Anybody got any ideas if the proportional valve from a 73 Cutlass can interchange with one from an 84ish full size Chev Caprice or Wagon? Redoing the entire brake system and have a brand new spare prop valve for the Chev. Looks identical except for the updated electrical connector.

Thx

 

I wouldn't do it - the 2 cars are nowhere near the same weight or balance!
Wagon probably has 50+% of it's weight on the rear, whereas the Cutlass has just the opposite!
Try to find one off another "A" body.

 

Although they look the same I was always curious about whether or not the inside was setup different.

 

Different part number prop valves have different internal spring preloads to change the amount of front-to-back proportioning based on the car's weight distribution. Now, in the real world, cars are designed to be driven at minimum and maximum loading, so there's some overlap in which prop valve can be used where.