Welcome aboard Mr. 41. I'm a bit confused...(nothing new). The only "caps" on the heat exchanger I'm familiar with are the end caps held on with a 1/2" or so brass/bronze bolt with a rubber gasket between the cap and the exchanger body. This "joint should NOT leak, at all.
The expansion tank has a radiator style cap designed with a specific "release" pressure (can't remember the # but about 7 psi, I think). On the side of the neck this cap screws on is a 1/4" diameter (or so) hose nipple which is the expansion tank overflow and there should be a small hose connected to that nipple. Thereafter there is one of two things....the hose is either open and drains excess anti-freeze away or the hose goes into a jug/reservoir.
IF your expansion tank is totally full, when the coolant heats up, pressure builds up in the cooling system, the radiator cap then allow excess coolant to vent through this above mentioned tube. So what's the problem?