Before I install a valve cover I lay it on the head without the gasket and look for gaps or bowed out sides. An extra head laying around the shop makes for an excellent anvil for hammering a bent valve cover into shape.
If you have a brass radiator, the leak around the cap may be from a breech in the solder between the tank and the part upon which the cap is screwed. I had to fix mine by heating it with a torch to remove it, cleaning up the brass mating surfaces, then solder it back on.
Carl
