just checked it out and the bottle has a seal on the edge of the cap but no diaphragm. I disconnected the hose and sucked on it slightly and then blew into it to see what passed vacuum (i really need to quit being a tight wad and buy a proper vacuum pump/gauge). it let air into the radiator but also let some out. next I cleaned the radiator cap for the 10th time and put it back on, now it lets air in but not out ( so coolant can go back in the radiator but has to be forced out at the pressure rating of the cap). I'm thinking that the radiator spout has some wear and isn't quite sealing up properly sometimes.