You remove the black cap which has the silver button by pulling it up and off. It'll resist but you won't break anything. Save them. Turn off the water.

Then get a T-30 star bit and a ratchet wrench that it will fit into and unscrew the two screws on top. Do this carefully and slowly and after taking one out, you'll see how much it takes to remove the other. There's a spring and little teeny valve gadget that'll fly out if you don't hold on to everything as you remove the second screw.

Remove the white plastic gizmo and the screws and the spring and the valvy thing. Pay attention to how they come out since you have to replace them the same way.

Now take a dishcloth or large rag and cover the hole that you see. Then turn the water back on and start the engine just for a second or two. The water pressure will blow out the piston. Turn off the engine and turn off the water.

Now you'll have the piston in the rag and you'll see it's probably got some 'crud' on it. That's the hard water calcium or lime or whatever that builds up after a while. That's what's making the piston stick in the cylinder. So carefully scrape that off (don't score or scratch the cylinder, use a nylon scrubbing pad that you get in the supermarket at the cleaning supplies area) and use some CLR (it's a product which removes calcium, lime and rust). Wear kitchen rubber gloves while doing this because it's a harsh chemical.)

Now, the really important part is this: go to your local hardware store and get some Silicone Grease. It comes in a little plastic can. And get a brush that'll fit in the cylinder BECAUSE THE CYLINDER HAS THE SAME CRUD ON IT!.

Pour a very little CLR into the cylinder and use the brush vigorously to clean out the cylinder. Use the nylon pad and CLR to clean the piston vigorously until there's no more crud visible.

Now here's where a disposable pair of surgical gloves would come in handy: put them on before you open the Silicone Grease. It's goopy and very sticky and very very hard to remove even with soap and water. Smoosh some onto the piston on its sides. The ends are not important to grease.

Then plunk the piston back into the cylinder. Then remove the disposable gloves and your hands won't be goopy. Put the spring on top of the piston and then the valvy thing into the spring with the brass part sticking UP.

Then put the white plastic retaining thingie back on over it and hand tighten the screws. Then use the T-30 star bit and the ratchet wrench to tighten the screws. Don't overtighten and break the plastic. Then put the black cap on and press on the silver retainer.

Turn the water back on and then start your engine. It make take a short while for the air in the system to blow out but pretty soon you'll have full pressure again in your Karcher 2400 which has this lovely design flaw.