Is there only one cap type to suit this expansion tank, or are there different caps set at different vent pressures according to actual engine type. To be honest I drained the coolant system, flushed it through, refilled with almost neat red anti freeze and then it seemed to blow out of the expansion tank. Bit by bit with keep refilling, the coolant is probably just water now, and maybe the reason for the rusty coloured coolant..Martin