SD-27-5028
Bendix® GSBC® FBM-40™ (Global Scalable Brake Control
Foot Brake Module)
INTRODUCTION
The Bendix® GSBC® FBM-40™ (Global Scalable Brake
Control Foot Brake Module), known hereafter as “FBM,”
is a firewall-mounted, pedal-operated dual circuit brake
foot valve. The FBM is the primary means of control for
the service braking system on a vehicle equipped with
Bendix GSBC.
The FBM accepts graduated driver input from the brake
pedal and provides a proportional request for service
braking to the GSBC system. It provides three means of
control for the braking system: electronic stroke sensing,
primary circuit delivery pressure, and secondary circuit
delivery pressure.
The FBM contains an electronic stroke sensor, which
measures the distance that the driver has depressed the
pedal by measuring the movement of the input tappet, and
by extension the brake pedal, during a brake application.
It has two separate, isolated supply and delivery circuits
for the control of the service brake system in an air-braked
vehicle, similar in design to a traditional Bendix Foot
Brake Valve. The FBM connects to both the primary and
secondary pneumatic circuits on the vehicle, and delivers
on both those circuits as long as the connected reservoir
is pressurized.
In most applications, the primary circuit is connected to
the steer axle, and the secondary circuit is connected to
the drive axle(s) unlike most conventional U.S. plumbing
configurations.
All three methods of control (electronic stroke sensing,
primary circuit pneumatic control, and secondary pneumatic
circuit control) are always functional when the system is
operating without active faults. In the event of a failure
(electronic or pneumatic), the other control methods will
remain functional and available to the driver.

Figure 1 – GSBC® FBM-40™(Foot Brake Module)

Incorrect identification or plumbing of the primary
or secondary circuit(s) at the GSBC® FBM-40™ could
lead to serious vehicle braking, instability and
performance issues, and/or potential collisions,
property damage, serious injuries, and/or
death. Contact your vehicle manufacturer for
up-to-date pneumatic brake system schematics if
work is being performed on the braking system of the
vehicle, or if the FBM requires replacement.
For the FBM, primary ports are 11 and 21, and secondary
ports are 12 and 22. See Figure 4.

GENERAL SAFETY GUIDELINES

WARNING! PLEASE READ AND FOLLOW THESE INSTRUCTIONS
TO AVOID PERSONAL INJURY OR DEATH:

When working on or around a vehicle, the following guidelines should be observed AT ALL TIMES:
▲ Park the vehicle on a level surface, apply the ▲ Never connect or disconnect a hose or line
parking brakes, and always block the wheels.
containing pressure; it may whip and/or cause
Always wear personal protection equipment.
hazardous airborne dust and dirt particles. Wear
eye protection. Slowly open connections with
▲ Stop the engine and remove the ignition key
care, and verify that no pressure is present. Never
when working under or around the vehicle.
remove a component or plug unless you are
When working in the engine compartment,
certain all system pressure has been depleted.
the engine should be shut off and the ignition
key should be removed. Where circumstances ▲ Use only genuine Bendix ® brand replacement
require that the engine be in operation, EXTREME
parts, components, and kits. Replacement
CAUTION should be used to prevent personal
hardware, tubing, hose, fittings, wiring, etc.
injury resulting from contact with moving,
must be of equivalent size, type, and strength as
rotating, leaking, heated, or electrically charged
original equipment and be designed specifically
components.
for such applications and systems.
▲ Do not attempt to install, remove, disassemble, ▲ Components with stripped threads or damaged
or assemble a component until you have read,
parts should be replaced rather than repaired.
and thoroughly understand, the recommended
Do not attempt repairs requiring machining or
procedures. Use only the proper tools and
welding unless specifically stated and approved
observe all precautions pertaining to use of those
by the vehicle and component manufacturer.
tools.
▲ Prior to returning the vehicle to service, make
▲ If the work is being performed on the vehicle’s
certain all components and systems are restored
air brake system, or any auxiliary pressurized air
to their proper operating condition.
systems, make certain to drain the air pressure ▲ For vehicles with Automatic Traction Control
from all reservoirs before beginning ANY work
(ATC), the ATC function must be disabled (ATC
on the vehicle. If the vehicle is equipped with
indicator lamp should be ON) prior to performing
a Bendix ® AD-IS® air dryer system, a Bendix®
any vehicle maintenance where one or more
™
®
®
DRM dryer reservoir module, a Bendix AD-9si ,
wheels on a drive axle are lifted off the ground
AD-HF®, or AD-HFi® air dryer, be sure to drain the
and moving.
purge reservoir.
▲ The power MUST be temporarily disconnected
▲ F o l l o w i n g t h e v e h i c l e m a n u f a c t u r e r ’s
from the radar sensor whenever any tests USING
recommended procedures, deactivate the
A DYNAMOMETER are conducted on a vehicle
electrical system in a manner that safely removes
equipped with a Bendix® Wingman® system.
all electrical power from the vehicle.
▲ Never exceed manufacturer’s recommended
pressures.
▲ You should consult the vehicle manufacturer's operating and service manuals, and any related literature,
in conjunction with the Guidelines above.

