pooling water at outlet

Bought my first house this year. LOVE IT. Came with a septic system, which was not a deal breaker for me. The previous owners had it pumped two months before we moved in. Within two weeks, there was about two inches of greywater pooling at the site of the septic tank and the washer wouldn't drain. We had it pumped again, and the guy offered to charge us $600 to 'explore'. Pass.
So we dug the hole, found:
homemade concrete tank, approx. 740 gallons. one leach line, looks like 4" PVC pipe. The water is pooling where the leach line meets the tank, it disappears after several hours of not using any water or it not raining. I flushed the toilet, ran outside and watched it trickle out instead of going into the pipe. The sewage is not backing up into the house. There are no wet spots or greener grass anywhere else on the leach field. We plan to repair this ourselves. Here's my series of HELP ME questions:
one of us thinks it's the septic tank itself, that the sides are corroded and it needs to be replaced, which is probably pretty valid, it's getting to be around 20 years old. I have been thinking about it though, and I think the problem may be that there's a leak where the pipe got displaced at the output after he drove over it with his loaded truck and trailer (whoops). It seems like it would be easier to try to fix this problem before replacing the whole thing and digging up a leach field.
What would I use to seal this? Does that even sound like a plausible explanation and fix, or should we just go ahead and replace the septic tank? I know nothing about this but I don't want to rent a mini excavator, dig up my entire yard, and spend a ton of money if it's a simple fix. Obviously, I will if necessary, but I need some third party opinions and not the kind that are profiting from pumping or fixing my septic tank. Tweet