Large truck clutches actually are two clutches. One is used when the vehicle is not moving - select gear to go forward/reverse and the pedel is depressed all the way to the floor. The other clutch is used for shifting - the clutch pedel does not go all the way down to the floor -if it does, you are wearing out one part of the clutch. Only takes 1/3 - 1/2 depression of the pedal to operate the clutch to shift gears. Takes someone with knowledge of these beasts to properly adjust them. I'm sure once you have your truck serviced, they will know what the problem is.
There are also lub points and oil levels that are important.
my $.02
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