I didn't read you long message, so you may have covered this point. If you stored your pressure washer in freezing temperatures that could ruin the pump/valves. I've done it myself. Don't think you can drain all the water out and just put them in a shed over winter in the Northern climate.

I think you need to replace Valve Kit 2.883.862.0. You should be able to find on internet if you Google that number. Here is a link that might be useful: Detergent Injector Valve

But, if the housing were cracked, it would be obvious (I think)....internal leak ??? IMO, due to high labor costs, cost effectiveness flies out the window.. And this is a primary reason for the workshop and garage to be in the basement , which should be huge... One cannot beat "free" heat.

make sure everything is clean;the inlet screen etc. Take a flashlight and look inside were the hose connects and make sure nothing is inside.To do a simple test;connect the hose,turn on the water,and WITHOUT starting the pressure washer,squeeze the trigger on the gun;you should have a steady stream of water coming out,about the same as a kids water gun;like a super soaker;If not,then your problem is something is restricting the water inlet,or if the washer is old,mineral deposits inside a passage.If it does squirt out pretty good,then your washer is tired,or you used a cleaner that's NOT compatible with pressure washer seals,or maybe the unloader is stuck,and won't allow the washer to build pressure...........If it's getting a good supply of water,the only thing that builds pressure is the seals,and the unloader.Other than that,there is a small passage were the outlet is,and that could be restricted with mineral deposits,but I would take apart the hose and check for debris in the line,to be sure Hope this helps.....marc