Good Morning,
I would start by replacing the high pressure ball valve which is leaking - a as you have 250 Bar of pressure on a WW965 I would go for a 500 Bar rated valve £15.30, once this is change you should loose the cavitation and see an increase in pressure.
It is always worth while replacing the lance jet in your machine when getting it ready for use as the jet size will have increased with wear giving a reduced pressure you should be using an 04/045 jet.
Servicing wise change the engine oil to any good SAE 30-40 motor oil, the same oil can then be used to replace that in the pump. When checking your pump oil it should be semi-clear but if well used a brown / black colour is fine. If the appearance of the oil is milk and thin then the chances are that your oil seals will need replacing, the colour and consistency of the oil is due to water leaking past the seals around the piston bases whilst under pressure. Replace oil every 4-6 months when in use in the pump engine twice per year. With direct drive pumps it is always worth undoing the bolts which hold the pump to the engine and checking / replacing the key steel as wear here will damage both the pump shaft and that of the engine, when reassembling pop some copper slip on the shaft to ease wear and heating.
It is also worth taking off your inlet filter bowl and checking for blockage or limescale build up which could potentially block valves if broken up and passed through the pump. if you find that the machine is still cavatating and they have been sat for some time it would be worth removing the three large caps along the base of the pump and removing the three caged valves checking that the springs are easily free and moving as these can stick when not used for prolonged periods.
Couplings wise, quick release is not the best system in the world once it has been left to sit, as you will usually find that the o-rings have stiffened up and shrunk allowing either water to leak or for air to be drawn in which is another potential cause of cavitation. A good stainless pair of quick release couplings on the high pressure side costs around £14-15 low pressure quick release on the inlet and return to tank are usually £7-9 each.
Once all that is set-up and working give it a good test turning the unloader valve down to minimum and back up to maximum to check operation.
Hope some of this helps...
Ben