2

SD-27-5028 Rev 001 Bendix® GSBC® FBM-40™ (Global Scalable Brake Control Foot Brake Module)

OPERATION
ELECTRONIC MODE OPERATION:

RELEASING: ELECTRONIC STROKE SENSING

If the GSBC System is operating without active faults, and
is not faulted due to electrical, mechanical, or software
reasons, it will be operating in Electronic Mode, where the
driver’s requests for brake force are measured by the stroke
sensor in the FBM and transmitted to the GSBC® EBS-90™
(Central Electronic Control Unit) known hereafter as “ECU.”

When the driver releases pressure on the brake pedal, the
internal components of the valve will shift back towards their
initial position. This displacement is continuously measured
by the FBM, and the distance that the components have
been displaced reported to the ECU, which then acts to
provide the corresponding amount of brake pressure to the
wheel-end foundation brake components.

While operating in Electronic Mode, the mechanical portion
of the valve will continue to operate as described below, but
the system will ignore those pneumatic inputs unless it falls
back to the Pneumatic Backup Mode state of operation.

NOTE: Regardless of the operational mode, air will always
be delivered by the FBM to the GSBC System.

The stroke sensor will only provide brake application
information when the FBM is operating in Electronic Mode.
See Figure 2.
APPLYING: ELECTRONIC STROKE SENSING
When the brake treadle is depressed, the internal
components of the valve will shift downward. This
displacement is continuously measured by the FBM, and
the distance that the components have been displaced
reported, via the stroke sensor, to the Central ECU, which
then acts to provide the correct amount of brake pressure
to the wheel-end foundation brake components.
BALANCED: ELECTRONIC STROKE SENSING
When the brake treadle is held steady in the applied
position, the internal components of the valve will remain
stationary, but they will be displaced from the initial position
of the valve. This displacement is continuously measured
by the FBM, and the distance that the components have
been displaced reported to the ECU, which then acts to
provide the corresponding amount of brake pressure to the
wheel-end foundation brake components.

IMPORTANT
Bendix®-brand Electronic Control Units (ECUs) are
not designed to store data for purposes of accident
reconstruction, and Bendix ® ACom ® Diagnostic
Software is not intended to retrieve data for purposes
of accident reconstruction. Bendix makes no
representations as to the accuracy of data or video
retrieved and interpreted from ECUs for purposes
of accident reconstruction. Bendix does not offer
accident reconstruction services or interpretation of
stored data. Bendix ECUs are not protected from fire,
loss of power, impact damage, or other conditions that
may be sustained in a crash situation and may cause
data to be unavailable or irretrievable.

Pneumatic Signal
Digital Control Signal
Driver Brake
Demand

Deceleration Demand

GSBC® EBS-90™
Central ECU

Electronic CAN Signal
Wheel Speed Signal

Electronic
Pressure
Demand

Wheel Speed Signal

GSBC®
FBM-40™

Wheel Speed
Sensor

Brake Control Pressure
Supply Pressure

Service Brake Pressure

GSBC® EPM-90™

Foundation
Brakes

Reservoir

Figure 2 – GSBC FBM Electronic Mode Example

SD-27-5028 Rev 001 Bendix® GSBC® FBM-40™ (Global Scalable Brake Control Foot Brake Module)

3

OPERATION (CONT.)
PNEUMATIC BACKUP MODE OPERATION:

BALANCED: PRIMARY CIRCUIT

If the GSBC System is not operating in electronic
mode and has experienced a fault due to electrical,
mechanical, or software reasons, the system will default
to operating in Pneumatic Backup Mode, where the
driver’s requests for brake force are interpreted purely by
the mechanical components of the valve. The electronic
stroke sensor will not be active or acted on by the GSBC
System. The mechanical components in the FBM do
not behave any differently if it is in Electronic Mode
(Figure 2) or Pneumatic Backup Mode (Figure 3).

