When Rare Spares had the radiator caps remade with the approved GM logo the part number 9928800 referred to a 15lb cap (518.15). The same cap was used on a HZ without overflow and all HQ Heavy duty. The HQ standard used a 13lb cap 9928799 (508.13). Part number 92006790 (534.15) is for HZ with overflow. That cap has a sealing ring for the coolant recovery system. Firstly you need to check that the cap is a coolant recovery type otherwise it will not seal the system. If you look at this web site you may get a better understanding of the difference. http://www.cpcauto.com.au/radiator_caps.aspx. You should ensure the cooling system has been flushed out thoroughly and check under the cap valve for any dirt. A radiator installer such as NATRAD can check the cap operation with a special hand pump, as well as your system. A coolant recovery system is a better option than just venting to the ground.