Old Simpson unloader valve issues

[84prerunner]

I acquired an old 1978 simpson pressure washer. Belt driven unit with a HF 6.5hp engine on it. Issue i am having is with the unloader. I took it apart, inspected everything, replaced a couple seals and put it back together. it no longer blows water everywhere, but it gets "stuck" in the unloaded position every time it unloads.
What i mean is, i start the unit and watch the pressure gauge climb to about 2500psi (within a second or 2) then it unloads and drops to about 1200, i pull the trigger and it stays at about 1200psi. If i shut the unit off, pull the trigger and start the unit with the trigger pulled i can slowly up the engine RPM and watch the PSI climb all the way 2500psi and run it all day long there AS LONG as i dont let off the trigger. if i let off the trigger, it goes into unload and the gauge drops to 1200 and stays there no matter what i do until i shut the engine off again. Ive taken apart the valve and scratched my head a bunch, i have 1 idea but looking to see if anyone else has experienced this.

 

[earthmover1980]

Put a stand alone unloader valve on it. They sell em.

 

[84prerunner]

This one is a stand alone unloader. At this point it's hard to justify spending $150ish on an unloader when the pump is 40yrs old and in unknown condition. Already spent $100 on a hf engine for it.