I bought a new trailer and when I went to pick it up the trailer plug was the small type and my truck had the large plug. So I bought the adapter and put it on and thought all was well but the trailers brakes stayed locked when we tried to drive off and we finally ended up disconnecting the brake wire to be able to pull it home. does anyone know what would cause the brakes to stay locked on the trailer? I have not pulled a trailer in over a year now so have not had to use the brake box,if that matters at all. Thanks!

Do away with that small plug and adapter. Get a trailer plug that matches the truck and use the wiring diagram below to wire it as it should be......

http://www.horsetrailerworld.com/home/wiring.asp

I agree, Just replace what ever is on the trailer with the newer 7 pin RV plug.  This is the standard that all trucks are coming with for the last 10 years.  Your connection will be much more universal and you won't have to worry about finding adapters.