I had the same symptom on my rig years ago. Just topping the fluid helped initially but where was that fluid going? Couldn't tell in the beginning as it didn't leave a very good telltale.
Eventually, the brake pedal went almost all the way to the floor and left me with manual brakes only. Kind of pointed to a bad master cylinder. And that's what it turned out to be. Replaced it myself after Freightliner quoted $2800 for the job.
It was on a '94 Bounder Diesel, on a 35' Freightliner chassis and had the Hydroboost system. Most everything in it was made by Bendix. All the parts had numbers embossed on the metal cases.
Fairly easy job once I figured out how to do it...
The hydroboost is that assembly just to the left of the master cylinder (the assembly with the white plastic fluid container on the top). They are really simple, have a plastic filter screen, a machined core with 5 large 'O' rings that moves in/out with applied brake, a couple hoses going to/from the pump, and a case it all fits in. And on the bottom is an electric motor that gives an assist if you lose engine power or it's not running. Takes the place of the power fluid pump.
Really not much too it. That device's screen just needed cleaning in my case. Nothing wrong with it...and not much can go wrong with it. All it does is takes some hydraulic pressure from the pump, and when you step on the brakes, it pushes on the master cylinders input shaft giving an assist. Didn't even need to remove it from the chassis as the shaft and filter screen was easily pulled out once the master cylinder was removed.
The master cylinder though...that was obviously leaking once I did a close up inspection. So the leak was brake fluid, not ATF supplying the Hydroboost device. They don't rebuild these big MCs generally so I had to buy a new one for ~$500.
On my chassis the MC was mounted in the drivers wheel well so everything was easy to get to, even with the tire still mounted.
Good luck with yours, hope it's something simple.
