Any John Bean sprayer experts here?

Just reaching out for ideas from anyone familiar with John Bean pumps. At my job we have a tree sprayer used only with a gun on a hose reel. I guess they've had it for 5 years with no issues. Las t fall it seemed to lose some capacity. It is a R20 four piston pump with a 11hp honda all truck mounted. A while ago I replace all the valves, seats, and springs in the head. It would only develop maybe 200 psi of what should be an available 500psi. So I put new cups in the pistons and the ceramic cylinders looked good. I did find a few small suction leaks on some sloppy plumbing from the tank. I redid all the plumbing joints from the tank and even upsized the ball type shutoff from the tank. I blew backwards in suction pipe with air and powerwasher, cleaned the tank, cleaned the filter screen. I had the head apart twice to make sure i did the valves and seats correctly. I had it doing 400 psi friday, but opening the gun to spray would give good spray and then drop off to 100 to 125. The operator of the unit says it always was able to maintain pressure. They used it saturday and and by 2pm it was not hardly pumping at all, almost 0 psi. How come something so simple is kicking my tail? I have been over this and over this. good supply, good rpms, tight belts, good connections. What am I missing. I havent been back to shop yet, I will look at it tomorrow.

Check the pressure releif . Depending on the type there is a leather and seat in there . Sounds like it not seating and will loose pressure .

Thats my first stop tomorrow. But i didnt mention that it has a hardened steel relief valve and seat and i put a new valve in thats seating squarely on the seat.

I have one also maybe a r 2020.. not sure with out looking.. if the same style head and valve assembly leave a bit to be desired as design. The flat type valves seem to me more problematic then the old style steel ball .. I use mine to spray whitewash.. highly abrasive on the cylinders and piston cups .. also did you change all the valves and seats . There is a few in pretty deep . Special toll was sent when I changed mine .

Years ago we had John Bean orchard sprayers with those types of Bean piston pumps. We had a fit with them. I had them out 3 times a year for rebuilds, cups, valves and springs not to mention ceramic cylinders, on and on. Bean made a lot of good stuff but this model of pumps was not one of their better ideas. Everything has to be right to build pressure and of course maximum volume is also an issue. I would suggest you put in everything new, cylinders, cups, valves and springs and check the regulator. Before you do that you might consider a good quality 2-stage centrifugal pump replacement if you can live with 150-200 psi.

Just a quick thought. Check the spray nozzle, was the size changed by operator or wear changing size. Pumped volume up = pressure down after reaching design capacity.