I have a 1991 CTD that just started to start hard and notticed that the pump is leaking at the back of the pump at the bottom. I have did some searching and sounds like there is a O-Ring that needs replaced. Is this something that a I could do or should I take it to a inj pump shop???? I know I will have r&r the pump, but I am not shure on how to change that oring. Thanks Justin