I think the brake system is something like this - Air boost hydraulic system. Except there is only one booster unit. This is a blurb from the document:
Air–Boost Hydraulic Brake System
(Air Brake Endorsement not Required)
An air–boost hydraulic brake system uses air pressure
to assist brake force. This is similar to vacuum–assisted
brakes on most passenger vehicles. An air–boost system
usually has the same components of a standard air
supply system including a compressor, governor, wet
and dry reservoirs. These components are found usually
in the same places as on a full air brake system.
The brake pedal linkage operates a hydraulic master
cylinder that sends hydraulic pressure to the booster
unit. Initially, at low pressure the hydraulic fluid passes
through the booster and begins to pressurize the wheel
cylinders moving the brake shoes out to the drums.
These booster units are similar in operation to
“Hypower” or “Hydrovac” vacuum boosters found
on most light and medium weight vehicles, but air
pressure is used to intensify the hydraulic pressure
generated by the master cylinder rather than vacuum.
Built into the booster unit is a hydraulically operated
air control valve.
This is where air from the reservoir is directed.
As the pressure from the master cylinder increases,
the air control section in the booster will open
and begin to deliver air pressure to the rear of the
air cylinder. The air cylinder pushrod transfers
pressure on a piston in the hydraulic section of the
booster, increasing the hydraulic pressure at the
wheel cylinders.
The driver has full control of the braking force as
the air control section modulates the boost pressure
in proportion to the master cylinder pressure.
If the vehicle was to lose all of the air pressure
the brake system would lose the air assist boost,
however the hydraulic system would continue to
work but at reduced effectiveness. An air brake
endorsement on a driver’s licence is not required
to operate a vehicle with this brake system. Consult
the operator’s manual for the vehicle you drive for
maintenance requirements.