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Could be the unloader valve but you could also have air in the line. You should always run the water through the pump, hose, wand and nozzle until all the air is purged first before starting the engine. The pump can't deal with air in the water, It can give you this sort of trouble. good luck.
The AR pumps used on the RD80746 are notorious for deteriorated o-rings in the unloader valve. Especially if bleach or high-alkaline detergents have been used with the injector. Bleach or high-alkaline detergents can also cause corrosion to clog the outlet orifice. Take it to your local ASC unless you or someone you know is very familiar with pressure pumps.
"HONK if you've never seen a gun fired from a moving Harley"