Mercury 35 hp 3 Ram Tilt/Trim problems

[mojobubba56]

I am having a problem with the pump on my Mercury 3 ram tilt/trim system on my 1988 35 hp outboard. The pump motor went bad so I had to buy a motor/pump assembly. I have burned up the new pump, replaced it with the original pump and burned it up also. I have bought a third. I have properly filled and purged the reservoir each time. The pump works for a few seconds ( not long enough to cycle once) and then quits pumping. Again to clarify the problem is now the hydraulic pump, not the electric motor that operates it. How do I assure that it does happen again?
Thanks, James