I have a Karcher pump with a Honda gas engine (Costco) that has a similar problem. The pressure comes and goes - it usually starts out good but I have to keep squeezing the trigger because if I letup it will never presssure up again.

I called a rebuild shop locally and they tell me that there is a 'spool valve' that goes out that should be rebuilt/replaced - they quoted me ~ $60. I've been limping along for a while but I'll need to do it eventually.

Call your local shop and see what they say about the service/rebuild.

Good luck -
James

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K