They make a valve that is specifically for that purpose. It's not a Y valve.
but has a hose connection that allows you to flush the engine when the valve
is the closed possition.

I think it's make by Forespar.. I have one on my boat but have reservations
about putting garden hose pressure on my water pump seals. (A good way to
get water into your engine oil.)


--
My opinion and experience. FWIW

Steve
s/v Good Intentions