Good brakes on a big rig are cheap life insurance, not only for yourself, but for the folks who share the road with you. If you aren't able to find replacement cylinders, it's possible to reline the old ones with stainless steel sleeves that will outlive a couple more owners of the truck. Don't discard a leaky cylinder- - - -someone here will be able to recommend a shop that specializes in relining them.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!