I have thought about leaving it an open system, but I'd really rather not. One other option is to put a check valve in the small hose and leaving it hooked up to the OEM resevior letting the OEM cap work for pressure relief. But I'm not sure if that will work because I don't know if a radiator cap is designed to let air back into the tank as the system pulls coolant back into the radiator. Hopefully somebody that is an expert on how a sealed coolant system works will read this and chime in. I found this on E-bay, but the guy doesn't seem to know what the pressure pop off is. - http://cgi.ebay.com/ebaymotors/NASCA...em360013108450