I have found several complaints on-line by people who say their Karcher pressure washer pulses and leaks when the trigger is off. I have even found videos posted on YouTube demonstrating the problem, one of which is posted below.
The consensus of opinion seems to be that the problem lies in internal cracks. Some people have found hairline cracks in the plastic pump cylinder head Karcher fit to many of their power washers.
If you fancy stripping down your Karcher pressure washer to see if you can find the problem and fix it yourself the second video below should be useful. It shows a step by step guide to dismantling a Karcher washer.
Spares for pressure washers from Karcher and other manufacturers such as Nilfisk and Bosch are available by mail order from BuySpares.
Karcher Pressure Washer Pulsing and Leaking
How to Disassemble and Reassemble the Pump on a Karcher Electric Pressure Washer