Petcock leaking o my 86 vf500 us version

Petcock problems. i disaseembled my petcock on the 86 Vf500 US version. I cleaned everything and the rubber diaphrap, spring and plastic plate were all in like new shape. I reaasembled the petcock, sealed and mounted to the tank mounted the tank and hooked up the gas. When i turned to "on" gas poured out of from everywhere from the housing including the two small vents. It leaks slowly on the "OFF" setting now.

What went wrong?