The problem you are having (I think) with a loss of pressure, when the trigger is pressed, is the same as one I had a good while back, on one of my other machines.
Pump & it's a piston/s, so it'll keep the pressure until it tries to push the water out.
Mine was on an Annovi Reverbi pump and it was easier to just replace the pump.
This is the spec of yours -
https://equipmentsuperstore.co.uk/cleaning/pressure-washer/diesel-pressure-washer/kiam-km3600dx-plus-industrial-diesel-pressure-washer-10hp-with-30m-hose-reel.htmlIt's a 3 piston pump, no branded name but at least it has 'German seals'.
If you buy this spec of machine, which is a brand - no disrespect, Like Loncin, Wolf & others they are virtually throwaway.
I'm very aware that budget is a deciding factor when people buy machines but that current price has had £1000.00 knocked off the price and they're still making money selling it.
You could have had a -
https://www.rutlandpumps.com/watertek-petrol-pressure-washers.html for £1300.00.
If I knacker an engine, pump or other part on mine it can easily be replaced.
Possible you could get a pump replacement but faffing around with non brand is downtime.
Two options -
Phone Julian @ Britclean (Biddulph, Staffordshire) to see if he can do anything with it (other repair shops are available but I trust Julian and he's repaired, serviced & replaced my machines for 15 years)
Bin it and go with one of the suggestions.