Help! Water pump pressurizing when using hot water tank.

Decided to be a little proactive and replace my water pump. After install of my SHurflo 4008 I determined that my check valve on the hw tank needed replacing. That statement is simplifying 2 hours of water system tests, looking for leaks etc) Went to replace it and of course couldnt get it out like most people have experienced - so pulled out the guts of the check valve with pliers and put another new plastic check valve in line. New check valve works fine. New pump works fine.
Here's the problem. When pressurizing up the system, everything works perfectly if I have the hw bypass 'on'. RUn a tap, water comes out, turn the tap off, pump shuts off immediately. If the hw bypass is 'off' - and i'm now using water from the hw tank - after closing a tap, the pump will run for 3-5 secs before reaching pressure and shuts off.
So - hw bypass on - the pump will shut off as soon as i close the tap. Hw bypass 'off', pump will run for 3-5 secs after closing tap. What's going on here? I'm at wits end after an 8 hr project. Help appreciated guys - I can't solve this one and we're trying to go out this coming weekend!