Water pressure question

[Ron Hill]

Dave : Yes, it could be the pump itself!! 
To easily check the hand shower - just clamp off the stern supply hose rather than tear every apart in the aft cabin!!  I have two pieces of 3/8" plastic hose that I slide on to some needle nose vice grips so I can clamp off a hose to temporarily stop the flow of liquid.  Also Like Ken mentioned for the pump check!!   

A few thoughts

[Jim Hardesty]

Are you telling me to shut off the supply to the pump from the tanks and then see if the pump cycles?

David,
Yes, the check valve on the pump may be leaking pressure/water back into the supply line.  If the supply is shut off no more leaking back.  Wouldn't let the pump run much with the supply shut off.  If it doesn't cycle with the supply shut down and pressure in the system it's the pump.  Sort the same check as Ken suggested with clamping off the line, just the other way around.
Jim

[KWKloeber]

Are you telling me to shut off the supply to the pump from the tanks and then see if the pump cycles?

David,
Yes, the check valve on the pump may be leaking pressure/water back into the supply line.  If the supply is shut off no more leaking back.  Wouldn't let the pump run much with the supply shut off.  If it doesn't cycle with the supply shut down and pressure in the system it's the pump.  Sort the same check as Ken suggested with clamping off the line, just the other way around.
Jim

Theoretically that works but not necessarily in real life -- it could give you a false negative result re: the pump being the cause (i.e, the pump still cycles.)  The test depends on sufficiently pressurizing the supply side using only leak-back, which may or may not happen. 
It also depends on where the shutoff(s) are (very close to the pump?)  BUT it's worth a try - if the pump stops cycling then it's a positive result (the pump check valve or pressure switch or diaphragm is most likely bad.)

The WH also introduces another snafu which I did not mention just to keep it simple for now (because it's a rare happening.)  The engine coolant (heatant?) coil in the WH can develop a pinhole that lets pressurized potable water push over to the engine coolant side (and vice versa if the engine is running w/ no pressure on the potable water side.  (So antifreeze can get into the potable water system - NOT good.)

BTW, (other than annoying) running the pump when dry is not an issue (I'm not suggesting for days at a time here.) 
(I asss/u/me it's a diaphragm pump?) so water doesn't lubricate any part that turns and nothing abnormally wears if run dry (unlike the seawater pump, which has a water-lubricated impeller that burns up.)