The EPR is a evaporator pressure regulator. If you have them on the suction lines there will be a dust cap you remove to access a allen screw. Adjusting will regulate the evaporator outlet pressure.
I dumped the 404 that was in it and pulled a good vacume and cleared the sight glass with new refrigerant. Im still getting an iced up line including the compressor but at least ive eliminated moisture being a problem. Im running 25 on the low side and 250 on the high side. Ive got a 10 degree line at the evaporator outlet and its frosted over the whole line. Still I cannot get the box temp below 30 in any of the 3 evaps. Im down to either having to adjust these expansion valves or finding this EPR valve you are speaking of. Do these types of valves go out of adjustments often??? Do you know if all systems of this type has this valve installed on them? I could not go further with this today because I had to go to a different restaurant and replace a condensing unit on another drawer system...lucky me.