Hello!
The nipple at the back on most fourstrokes is not a pressure nipple!
It is a nipple for getting out the exess oil in
the crankcase (the same for all other fourstrokes,cars etc).
The nipple on the muffler/exhaust tube is the one you you should use for pressurizing the tank, if you want to pressurize that is...the engine will work okay on no tankpressure!!
Regards!
Jan K
Sweden