Most of the time when this occurs you have to replace the entire pump.
These are very inexpensively built pumps and usually will cost more to repair them that replacing them.
What usually happens is that customers tend to put the pumps away without using the pump conditioner liquid in them. The causes the pump to corrode inside and the result is that the pressure relief valve inside the pump gets pitted and won't close. Also the corrosion causes other parts inside to not mesh together correctly.
Here is a website that has the pumpsaver conditioner that must be used before storage.
http://oldmowerparts.com/PUMPSAVER-CONDITIONER-P518522.aspx Also, some of the time you can add this pump conditioner to the pump that doesn't work and it will loosed the valve and other corroded parts inside the pump. this only works about 20% of the time, but it might be worth a try. It would also just be a temoprary fix because once the pump gets corrosion inside it, it usually is on it's last legs.
If this pumpsaver additive is added correctly before storage, you cab usually prevent the problems you are having.
As much as I hate to give this kind of answer, it is usually the correct answer. Buy a new pump. You probably can get the new pump assembly at one of two places. Any good MTD parts dealer.or
http://www.partstree.com/ They are an online parts supplier.
In many cased it might be best to just replace the entire pressure washer.
Wish I had a better answer.
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