I have an old system where the ports are labelled A,B,C. Which port is the common port? I am familiar with A,B and AB but need to identify this valve so I can replace the actuator. The actuator is a Powers M/N 598-028 and I can not seem to find a cross. Belimo can supply an actuator if I can give them enough info. I will also need to replace the controller as it is a Johnson M/N TC-4100-31 operating on 24 volts with a 0-18 vdc signal. Does anyone know of a company who could provide a cross reference for these parts?