Water system Mystery Valve

Ian Townsend

Friends, I am having a devil of a time with installing a new freshwater diaphragm pump with adjustable pressure switch from 17-55 PSI. Even at a low setting for the switch, it is overpressurizing the system. This should not be.
In order to prime the system, I opened what I call the mystery valve (picture below). System then primed properly primed and the pump cycled off. After two hours, suddenly the water heater tank release valve blew. So I then joined the hoses and bypassed the water heater. Then the pressure gauge above the mystery valve developed a pin hole and is also leaking at its connections. 

Any ideas?

Loca Lola II
SM 153