on my 99 I changed the head gaskets, and the overflow system worked ok, even with a leaking water pump. I changed the water pump, and now the coolant does not return from the overflow tank. 

No bubbles in the overflow tank.

Tried a coolant pressure tester, and could not get a good seal, so I hooked up about 10 psi of compressed air to it, for about 20 minutes looking all over, and can not find a leak, and not loosing coolant. 

I am going to get a new radiator cap. it was new before the head gaskets went bad, and the same one from before the water pump change.

could the head gasket failure ruin my cap? it was shooing water out of the radiator. and if so why did it work with a leaking pump?

 

 

Did you burp it well?

Check the radiator fluid level with the car cold.  Is it full?

Also make sure the end of the overflow hose is cut at a 45 degree angle so it doesn't get sucked down to the bottom of the tank

Rad cap likely suspect, make sure it's a Subaru one.

Could also be a leaking connection of the hose at the radiator (I usually put a tyrap on the hose to make sure it doesn't loose vacuum.

I thought I had it burped well. If i top it off it is only about 1-2 cups. every time. It seems to spit it into the tank and not pull it back. I have trimmed the hose at the radiator connection, and I have a hose clamp on it. the hose at   the bottom of the tank is cut at an angle.

A pinhole in the hose will leak enough to prevent backflow.  A 19 year old hose has likely degraded and needs to be replaced.  The barb for the connection to the radiator needs to be clean and smooth so air doesn't leak there. 

Thanks for the help. I let some water out of the bleed screw on the top of the radiator, and tightened it back down. That seems to have fixed it, Something caught in the o=ring seal or something.