Thanks everyone for the advise.
powderman426, I wasn't aware a valve of this type could fail but I really suspect that is what is happening. With all others in their normal operating position, as I open the hot side valve I start to get flow then it suddenly stops.
enblethen, its definately not on the supply (cold) side because I have plenty of flow when opening the blow off valve outside.
Old-Biscuit, I plan to investigate the output fitting on the tank closer but I couldn't find a flow direction stamped on it so I didn't think it would have a check valve in it.
dbbls & RoyB,
The input/output (cold/hot) valves are not the 90 degree shut off type valves. These turn approximately 360 degrees from full open to full close (much like a standard outdoor faucet.
It will be a couple weeks before I can look at it again but hope to solve the issue soon. Any advice on dealing with Pex tubing since I don't have a crimp tool for it?
Michael