Brakes 101

Short answer: No — you can’t keep the drum master cylinder. Drum systems use a residual check valve (5–7 psi) to keep the wheel-cylinder cups seated; a disc/drum system only keeps that valve in the rear (drum) circuit, because residual pressure on discs would drag and overheat them. So use a disc/drum master cylinder (or a 4-wheel-disc master for 4-wheel disc), plus the proportioning/metering valves and warning-light switch appropriate to the system.

On a correct 1969 disc system: a front-brake metering valve (the rounded piece under the master) holds off the discs until the rear drums take up their clearance; a distribution block with the warning switch is down the line; and some cars (A/C V-8, etc.) add a rear proportioning valve on the left subframe. The 67–68 four-piston and 69–70 single-piston systems use separate valves; GM went to the one-piece “combination” valve in 1971.

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Content last modified: June 13, 2026 at 10:06 am

Categories: Wheels, Brakes, and Axle - All, Wheels, Brakes, and Axle - Brakes

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