Incidentally, I thought those check or proportioning valves were mounted seperately in front of the Master Cylinder on the chassis rail on the Motorhome chassis. I might be wrong because I am not familiar with the age of your coach or whether it differs between a 350 and 454 set up.
1977 Service manauals
ST330-77 1977 Chevrolet Light Truck Service Manual
ST333-77 1977 Chevrolet Passenger Car and Light Truck Unit Repair Manual
1977 GM CK Light truck Wiring Manual
ST-331-77 1977 Chevrolet 40-60 Medium Duty Truck Service Manual
ST-332-77 1977 Chevrolet 70-90 Heavy Duty Truck Service Manual