Hope for Pressure Washer?

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I have an old Advance Model 600 pressure washer that recently quit pressurizing. I only use it for blasting grease and paint off of old tractors, and it"s great for that. But now, water runs thru, the pump turns, but no pressure. The unloader valve is an internal type . . . it fits inside the casing with the three pump pistons (far right side). It appears the the unloader valve isn"t closing . . . the plug (fits against seat) is badly rusted and no longer seals, so the high pressure water just flows back to the low pressure inlet. Problem is that when I called Advance I discovered they don"t offer replacement parts for this old model. Has anyone repaired an old pressure washer? I thought it might be possible to plumb an external valve to do the same function. But I"d have to get the right pressure rating, and also plug up the passage where the original internal unloader valve used to be. Any suggestions or experience would be welcomed.

Thanks Willy, but I did get it out and apart. This photo isn"t very clear, but the plug (on left)of the valve stem is really corroded, and looks like it won"t seal against the seat on the other (right) side.