Low Pressure with no hose attached, pressure is good, attach hose without wand and pressure drops 50%. Have had pump apart 10 times, have checked bypass valve, have even removed bypass and filled passage way so water could not bypass wihout any change. Am down to thinking pump casting is cracked allowing bypassing to happen. Does each of the three piston chambers have a check valve so one piston is not pumping back into another piston chamber?

I worked at a tool rental while attending college and had the opportunity to work on a variety of pressure washers, however, they were of the commercial type. Commercial washers are slightly different in that they usually have seperated components for easier service. Attached to the pump by fittings or rubber hose were the pressure regulator (which has the bypass unloader inside), chemical injector, pressure gauge as well as the hot water release safety all external to pump. Your pump has all these components in one case. By far the most troublesome component is the pressure regulator/unloader which when failing will result in low pressure because it will bypass water back into the intake as you suspect is happening to yours. Here are a few things to check. I believe that your pump is 2500 2.4 gpm. Test gpm water capacity by running pump normally with hose and proper tip attached. Spray water into bucket for one minute and verify that you are pumping 2.4 gallons of water (or what ever gpm your machine is rated). Measure the amount of water that your machine delivered. If it delivered proper gpm and pressure is low then problem could be worn or improper tip. The pressure is created at the tip and pressure will drop as the tip wears. As the unloader/bypass wears, water will bypass at a lower pressure. (Try lowering the pressure regulator all the way ) If water measures less than rated delivery than the machine is bypassing water due to worn regulator/bypass, checkvalves pitted, oring in front of checkvalve worn or cut or missing, or body of pump where checkvalve/oring seat is pitted allowing water to leak past valve. The cap that is removed to access checkvalve must presses against checkvalve with enough force to prevent water from leaking thru oring and body of pump. Examine checkvalves and orings with maginfy lens and look for pitting. Valves surface and orings must be very smooth and tight fitting to work properly. Body of pump where checkvalve oring snuggs against must be very smooth. Any surface where oring seats must be smooth. As water bypasses it cuts metal and orings just like a leaking faucet so look for cuts in metal and rubber. This info and my experience is better suited for larger pumps and higher pressure but maybe it will help. Good luck with your repair.