F-150 transmissions are very tough--I've seen people really neglect them and they keep on going. You may have just had a boil over situation, where you were hauling a large load in the mountains in the summer heat. The rubber plug is for access to the drain plug in the torque converter. There may also be a breather or overflow tube under the bell housing.
I would drain and clean the pan, change the filter, drain the torque converter, and refill with Mobil 1 Synthetic Multi-Vehicle ATF. It may allow the transmission to run up to 50 degrees F. cooler, which is huge. Here's an even better alternative approach to changing the fluid:
http://www.askmehelpdesk.com/cars-tr...tml#post552241
You should notice a substantial improvement in performance and it should eliminate boil over. I would also install, if you don't have one, a supplemental transmission cooler--easy job.
Linear Mode
