I have a briggs gas power sprayer - new find (See label of sprayer pump)

Problem is it builds too high pressure from engine side that the engine stops. It only runs a split second when it starts and the water pressure goes up. It is very hard to pull start at this point until i let the pressurized water out via the sprayer "gun". What do i check in this pump that would "release" the shaft so the engine can spin freely(thus not stall) when pressure is high enough?

TIA.