 piece rear line


application: 1971+ GM vehicles, or modified systems

location: variable. usually near master cylinder, or on frame

part # variable

type: combination

purpose: hydraulic control of either or both, front and rear brakes, distribute lines, and house the warning switch

operation: depending on application, this valve may contain both the above described front brake metering valve and rear brake proportioning valve, or just one of them. Different vehicles had different needs, and the valves also would have different pressure specifications.  A disc/disc car typically would not have the front metering valve, but may have the rear proportioning valve. Many disc/drum cars might have only the front metering valve. All of these would have the warning switch. 

note: new brass combination valves contain the entire rear proportioning valve inside the rear proportioning nut. The old cast iron ones were integral. 

Picture is of one style. There were several.


application: various

location: various

part #: various

purpose: to regulate rear brake pressure in proportion to front to compensate for weight transfer and/or rear drum energization.

type: adjustable pressure operated proportioning valve

operation: fixed slope, adjustable "knee point". The slope is fixed on this valve at approximately 40 psi out for every 100 psi in over the knee point. The knee point is adjustable.

ports: 2. various. most after-market come NPT thread with 3/8x24 SAE adapters

notes: The only GM I am aware of which came stock with an adjustable proportioning valve were some early C3 Corvettes with dual pin heavy duty brakes.

ADDITIONAL NOTES: depending on where you look, any and all of these control valves may be termed metering, proportioning, delay, or hold-off, many times with improper descriptions of functions. Automotive textbook brake definitions describe a front brake delay valve as a metering valve, and a rear brake pressure regulator as a proportioning valve. These valves are NOT interchangeable with each other.

I have not found any residual valves in the system, but they are listed for the 1967 Firebird, located behind the brass seats in the master cylinder outlet ports for drum brakes. Residual valves are used to keep the wheel cylinder cups pushed against the bore to avoid air being sucked in, not to hold the shoes out (the shoes require about 50+ psi to even begin to overcome spring pressure, residual valves are 2-15 psi). They are also used on floor mounted master cylinders to prevent fluid drainback.

Copyright 2006 Scott Korb.  All rights reserved.