on that bypass valve how does it work? where do you insert it in the system and where do you connect the vacuum? sorry for the questions im just trying to figure out an easy way to bypass the heater core. im thinking i may use 2 shutoffs and a bypass tube with 2 tees before the shutoff.
On the bypass valve that I installed, it's a vacuum-operated butterfly valve. If you take a look at the crude diagram I attached to this message, you can get an idea of how it works. When the valve's in one position, coolant's routed from the bottom right to the top right and from the bottom left to the bottom right. When the valve's in the other position, coolant's routed from the bottom right to the bottom left, and from the top left to the top right.
I connected mine to the vacuum line that goes to the air recirculating door (which determines whether air's sucked from outside the truck or recirculated in the cab). I simply put in a T-fitting so that, whenever the door's closed (i.e. the system's OFF or in MAX A/C), the valve would also be closed, and the valve is open at all other times. I THINK the line's black with a white stripe, but I can't say for sure...it's right by the heater core fittings at the back of the engine compartment, and I think it's either the only vacuum line present in the area or, if not, it's the one furthest outboard.
As to whether coolant needs to loop...I quite highly doubt that simply shutting off coolant flow to the heater core will make the engine spontaneously explode
but I do recall some people saying in the past that it's better to let the coolant loop back to the water pump than not...and that it was under debate at the time and (AFAIK) never resolved. Frankly, I don't really know myself, but I figured it'd be better to be safe than sorry, and since the valve I used facilitated it as it was, no harm no foul, right?
Russ is of course correct that the heads do NOT get their cooling from the heater core hoses...