Abstract: A pressure-tight cap assembly providing a leak-proof seal against positive nternal tank pressure. Tank pressure may be vented prior to unlocking and removing the cap. The cap assembly incorporates an external adaptor with ears and slots in its outside circumference to be permanently threaded onto the tank outlet pipe and a quick-disconnect cap containing a spring. The spring fits inside the adaptor and the cap slides over the adaptor and locks onto the adaptor by means of cylindrical pins on its inside diameter received by the slots of the adaptor. A rubber O-ring fits between the outer diameter of the adaptor and the inner diameter of the cap and cooperates with the spring to provide both a leak-proof seal and a means for venting tank pressure.