[JRTJH]

I agree with Rob (NH_Bulldog). It's probably a defective bypass valve (unlikely) or a calcified (stuck open) backflow valve on the hot water outlet.

Essentially, I winterize using the same process as Rob. The exceptions are that I use a 4' piece of 1/2" clear vinyl tube with a 90 degree fitting to draw antifreeze into the pump. For me, I wouldn't install an additional 3 way valve to replace that device. On my system, and yours if you have a strainer/inlet screen (and you should if you don't), to protect the pump from debris in the fresh water tank, you'll still need to remove that strainer/screen to clean it every year. Seems to me "self defeating" to add a valve that might leak if you're going to be removing the pump input to clean it anyway...

Either method will work, but since I'm going to be unhooking the pump to clean the strainer anyway, seems simpler not to add another valve to the system.