Rear brakes locking up after Double MC/Booster install

[HackJob65]

Captainfab correct me if I'm wrong but on the 67-70 drum/drum dual pot master cylinder trucks the distribution block containing the brake switch internally have a piston. This piston will actuate if a leak develops in a front or rear line. When actuated it will block fluid flow to the line which is leaking. Internally master cylinders typically have a center floating piston which requires fluid on both sides/ in both reservoirs to work correctly. Thus the real safety component of any dual reservoir master system in actually in the proportioning valve / brake switch block. I believe this is a majorly overlooked component.