I got a used Lesco 200 gal. skid sprayer with a Hypro D30 pump recently. The pump doesn't build pressure unless I close the agitation valve. Then it's got all the pressure in the world. The pump has the regulator with the handle and 4 settings. I've tried all 4 settings, as well as turned the brass adjusting knob. It doesn't change the pressure until I close the agitator line. Do I need a new regulator? I just changed the diaphragms and valves today. It didn't help. They were pretty nasty, though. Thanks for any help.