work 3 jobs huh?
when do you find time to sleep or spend time with family and friends? hehe ;o)
i could not imagine working 3 jobs, i worked 2 for quite a few years but i got my fill of it and decided that my day job was enough
cudo's to you man!

Sure the lift is old, and due to the age i think the term 'obsolete' is meant because of its age, the technology in it is outdated and alot of those trucks have been graveyarded or mothballed (besides the ones that get sold off at auctions and sold to the 'little guy') These 'mom and pop' businesses can get by purchasing an old lift like this because it isn't used very much so cost is fairly minimum when it comes to operating costs so i can understand bbforks response, he has made many claims about his dissatisfaction on the way the industry has gone and his hands are tied when it comes to the newer 'computerized' lift trucks.

Anyway to stay on topic here
stuck (or rusted) wheel cylinders have been mentioned
broken hardware (springs etc) which have been deemed not the problem now)
possible master cylinder? hmmmm.... duno bout that one
hydraulic controlled brake system? on a 4FGC25?... dunno bout this one either, never seen that on one of these this small, this old

What i suggest is to verify that the shoes are actually applying pressure to the hubs. Reason i say this is maybe they are glazed over or contaminated by brake fluid or oil? Just not enough friction to grab and hold well? I know from experience dealing with wet shoes that once they get oil or brake fluid in them they are useless, you will NEVER get them back to the condition they need to be to stop the machine properly no matter what you do to them. I have tried many methods to remove the oil or contamination and it just never does get it all out and it eventually comes back to the surface. Ultimately having to replace the shoes in the end.
So make sure those brake shoes are not contaminated, and if they are not then take some sand paper and rough them up and rough up the hub surfaces and make sure to re-wear them back in.
Another thing to pay attention to is the backing plates where the edge of the shoe seats against the raised surfaces of the backing plate, if there are grooves worn in the backing plate this will keep the shoes from moving, basically locking them down, no matter how hard you press the brake pedal they truck will not apply the right pressure to the shoes and the truck will not stop.
So make sure there are no grooves worn in the backing plates.

Slave cylinder? most brake systems do not have one , if your speaking of the cylinder on the transmission (some do have one of those) that is the inching valve that releases the transmission when the brake pedal (usually the left one) is pressed to allow for the transmission to disengage and let the operator feather the drive so he can 'inch' up to his load slowly. The only problem i've ever seen those have is leaking fluid into the transmission, (you keep having to add fluid but don't see where it is going)

And yes sometimes threads here get pushed down to the bottom if alot of time goes by and they get forgotten about. One would have to go digging around in old threads to find these unanswered ones like this one. Unfortunate but it happens :o/

Anyway plenty of suggestions to check here
good luck