Brake Master Cylinder

Drove truck home from work with no problems, parked it until the next day. In the morning, the brake pedal goes all the way to the floor board and the light comes on. The MC reservoir was dam near empty with absolutely no signs of external leaks anywhere. Refilled the MC,the light goes off but the pedal is still low and mushy. MC leaking into PB???

Probably.

Remove the bolts holding the master to the booster and pull it away from the booster and have a look. if the master is leaking that much in such a short time, you should get a river of fluid from between the booster and the master.
Might also want to look under the truck, OEM brake lines seem to disintegrate pretty quickly these days, and bad brake lines seem to run in the Dodge truck line pretty often. Gene