See this diagram:
http://www.mastertoolrepair.com/images/DPD3100_pump.jpg?osCsid=1f3m03ijp5lcht6569kran6rk2
Remove Items 122 (Un;loader) and 138 (Thermail relief). Clean them up as best you can with a nylon brush under some slow running water. Dis-assemble the unloader valve completly to clean (both ends). Then lubrcate them with
White Grease and reinstall both parts AFTER placing White grease in the cavity the valves mount into. Very briefly... Test water flow with the engine not running Then start the unit with
water off and let run 30 seconds. Turn off engine. Turn ON water supply and test water flow engine OFF. Note you will get a certain amount of white grease initially that passes through the unit. Start unit and rapidly actuate the trigger 1/2 second on 1/2 second off multiple times to force the unloader valve to cycle.
You can get parts here if either of the valves have completly failed:
http://www.mastertoolrepair.com/dewalt-dpd3100-n001194-pump-p-59405.html
The above steps should get your presure washer working properly again.
Thanks for choosing FixYa,
Kelly