2014 Hino 195H hydraulic Brake Booster failing?

[Berlin]

Hi all,
Canadian 2014 Hino 195H hybrid here, second owner. with a brake problem.
A seldomly used truck for my company. 12000km(7500mi)/yearly.
Still drivable but i feel something is off.
Anything helps thanks ahead.

Symptoms (Chronological):

1. Squeaking at Start-Up:
   • When starting the truck (warm or cold) and accelerating, there’s a squeaking sound, like a belt rubbing a pulley.
   • The squeak disappears after a few seconds after acceleration(maybe pulley catch up speed), and the truck drives normally.
2. Fluid Leak from Brake Booster Reservoir:
   • After parking the truck for a couple of months, I started it (while pressing the brake pedal). Fluid poured out from the hydraulic brake booster reservoir. I later topped off with
3. Brake Pedal Behavior (Engine Off):
   • With the engine off, pumping the brake pedal doesn’t stiffen it—it stays the same resistance.
   • The brake fluid level in the reservoir rises when I release the brake pedal.
   • When the engine starts, something in the system bounces back, the fluid level drops, and you hear a sound.
4. Beep Alarm During Auto Stop at Red Light:
   • When the truck’s engine goes into auto stop at a red light, after a minute (without changing brake pressure), there’s a long beep.
   • Lifting off the brake pedal starts the engine again, and the alarm stops.
5. Brake Pedal Movement Causes Short Beep:
   • During auto stop, if I slightly press or release the brake pedal, I hear a short beep.

Possible Causes I Suspect:

1. Hydraulic brake booster failure (expensive fix).
2. Check valve failure or hydraulic accumulator failure (cheaper fix).

Again, anything helps, THANK YOU