the tank may be fine...but the "pressure switch" may be acting up. Can you describe the symptoms ?
The way it is supposed to work...is as water is used the pressure will drop...this triggers the pump to run which fills and pressurizes the system. it should operate within a "band" of pressure ...say 50 to 70 pounds. when the pressure drops below the low set-point it comes on and runs until the high point is reached.
These tanks normally have a "bladder" type device in the head space that holds air. Water is NOT compressible so the tank can not be 100 percent full, the pressures would spike dangerously high. If this bladder is leaking then water will displace the air space and cause the pump to cycle more frequently.
With all the valves closed the system pressurizes fine to 50 PSI. When a valve is opened and left open the system will drop to 30 PSI and the pump will turn on. Then the odd thing is that when the pump turns on the pressure drops to 15PSI. If the valve is then closed the pressure will jump back to 30PSI and then slowly climb back up to 50 before turning off. What are you thoughts?
Does your pressure tank have a "bladder input" location? This would be at the top of the tank and look like the air fill valve on a regular car tire (schrader valve stem)
yes. I have already turned the pump off, drained the tank and ensured the pressure was 2 psi below the pump cut in pressure. But still behaves as described above.
Have you checked the pumps output directly flow and pressure? The pump motor may run just fine but if the flow is not there it could be a problem with the pumps head...do you know how old the pump is and when it last had service?
Normally in these type is situations you start at the beginning of the system and verify each component sequentially.