Last weekend I put a new trans pan on my truck. I did the usual procedure after refilling it but did not drive it. I just tryed to drive it and in reverse it goes just fine but when I put it in drive and push the gas the truck does not want to move. It seems like the trans is loading up like i was holding the brake. I checked to see if the parking break was holding and it is not. The truck does move but very slow? What could be the problem??