Metering Valve vs. Proportioning Valve
Short answer: The “round piece” under the master cylinder is the metering valve (front) — it holds off the front discs until the rear drums engage. A separate proportioning valve (rectangular, on the frame rail under the driver’s seat) was fitted only to 1967–69 V8 + A/C + disc cars to limit rear pressure and prevent the nose-heavy car’s rear wheels from locking.
From the 1967 Service Manual: “The Proportioning valve is used on disc brake cars with V-8 engine and air conditioning. Basically the valve works to limit hydraulic pressure to the rear wheels. Up to 380–420 psi the inlet (master cylinder) pressure will equal the outlet (rear wheel cylinder) pressure. Above this figure the outlet pressure rises slower… braking effect of the rear wheels is reduced in comparison to the front.”
Don’t “meter the front” to balance a system — that only encourages rear-wheel lockup. And don’t mix systems: use all the model-specific parts of one era (don’t combine the later combination valve with an earlier metering valve). The 1967–69 hydraulics are basically the same, so 1967–68 4-piston and 1969 single-piston calipers can be swapped. Use one donor’s complete valving (members prefer Firebird, not Camaro/Nova, lines and switch), and always replace the rubber hoses.
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Content last modified: June 13, 2026 at 10:06 am
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