The Stovebolt Site Forums The Shop Area The HiPo Shop Brake questions: modified '57 3600

I'm chasing gremlins on my latest acquisition, a 1957 3600 that was built with numerous modifications some time ago and then driven very little. The truck has a front frame clip from a 1975 C20 (PS, power disc brakes, etc.). The rear axle is a 14 bolt from the same C20. The master cylinder is a Classic Performance Parts frame mount kit with booster, an inline 2 pound residual pressure valve in the front brake line and an adjustable proportioning valve in the rear brake line. My problem is the brake lights won't turn off after releasing the brakes. I suspect the problem is caused by the brake light pressure switch being mounted inline between the residual pressure valve and the disc brakes, thus being kept on by the residual pressure valve.
Questions:
Is my diagnosis correct?
If so, where should the brake light pressure switch be located? between the master cylinder and residual pressure valve? or relocated to the rear brake line (drums on the rear)?