Field King Backpack Sprayer Trigger Valve Gasket

If your spray wand leaks continuously, it may be this part that is broken. You can buy a new trigger assembly online for $17+ shipping which isn't too bad but I didn't want to wait for it. When you take apart the wand assembly, the portion that has the trigger on it you will be able to see a hexagonal depression with a hole in the middle down the tube. This takes a 9.5mm hex wrench (I include a print here that works) and unthreads to bring out a cap. The cap when it is threaded down compresses a part that is made of some kind of rubber to create a seal to the sides of the tube. Depressing the trigger moves a metallic cone that normally pushes into the rubber part to plug the hole in the middle which allows what you're spraying to pass through. On my unit, this rubber part had degraded and had a crack all the way through it which allowed the wand to leak. 

This replacement part allows 3 rubber O-rings to be used to recreate those seals and replaces the rubber part.  When I tried to remove the rubber part it broke into many pieces.

I printed this at 0.12 layer height and used 100% infill with 4 walls, 5 top and bottom and tree supports in ABS. Have the seat for the small O-ring side facing up. It looks a bit rough coming out using a 0.4 nozzle, if you used a 0.2 it would look a lot better but is still functional. The O-rings are one 5x2 mm and two 10x2 mm. I used some silicon O-ring grease when seating them but I don't think its required. A bit of cooking oil will help with installation back into the spray trigger body just as well I think.

Put the part with the three O-rings seated into the trigger body with the small O-ring going in first. Use the cap with the hexagon to chase it down the tube and depress the trigger to get it down a ways. Let go of the trigger and the spring loaded cone inside should push the part back out a little. Use the 9.5mm printed hex wrench to start tightening down the insides of trigger tube.  Put some water in the sprayer and pump it up. Adjust the trigger so it has enough movement to work properly while also preventing leakage when not depressed by threading the internal cap in and out until you find the correct balance.