I found the answer to this somewhere else on this site but now can't locate it. A common problem appears to be that the pressure valve gets gunged up after a while and this needs to be cleaned. This is a plunger valve that switches off the pump motor when the trigger is released. To fix it, take off the handle (two clips), then remove two screws and unclip the blue front panel. Now locate a black plastic box attached to the pump by a nut. The box will have a label on it COD.PREM.TSS and a number.
What you have to undo is the 13 mm nut hold the two together. You need a thin 13 mm spanner to do this (I found loosening it first with a 14 mm spanner helped). Because the nut is between two components it isn't obvious which way to turn it. It screws into the pump head not the box, so unscrew it from the pump head.
Once undone, pull the box away carefully and remove the spring from the plunger.Then withdraw the plunger using long-nosed pliers. Clean up the plunger and rubber grommet and regrease it before reassembling everything.
Greyflannels, June 2018
