sounds like contaminated or wrong fluid
like tom said, these trucks used DOT3 which is yellowish to clear depending on the brand.

you probably should go through the whole brake system
if someone did put transmission fluid in it then it will have to be drained, lines cleaned, wheel and master cylinders replaced.
If it has a slave cylinder for the inching that too.

Brake fluid only (DOT3), if the system was filled with oil of any kind you may have swollen or damaged Master and wheel cylinder seals.