Master Cylinder Valve Question

[57chevyman]

Hey Guys,
I recently upgraded my 1957 truck to a dual master cylinder with no booster... I got the "corvette style" MC from CPP....

I have drum brakes all around.

I had my mechanic who specializes in hot rods and muscle cars install the new master cylinder. He did a real nice job and ran all new brake lines and the brakes work like they should.

Here is my situation that I want some advice on:

I was advised by CPP to not use the proportioning valve that came with my master cylinder and use residual valves instead for a drum/drum application.

My mechanic hooked it up with the proportioning valve and says that its fine... but apparently its not fine because the thing is leaking. it seems to be leaking from one of the plugs that's screwed into the drivers side of the master cylinder. ( the brake lines are attached on the passenger side of the MC). I looked close and saw that the plug that is leaking was screwed in using some white plumbers tape. im bringing the truck back to the mechanic tomorrow morning.

Is the MC leaking because its defective? is it leaking because he hooked it up with the proportioning valve?

Is it OK to use the proportioning valve with all drum brakes ?