The conventional H-5 Relayair valves configuration is obsolete and has been replaced with the A-1 Charging and Cut-Off valve in the industry since many years.
Obviously the fact that the cars are not equiped with #8 vent valves doesn't help at all. However I assume that the locomotives are equiped with at least one vent valve at rear end.
Suggestions:
1. Make sure port #1 fo A-1 C&CO valve is pipied DIRECTLY on Brake pipe trainline with 3/8 OD copper tubing (type K I guess).
2. Make sure there is no restriction at all between A-1 C&CO valve pipe #1 and traniline.
3. If not existing, equip locomotive with Low Brake Pipe Pressure Switch (LBPS) (EMD part 9518549 or equivalent). LBPS shall be piped on Brake Pipe close to trainline (not on A-1 C&CO pipe). LBPS is normally wired on PCS circuit in order to initiate power knockdown when Brake Pipe pressure drops at switch drop out set pressure (39 psi). Power can be restore through LBPS when Brake Pipe pressure is increased above switch pick up set pressure (45 psi).
4. An Air Flow Meter is also a good mean to monitor Brake Pipe pressure along the train by loco. engineers.
Good luck !
Clyde