[Tony Z]

My first thought is also the cap

But I do have another idea.... what about vibration?? Is you header tank mounted on rubber mounts or straight onto the firewall?? You could be rattling the thing apart.

Try this. Make a plate to mount your tank onto. This plate... mount it to the firewall using 2 flexible rubber mounts (used for mounting small motors and pumps). Mount the plate onto thie mounts and mount the tank onto the plate. Then get some (2) rubber straps and tie them around the plate and the tank. This will allow the tank to be fairly rigid to the plate, while not recieving the vibrations from the car.