The Stovebolt Site Forums The Parking Lot Big Bolts Dual master cylinder conversion

Has anyone mounted a dual master cylinder on their big bolt chevy truck? This topic has come up different times but I'm hoping I can run into someone who has done it successfully. I'm using the original 216 engine with drum/drum brakes. All rebuilt wheel cylinders, a mid 70s chevy vaccum booster mounted under the cab, orginal vaccum line that went to the hydro-vac system I now have piped into the vaccum booster, new brake lines but just cant seem to get bled. The dual master cylinder I'm using is from a late 70s g30 or p30 chevy like a one ton box van with 1.25 bore size advertised as power drum....both resivore are pretty even one side might be just a touch bigger....any suggestion or thoughts to help me out, thanks!!

Do you have residual pressure valves installed? Drum/drum split systems need to maintain 10 PSI static pressure in the brake lines to prevent air from being drawn past the wheel cylinder pistons. That much pressure on a disc brake will keep the calipers applied all the time and cause the brakes to overheat, but since the wheel cylinders are much smaller diameter and the shoes are retracted by springs, more static line pressure can be tolerated.

You'll also need to bench-bleed the master cylinder well before installing it.
Jerry

Yes sir I actually do have 10lbs wildwood residual valves in line like almost directly out of the mc. I ran my brake lines out of the master cylinder somewhat a 90 degree angle (without pinching them ofcourse) up overtop of my driveshaft close to the cab then over to the passenger side frame rail where they tied into the existing lines front and rear....with that being said I was afraid the lines going above the master was trapping air so I took them off and bent the line to where it wasnt such a hard angle....so right now I dont have any residual valve in the front but it still didn't make any difference when I tried bleeding them..

Here's a sneaky trick- - - -while you're bleeding the brakes, adjust the shoes out to a firm contact with the drums. That pushes the pistons all the way back in the cylinders and allows the fluid to completely fill the space between the seal cups more quickly. Once you're getting a solid stream of fluid from all the bleeders, then go back and adjust the brakes for normal running clearance. Worst-case scenario, you might have to rent or buy a pressure bleeder.
Jerry