If your proline gun is leaking and it isnt because you've let some fuel over flow the vent tubes.....take the gun apart and check the entire length of all the vent tubes. There may be a crack or split in one or more of the vent tubes maybe half way up.
I had this happen to my gun and found out it was a split in 2 of the tubes. A bit of solder and now its leak free.
The oring on the nozzle of the proline gun us usually good.