Why wont pressure washer keep pressure?

There is no single simple answer. Your manual, or a manual from your manufacturer's website will give you some troubleshooting suggestions. I can detail a list of the most likely problems, but you are not likely to understand or be able to act upon the information without extensive step by step information. And, I don't even know which model and pump you have. Several different pump designs may have some problems more likely then others. Check the nozzle on the end of the wand to make sure it is not clogged. Check that your hose delivers a proper water flow. Most other problems require significant pump knowledge... the unloader can be stuck, the valves can be damaged or full of trash, the key-way could be sheared, the oil end could be "fried"..ie bad bearings, broke pistons or retainers. Seals are not usually the cause of a complete lack of pressure..especially if there is no leaking water.