This just came up in another thread, but I didn't want to hijack it too far...


What kind of setup is needed to keep the coolant topped off and free of air pockets ?

My guess is that a surge tank/reservoir/level sensor in the engine bay with the small vent tube connected to the high point (the aluminum water pipe on the engine) would take care of that area, and a bleeder valve of some sort at the top of the radiator up front. I'm assuming the top of the radiator is lower than the top of the engine, right ?

Is this the kind of thing that once you bleed the system, everything is fine, or do you have to keep checking for air pockets ?

If you go with a renegade radiator it has a bleed screw built in, if you are going with a subie as your avatar suggests bleed it from the throttle body heater hose.
Oh yeah don't be stupid when you route your coolant hoses, avoid un-bleedable high spots.
You shouldn't have to constantly check for bubbles but after driving you may shake a couple loose so bleed every week or so when you first get the car going.
The stock subie engine has a nice water filler tank that has a nipple that is supposed to attach to the overflow bottle, use that.

There's only one way to keep the air out. you have to have an expansion tank (as high as possible in the engine compartment) that will let the coolant expand into it than get sucked back in when it cools. Eliminates all air. this means you have to have the "radiator fill" cap back at the engine (high point) with the 3/8" outlet connection and the right type of radiator cap that allows the system vacuum to pull the coolant back in. The tank has to be pretty good size since there quite a bit of coolant with all the hoses. If you PM your email I'd be happy to send some pics of a typical setup.