check valve
The check valve in your truck is in the rear fitting in the heat exchanger. Remove the line that runs from front of trans to rear of exchanger, remove fitting from exchanger, push on side of little sheet metal retainer in fitting to cock it, catch edge with little screwdriver, pry it out, spring and ball will fall out, reinstall fitting and line. You will lose some fluid doing this so be prepared. The side effect could be some converter drainback overnight, the plus side is less restricted cooler flow. If you don't have a modified valvebody that fills the converter in park, then just start truck and let run in neutral for 30 seconds to a minute before trying to drive away and you'll not notice the drainback. The benefits outweigh the side effects. I have seen these balls stick and cause melt down of a trans. My. 02. HTH
James