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


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