You say you have a heavy pedal? I'm assuming you mean your stopping power is not sufficent and have to press extra hard on the pedal to get the truck to stop?

before you go putting the wrong master cylinder on the machine have you checked the following?
- check for external leaks in the braking circuit (wheel cylinders included)
- check the booster assy for internal leaking (if equipt with a booster)
- Pedal freeplay is set correctly
- sticking components or fluid blockage in brake circuit (lines or junction blocks, etc)
- is the correct fluid in the system?

These machines have the vertical cylinders mounted underneath the dash between the inching and brake pedals if i'm not mistaken and according to the diagrams i'm looking at.
If this is true to your machine i can say they can be very difficult to bleed out and setup. IMO one of the worst designs clark came out with ever!
All i can say is good luck sorting this out, it will take some time and you'll probably need a pressure bleeder to bleed the system correctly.
Make certain you do not have fluid leaking into the brake chambers in the wheels, wet shoes is a common problem that cause poor braking issues.