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

 

[greasytshirt]

There is a hydraulic fluid reservoir at the left rear of the engine bay for the booster pump. The reservoir stains and is impossible to see the actual fluid level through the gauge on the reservoir. This reservoir gets Dexron III atf. It'll alarm if the level gets low.

Pumping the brake pedal with the engine off quickly depletes the accumulator so you should feel boost drop off. Sounds like that isn't working.
When you hit the brakes, engine running, does the pedal kick back at you?
I've replaced a number of brake boosters on the cab over trucks and it's very involved and time consuming.