Gravely Sprayer Question

I have a tank sprayer for my Gravely. It had a Hypro piston pump that sort of worked but leaked and was totally rusted. So I bought a new Hypro pump to replace it. I replaced the lines and added a Hypro pulse damper (precharged to 800 psi)on the output. The new pump is rated 500 psi and 2gpm. The pump will quickly pump up to 500 psi and when I pull the spray trigger I get a great spay pattern but the pressure immediately falls to zero. I thought I might have received a bad pump so I borrowed another and installed it with the same result. I am at wits end to know what is going on. Would appreciate comments. Thanks