how tight is the alt belt? which motor is fitted? SN?
the gp40 is a different series/size/frame of truck than the FGC25K that L1ftmech had. there was actually a PSB, with PN for shims, if I recall, about the problem he was having, but this truck may have a pump mounted to the back of the alt?

Brake drums have a maximal diameter for worn. If this diameter is exceeded shoes don't make contact, or poor contact.
Have you perform adjustment of shoes (from behind brake plate) after installing drums?
Drum brakes on cars work ONLY by the same principle; industrial machines are smth else.

Just went though the same problem. Sounds like you have contaminated brake shoe linings.Did you happen to get dirty hands on shoe linings or drums (grease,oil or dirt ect.)? If you have bled all air from system and pre adjusted brakes where the pedal is hard and the emergency brake handle feels like it has correct resistance there is no thing else it can be. If all this has been performed pull the brake adjuster grumets out of backing plate and blow a little baby powder in the brakes and test. An old timer had taught me this trick and has worked for me in the same situation. I know it sounds crazy but baby powder has talcum in it wich will absorb the contaminates. hopefully this will help you out.

Good job- L1ftmech- sounds like a ton of work. How far out of spec were the frame extensions when you started?

Do you work for a nationwide trucking outfit by chance? I had a customer in that field & man were they hard on their lifts! Some of the things they did were unbelieveable. For a while they had Nissan CPF02's that held up the best (IMHO). They just seen to destroy the newer stuff

I'm not familiar with this Caterpillar forklift, but we had a Mitsubishi FGC25K that had the same (or similar) complaint and symptoms.
The truck was shipped to us from another terminal because a local vendor at that small location had been working on the truck for a couple of months and still couldn't get any braking power.
The brake pedal was very hard......too hard actually.
And if you tried to move the truck, it acted as though the brakes were dragging (they were, but not in the normal way).
When I began working on this truck I noticed that it seemed like the wheel bearings were adjusted too tight or something because you simply could not turn the drum by hand without a pry bar.
Pulling the drums off it was apparent that brand new shoes were in place, and showed signs of just barely having made contact with the drum at the top of each shoe. But the brake drum showed a groove worn into it by the side edge of the brake shoe metal table, not the brake lining material. My first thought then was that these shoes were too wide for the application.
Our local Mitsubishi/Caterpillar dealer thought so too until we laid one of the suspect shoes next to one of his brand new shoes and they were an exact match.
Back to the drawing board.
It was determined (by painting the edges of the shoe tables) that in fact the shoes were binding into the brake drum where the drum vertical wall meets the brake shoe plane.
I finally pinpointed the cause of this to be the frame had been bent (spread apart actually) by what I imagine was a very hard impact to one of the drive wheels by another forklift counterweight at some point in the past.
The drive axle is mounted as a free floating member on the axle mounting trunnion brackets. The brackets are bolted to the frame extension flanges but the axle is free to float inside the brackets. However, the brake backing plates along with the shoes are bolted onto the axle mounting trunnion brackets.
The mast rides on the axle in the standard fashion and mounts up with trunnion caps.
What happened in the impact that caused the brake shoes to dig into the sides of the brake drum is explained here.
Because the drive axle floats inside its brackets, the side-on impact on the drive wheel drove the axle assembly toward its stops on the OPPOSITE side of the truck, and this caused the frame extension flanges to bend outward on that side.
Because the axle trunnion brackets are bolted to these extension flanges, when the frame spread wider, it removed all of the side clearance between the shoe tables and the drum because the brackets, backing plates, and shoes all moved closer to the drums.
After that, the shoes were "clamped" between the brake drum side walls and the brake backing plates.
You may not be happy to learn what we had to do to repair the truck because we actually straightened the frame extension flanges.
The process included having 2 large plates made in a machine shop out of 1' thick steel plate. On one edge of each plate we had the mounting bolt hole pattern of the axle trunnion brackets drilled so the plates could be attached to the truck.
Opposite edge of plates I had the machine shop put 3 holes of 1-1/4" diameter.
These 1-1/4" holes were to accept 3 all thread structural steel rods 3 feet long and 1" in diameter to "pull" the frame extensions back into proper alignment. The "oversized" 1-1/4" holes allowed for the 1" rods to fit into the pulling plates despite the 2 plates not being parallel to each other, and to allow for the plates to deflect when pulling without binding on the threaded rods during the process. I had to fabricate an adjustable "jack-brace" in our shop to brace the frame beyond the extensions area so the pulling forces would not bend or distort the frame in the steering axle mounting area at the rear of the truck.
Oh, that might sound like it was too easy.
In order to do all this...........the counterweight was removed to lighten the overall mass, then the mast had to come off and we removed the drive axle/transmission as an assembly leaving the engine in place suspended by a chain from the overhead guard. Then we positioned the remaining carcass sideways across our drive-on forklift work rack (considering it to be a super work table at this time) with the front of the frame at an easy height to work on.
It took me and a helper about 4 hours to do the straightening (after positioning it on the rack) because we had to make a pull, and then back off all the nuts on the puller rods to measure our progress using a large "purpose built calipers).
Proper placement of the adjustable jack brace was also key to achieving an accurate "pull" to the exact point you desired.
We were aiming for a finished job that would leave about 1/8" to 3/16" clearance (1/16" to 3/32" per side) between the drive axle mounting brackets and their mating stop ledges on the axle housing itself. We were happy to have it turn out at 3/16".
And after reassembling the truck it had good brakes.
Anyone wanting pics of this can e-mail me.

i did rough up the drums. and the backing plates are fine. the shoes move as they should. should i be able to feel a difference in the pedal with the engine running as opposed to not running like in a car? because i can't. thus i swapped out the booster but still no diff. from engine off to engine on. the shoes came from the dealer but is it possible that the linings are harder than spec?

did you rough up the drum surfaces before reinstalling them?
in some cases having them turned and trued up a bit helps with that.
what sort of condition were the backing plates? those were notorious for wearing a pattern in the backing plate and that would actually keep the shoes from moving correctly. if the backing plate has ANY indention worn in it that would probably be the cause of your shoes hanging up creating the no brake power situation.

couple of things to ponder while scratching your head ;o)