yes, extra pedal effort needed- it works but takes too much compared to others ive driven.

no booster on these trucks, all manual, put new wheel cylinders and new shoes a few days ago, old ones appeared oil glazed(seemed likely axle seals, but they looked dry- maybe they just didnt clean the stuff or spilled a little axle oil), both wheel cylinders were starting to seep into the boots, one boot had just started seeping into the brakes(thing sat abandoned outside 2 yrs, surprised wasnt even worse)...anyways all new, cleaned,adjusted, bled, bleeding the inch cyl was a hassle but brakes were easy to bleed.

everythings good now, just takes more pedal effort than i think it should...as the only remaining old part is the brake master, went ahead and ordered a 3/4"(still betting the wrong one was put on long ago)

other projects to get done around the house before winter, doubt i'll get back after it for a few weeks- its fine till whenever...agree this is one not so great design

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.

hard to find info... but, found(according to the internet...) that:
part number 2779923 = 15/16 bore (~.69 sq.in. piston area)
part number 2812896 = 3/4 bore (~.44 sq.in. piston area)

both numbers were found as replacements for a GPX truck

so *if* its got the bigger one, changing to the 3/4 will give ~56% increased PSI at same pedal effort...sounds likely the issue think i'll order one :)