Number of reasons for the primer not filling, But i'll go with this one as you say the tank is under pressure.
Faulty primer. The primer has a couple of one way valves in it, so it sucks up the fuel while expelling the air in the line. Id say that the valve in the primer has failed and instead of pumping up the fuel you're pumping air into the tank. The replacing the primer. part number 545 00 80-30
Answers & Comments
Number of reasons for the primer not filling, But i'll go with this one as you say the tank is under pressure.
Faulty primer. The primer has a couple of one way valves in it, so it sucks up the fuel while expelling the air in the line. Id say that the valve in the primer has failed and instead of pumping up the fuel you're pumping air into the tank. The replacing the primer. part number 545 00 80-30