Brakes 101

Q: Brakes 101

I was wondering, when I switch my front drums to disc, can I use the same master cylinder and power brake booster? Do I just need a different proportioning valve or the whole setup? 1969 firebird, hdtp, 350 2 bbl, drums all the way around (for now)

A: You need to understand a couple of basics here. Drum brakes require a check valve in the master to keep the cups in the wheel cylinders upright, which uses about a 5-7 psi pressure maintained in the system. Disc/drum combo systems only have the check valve in the chamber that feeds the rear brakes, as that slight pressure kept applied to discs will wear them out and/or overheat them. You will need to use the disc/drum master cylinder for disc/drum systems, or a 4-wheel disc master cylinder for 4-wheel disc systems. You will also need to use the proportioning and metering valves and light switch appropriate to whatever system you use. Those are separate pieces on the early cars (67 and 1968 GM) with the fixed-position 4-piston calipers, as well as the 69 and 70 GM cars with the newer single-piston system. In 1971 GM changed to the one-piece “combination” valve.

If using a correct 1969 system, it will have a front brake pressure metering valve, mounted under the master cylinder – that’s the rounded piece. This prevents the pressure from going to the disc brakes until the rear brakes have made full contact, since discs are zero clearance while drums have to take up some clearance. Then down the lines a bit will be the distribution block with the warning lamp switch. Some applications (AC-V8 cars, among others) will have another valve mounted on the left side of the subframe, this is the rear brake pressure regulator, also known as the proportioning valve. This slightly reduces the rear braking on nose-heavy cars.

On my 1969 drum brake car I removed the check valves and stayed with the non-power master cylinder when I installed the front discs, and even removed the rear brake check valve after adding the 79 WS-6 disc rear, but I will next add the master cylinder and combination valve from the 1979 car. While it’s drivable, I don’t recommend this tactic for the novice. Of course, this plan could change if I decide to go back to a 69 drum rear end. The disc rear is much heavier and complicates the car far beyond the benefits of 4-wheel disc brakes due to parking brake issues and other things.

I guess if somebody wants a 79 posi rear with brand-new Richmond 3.70:1 gears, reman calipers and the 1967, 1968, and 1969 perches I would probably bail out of it. I don’t think I drove it 1,000 miles before I laid up the car. At this point it might be easier for me to swap the rear back to a Pontiac 3.55:1 than proceed with the rest of the hydraulics and the parking brake cable bracket work.

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