91 Galant auto transmission problem

I have a base model 91 Mitsubishi Galant with auto trans. It has a sohc 2.0 liter engine with about 130,000 miles on it. The car is in good condition other than the transmission problems. I have tried to keep the maintenance up on the car over the 6 or 7 years that I have had it. I even kept the fluid and filter changed in the transmission according to the maintenance schedule in the owners manual, but it still went out. I ordered another transmission online awhile back and installed it myself. It turned out to be not all that hard of a job. I even change the filter before I added factory fluid to the recommended level, but when I started the car and went through the gear selection, nothing happened. I have the front of the car off the ground so that the wheels can spin freely, but they don't move in any gear including reverse. Also, the fluid does not go down or move at all when the car is in gear and the fluid is viewed through the fill hole. After I called and talked to the guy that I bought it from and his mechanic, they told me that I broke the pump when I put the torque converter in, because I probably pushed it in to far. They also told me that this was quit common with Mitsubishi transmission. I want to know if this is true and if it is possible for me to just replace the pump myself and about how much should this cost? Thanks, in advance for any help.