Depending on the Karcher model, there may be several reasons:
1) most likely - the relief valve on models k 720 and k 520 blocked
tip: remove, lubricate with water silicone (not to be confused with construction silicone) and put it back on if the O-Rings on it are whole and not used
2) pressure valves are to be replaced, i.e. their seats or plates (made of Teflon) are scratched, regeneration is not an option for about PLN 17, you should buy new ones
3) as mentioned before, sometimes only impurities between the valves prevent pressure build-up (valves do not close fully)
4) check high-pressure seals (black rings in the pump head where the pistons "run") and the pistons themselves for wear
5) check for leaks at the pump or in the section of high pressure hose with a gun
6) A rinsed lance nozzle can also be the cause
PS Regarding oil, it is best to make bz silicone grease, but it can also be engine oil (better quality)
If you have any questions, please write: geex (at) o2.pl - topic: Karcher serwis