When asking such questions, it is a good idea to state the model and serial number.
Based on your wording, I think that you have a tractor that was made after 1985. You are confusing the holding valve end cap with the actual drain/test port.
The true drain port is a small pipe plug that is 1/4" NPT. This plug has a hex shaped HOLE in it that must be thoroughly cleaned out before VERTICALLY inserting the correct Allen Wrench into it all the way to the bottom. This hex hole faces downward to the ground.
Failure to properly clean that hex hole and use a quality Allen wrench that fits tightly will result in damaging the hole and not being able to extract the drain plug.