904 tranny problems

Not sure what's going on but I just got my car running and just put fresh transmission fluid in and I can't get power out of my car I push the gas and it seems like something is holding me back I have to push down on the petal pretty hard for it to move. Could it possible be the brakes someone help I just wanna cruise already. oh its a 1973 318 with a 904 tranny I'm putting in a 59 Plymouth belvedere