The Stovebolt Site Forums The Parking Lot Big Bolts Master cylinder swap

Has any one found a good dual curcuit master cylinder to use in the bigger trucks with the original drum brakes. I know many have done it on the light trucks, but the larger trucks use larger cylinders. My '41 for instance uses 1 1/4" master, 1 1/4" front and 1 1/2 rear cylinders with no hydro-vac. I would expect the smaller dual master cylinders popular to use in the light trucks would not move enough brake fluid to operate the brakes properly.

Try a late sixties C-50 or similar. They had drum brakes and a dual master cylinder...~I think!

No hydrovac? First for me, but I'm usually into 48 and up trucks. Most MC's are gonna be 1". I have seen up to 80's C-60's with single line MC, but they always had Hydro too. Wonder if you could adapt one of those later ones? Wait, you wanted 2 lines, I forgot! Not sure what you could do really!

On my '48 1.5 ton, I used a master from a '66 Pontiac Catalina. It was designed for non-power, 4 wheel drum brakes without a proportioning valve. If your brakes are properly adjusted, you shouldn't use much fluid from the reservior. Besides, dual res. masters hold more fluid than the originals did. I did incorporate a booster into my system (from a '75 GMC truck). You need to fabricate a mounting bracket though.
That's what I did, Scott

The original brakes work OK on the '41 but I've never had much of a load on it. I thought of adding a hydro-vac, but for the price of a new one I thought adapting a later dual master with a booster with a bracket would achieve the same effect and be safer. I'll have to study on it a bit more. Thanks.

Looking in my catalogs, I can't find a dual-circuit power drum/drum master in 1-1/4" for a GM truck. Looks like they went to disc brakes in front at the same time they went dual circuit in the medium trucks.

Late '70s and early '80s 3/4 ton trucks with the big rear brakes used a 1-1/4" dual master. It's a disc/drum master with large/small reservoirs, but it would work fine on drum/drum--you'd have to add residual valves in both lines, but that's no big deal. Some of these cylinders are machined for residual valves under the tube seats...I've got a few of them in my core pile.

The '70s-'80s boosters are pretty big and the mounting is sort of specific for the firewall, so the booster might be difficult to mount under the floor. If 'twere me, I believe I'd get a 10" or a dual-diaphragm 8" booster from a street rod supplier. It may be that an under-floor bracket for a light truck would fit the larger truck or be easy to modify to fit. I'm not sure about '41-46 trucks, but I know that '47 to '59 larger trucks use cylinders that have the same shape and mounting as the light trucks, with the larger bore. The earlier ones are probably the same. So the bracket might need modification to fit the frame, but the cylinder and pedal mounting would work fine.

Joe