Sprayed last night and had no issues. (???) The pump primed immediately.Where you describe your normal start up procedure: I question if step 2 is causing some issues. With water in the tank, and above the pump, you should be able to start the PTO pump and it should prime. It will (and should) send all the fluid back to the tank via the by pass valve with the spray boom off. The by pass does two functions: you need a certain volume of spray to satisfy the volume and pressure requirements of the spray tips at a certain speed. The pump will (or should) be capable of putting out more than whatever that requirement is, so it has to go somewhere. Hence- the by pass. Sends it back to the tank. Second function of the by pass is to agitate the spray mix. Some stuff mixes and stays mixed pretty good. Others want to turn to cottage cheese. You need good agitation either way. The pressure gauge should read pressure (I tried to trace the plumbing, but I'm 99% that's factual) whenever the pump is running. No matter if its by passing into the tank or spraying via the boom. You need to be able to see the gauge easily from the seat of the tractor. If you can't, either plumb another gauge into the system and put it where you can see it, or relocate the one you have. When your spray solution is almost out, and the by pass is still on, the pump can start to cavitate (or, "starve"). Your gauge will start jumping around erratically. You need to either shut the pump off or , if your goal is to empty the spray solution 100%, shut the by pass off. Otherwise, even though you might think you didn't run the pump dry, you may in fact momentarily done just that. Potentially causing air pockets, or pump damage. A gauge is your friend here. You asked about brass fittings. DON"T DO IT! The poly fittings are generally impervious to most any chemicals you'll use. Use poly anytime you change plumbing.
Thank you for you explanation of the bypass system. Didn't occur to me that fluid is still going through the bypass when the booms were open. But, that does make sense now.
For emptying the tank, I have been running it until I see that the fluid coming out of the boom nozzles starts to sputter. Then I stop the PTO and return to the barn. I add a few gallons of water and run it again until it sputters. I repeat this once more to further dilute any pesticide/herbicide. There is always residual fluid in the tank, so i'm not sure how to empty that. As you pointed out, don't want to run the pump dry. There isn't a way to shut off the bypass completely as far as I can tell. There is a valve on the bottom of the tank that shuts off flow coming out of the tank to the pump. I guess that is there so you can disconnect the pump and not have the tank fluid pour out.
The pump is all stainless and the sprayer is stored in a heated barn during cool months. Would you still store it with RV antifreeze, and why?
Ordering those Banjo fittings today!
Thank you for your advice!