What does it do if you let the clutch out suddenly, does that kill it, or does it pull down a lot then recover? When you jam the brakes in 4th you are pretty effectively locking up the drive train, especially if your disc brakes are working correctly. The engine is not going to be able to override the brakes if applied fully, unless you lift your foot in time for them to release before your engine stops. Remember, you are operating a tractor that weighs approximately 3500 pounds, and a Super H engine unless modified produces around 40hp. Once you get it stopped or nearly stopped moving, that is a lot of inertia to overcome with an engine that may be down to very low rpm (also very low hp at that point) when the brakes release. Plus, I do not think the disk brakes with the ball and slot system they used release very quickly.