If sprayer doesnt seal against pressure, I suggest check cover gasket A (image 1). Check pump cylinder gasket B (image 2) and additionally, check the procedure if pump dont build pressure. on how to remoce pump cylinder on follow lines.
If pump doesnt build pressure... in most instances, failure of the pumo to build air pressure in the sprayer is due to dry or worm plunger cul leather. To work on cup leather, always remove plunger from pumo cylinder. Do not attempt to oil cup leather through breather hole in **** cap.
First, unthread pumo cap A (image 1). If cap is too tight, and if pumo will build even slight temporary pressure, pressurizing the tnak will make it easier to loosen cap. Once cap is lossened, release tank pressure. Hold pump cylinder by inserting hand through tank filler openning (image 2), and unscrew cap all the way. Pull plunger from cylinder, and remove cylinder from inside the tank.
Place a few drops of clea oil (not heavier than SAE 30) on leather, and massage into leather until it is soft. Replace plunger cup leather if damaged (image 3). to remove plunger leather assembly, use coin rather than screw driver in the slot of the plunger cup retainer. To reassemble, hold pump cylinder in tank openning, with squared edge of flange, at top of cylinder, toward tank wall.
Hope this helps to solve it.