Hi,
There is normally two things that can cause this problem (depending on model no). There is a spill valve (http://www.candmdomesticappliances.co.uk/product.php/1970/483/spill-valve-to-fit-karcher-jet-pressure-washers) this sometimes gets stuck and the spring loaded mechanism dosen't work correctly. The other thing that can cause this problem is inside the black plastic outlet elbow there is a small red & grey coloured bottle shaped piece, If this dosen't hold the pressure it can make the body keeps pulsating.
if you have any problems you can contact for advise or spares.
www.candmdomesticappliances.co.uk
paul
April 2009