I have a 3600 psi Devilbiss powerwasher. It will start but shuts down within 10-15 seconds. When you try to restart there is a lot of back pressure which will not allow you to start the unit. If the pressure is released at the gun, it will start briefly and stop. I have taken the underloader apart and the piston that slides back and forth does not move freely. Should I be replacing the underloader valve if I can find a match on line?

Yep, sounds reasonable.