early 60s disc brake swap master cylinder

[66gmcguy]

Forgive me if this has been talked about already. I couldn't find any threads on this topic though.

I have a 1961 Suburban 4x4. It has drum brakes in the front, and it has the combo brake/clutch master cylinder. I am wondering if anyone has swapped in a disc brake axle and kept the stock master cylinder without any problems. I know typically you are supposed to change master cylinders when doing disc brake swaps, but since this is kind of an oddball master cylinder my options are limited. I've seen the adapters to mount a brake booster and newer style master cylinder but something like that is not in the cards for me yet. I am trying to do this as cheap as possible to hold me over until I decide whether or not I'm going to keep this rig.

[AcampoDave]

Sorry wrong post

[theastronaut]

The stuck drum brake master cylinder has a built in residual valve that will cause discs to drag. You'll need a master cylinder that doesn't have one for the front discs but does have one for the rear drums. Or, you can use a disc/disc master cylinder and add an external 10 psi residual valve in the rear line.

You'll also need to add a hold off valve in the front line, and a proportioning valve in the rear line. You can get a combination valve that has both built in, but I prefer separate valves with an adjustable proportioning valve.

Power brakes don't stop any better, they just make the pedal lighter, but that's not the best idea without ABS- you can make the pedal too light and lose the ability to modulate the brakes in a panic stop situation.

My setup is a 1" bore disc/disc master cylinder (no internal residual valves), a hold off valve in the front line, a 10 psi residual valve and a Wilwood adjustable proportioning valve in the rear line, no booster. It has the usual squarebody sourced front disc swap and stock rear drums. It has plenty of stopping power, enough to lock up the tires at any legal speed but also is easily to modulate to get right up to the point of lock up for the shortest stopping distance without locking up.

[theastronaut]

Your "cheap as possible" option is to disassemble the current master cylinder, remove or disable the residual valve (if possible), and add one into the rear line. You'll still need a hold off valve and a proportioning valve to balance out the timing of the application of the front and rear brakes, and a proportioning valve to keep the rear from locking up before the front. The stock drums have staggered wheel cylinder sizes to naturally balance the front to rear bias, which keeps the rear from locking up first. When you swap to discs it upsets that balance, so a proportioning valve and hold off valve are needed.

[theastronaut]

Another option- make your drums work like they should and they'll stop as good as discs. Discs aren't any more powerful, often they're not quite as strong since drums are self energizing, discs are just less maintenance and don't overheat and fade as easily. But when drums are right, they work very well.

[PM Garage]

Yup, you can run discs with the stock MC, but don’t expect great pedal feel. Did it on my ‘63 C10 for a while. Braking worked, but it was real soft and kinda sketchy under load. If you’re not adding a booster or proportioning valve, just be ready for uneven braking until you upgrade later

[AcampoDave]

Originally Posted by theastronaut

Another option- make your drums work like they should and they'll stop as good as discs. Discs aren't any more powerful, often they're not quite as strong since drums are self energizing, discs are just less maintenance and don't overheat and fade as easily. But when drums are right, they work very well.

This is good advice right here. I did that on my longbed. New shoes, New rubber lines, and turned drums go a long way toward throwing down a skid. I also made my E-brake work well and yours should too with a single pot MC.

[theastronaut]

Originally Posted by AcampoDave

This is good advice right here. I did that on my longbed. New shoes, New rubber lines, and turned drums go a long way toward throwing down a skid. I also made my E-brake work well and yours should too with a single pot MC.

My first car was a '64 VW with 100% stock manual single pot drum brakes and it stopped great. I had swapped to 15x5.5" Porsche 356 wheels to run wider 195/50 and 195/60 tires for more grip and lowered it so it would handle better. I had a g-meter and it would pull .97g braking.

My sister's first car was a '99 New Beetle with four wheel discs, abs, and wider 205 sport/summer tires. It would pull .99g braking. Dad's 97 S10 was in the low to mid .90s with disc/drums and abs.


If your factory brakes are working correctly, tires are the limiting factor in how short it'll stop, not the brakes.

[theastronaut]

Originally Posted by PM Garage

Yup, you can run discs with the stock MC, but don’t expect great pedal feel. Did it on my ‘63 C10 for a while. Braking worked, but it was real soft and kinda sketchy under load. If you’re not adding a booster or proportioning valve, just be ready for uneven braking until you upgrade later

The stock drum brake master cylinder has a residual valve so it shouldn't be used with discs without removing or disabling the internal residual valve. The calipers will drag if its no removed.