I don't know if I'm losing the plot here but isn't the fuel tap suppose to work under vacuum? i.e. if you suck on the pipe connecting to the tap, it will release fuel.....
Ive just gone to drain some old fuel out of the tank from last summer and it won't release the fuel when sucking but will when blowing on the vacuum pipe......

lee

There are two ports for the diaphragm chamber, one is a vent, the other the vacuum port. The vent port points down, unless the tap has been assembled incorrectly.

the tap hasn't been touched in the ten years I've owned it. I put the tank back on with fresh fuel and connected the vacuum hose the turn the engine over and it released the fuel so I'm guessing its ok. BUT I've got a bigger problems with the carbs now which has been ongoing... ill write another thread for this..