What does the EPC Limit and Converter relief valve do?

[skepsisxx]

My car was lagging in First Second and third gears, one day i stopped and it just wouldn't go. After digging around I read that its possibly the valve body gasket. I replace the valve body gasket and put it back up and all of a sudden I have no reverse. After more reading on these forums I realize i'm missing the metal EPC limit valve but the converter relief valve is still there. I later found it in between the gasket and the valve body. I'm going to put the valve body back up to see what that resolves but now for curiosity Id like to know what it does? anyone that can let me know?

also if this doesn't fix it, any ideas to fix forward and reverse without going into the transmission