Initially it was because I had used BUNA petcock gaskets, which disintegrated under the influence of petrol.
Tom
I use Buna N quite sucessfully. Problem is most Buna N that is easily available is around 40 durometer in hardness, that will tear as Tom describes. I found some that is 70 durometer and has worked on several petcocks just fine. Most petcocks leak when there is a lack of use with turning them on and off. The gasket swells over time into the divot of the on/off device and gets hard. When you turn the petcock later, it cocks because of this swelled lump and of course, leaks. JB