Since you say "brakes" I'll assume you mean both front brakes are locking, not just one side. And, again assuming, they don't stay locked, it's just that the fronts skid and the backs don't.

Not sure which brakes you have, the original Lockheeds or later Bendix's from, say a Ford pick up. Did you change the shoes or just do the M/C-pedal swap?

I don't understand why you put a proportioning valve to the rears. With drum front and rear that's not necessary, and if you have it adjusted to reduce the rear brake action then that's probably why the fronts lock first.

Streets' comment would be compounded by your restricting flow to the rears as hydraulic levereage is a function of M/C piston size vs wheel cylinder size.