When the primary delivery pressure acting on the piston
equals the mechanical force of the brake pedal application,
the primary piston will move and the primary inlet valve will
close, stopping further flow of air from the primary supply
line through the valve. The exhaust valve remains closed,
preventing any escape of air through the exhaust port.

NOTE: Pneumatic backup mode is similar to conventional
pneumatic brake systems commonly found on
FMVSS 121-equipped pneumatically braked vehicles.
APPLYING: NORMAL OPERATION PRIMARY CIRCUIT
PORTION
When the brake treadle is depressed, the pushrod exerts
force on the tappet and primary piston.
The primary piston closes the primary exhaust valve. As
the exhaust valve closes, the primary piston allows primary
air to flow out the primary delivery port.
APPLYING: NORMAL OPERATION SECONDARY
CIRCUIT
When the primary inlet valve is moved off its seat, air is
permitted to pass through the bleed passage and enters the
relay piston cavity. The air pressure moves the secondary
piston, which contains the exhaust seat, and closes the
secondary exhaust valve. As the secondary exhaust valve
closes, the secondary inlet valve is moved off its seat
allowing the secondary air to flow out the secondary
delivery port.

BALANCED: SECONDARY CIRCUIT
When the air pressure on the secondary side of the
secondary piston approaches that being delivered on the
primary side of the secondary piston, the secondary piston
moves, closing the secondary inlet valve and stopping
further flow of air from the supply line through the valve. The
exhaust remains closed as the secondary delivery pressure
balances the primary delivery pressure. When applications
in the travel range are made, a balanced position in the
primary circuit is reached as the air pressure on the delivery
side of the primary piston equals the effort exerted by the
driver’s foot on the treadle. A balanced position in the
secondary portion is reached when air pressure on the
secondary side of the secondary piston closely approaches
the air pressure on the primary side of the secondary piston.
If the brake treadle is fully depressed, both the primary
and secondary inlet valves remain open and full reservoir
pressure is delivered to the system.
RELEASING: PRIMARY CIRCUIT
With the brake treadle released, mechanical force is
removed from the tappet, helical springs, and primary
piston. Air pressure and spring load moves the primary
piston, opening the primary exhaust valve, allowing air
pressure in the primary delivery line to exhaust out the
exhaust port.

APPLYING: LOSS OF AIR IN THE SECONDARY CIRCUIT

RELEASING: SECONDARY CIRCUIT

Should air be lost in the secondary circuit, the primary
circuit will continue to function as described above under
Normal Operation Primary Circuit Portion.

With the brake treadle released, air is exhausted from the
primary circuit side of the secondary piston. Air pressure
and spring load move the secondary piston, opening the
secondary exhaust valve allowing air pressure in the
secondary delivery line to exhaust out the exhaust port.

APPLYING: LOSS OF AIR IN THE PRIMARY CIRCUIT
Should air be lost in the primary circuit, the function will be
as follows: as the brake treadle is depressed – and no air
pressure is present in the primary circuit supply and delivery
ports – the primary piston will mechanically move the
secondary piston, allowing the secondary piston to close
the secondary exhaust valve and open the secondary inlet
valve, allowing air to flow out the secondary delivery port.

4

SD-27-5028 Rev 001 Bendix® GSBC® FBM-40™ (Global Scalable Brake Control Foot Brake Module)

Pneumatic Signal
GSBC® EBS-90™
Central ECU

GSBC®
FBM-40™

Wheel Speed
Sensor

Brake Control Pressure
Service Brake Pressure

Supply Pressure

GSBC EPM-90
®

Foundation
Brakes

™

Reservoir

Figure 3 – GSBC FBM Pneumatic Backup Mode Operation Example

FRONT

Port 11 - Primary Supply

Port 21 - Primary Delivery
Port 22 - Secondary Delivery

Port 12 - Secondary Supply

“Port” 3 - Exhaust

Mounting

TOP

BACK

Electronics

Pedal Input

Electrical
Connector

Figure 4 – FBM Primary Features
SD-27-5028 Rev 001 Bendix® GSBC® FBM-40™ (Global Scalable Brake Control Foot Brake Module)

5

GSBC SYSTEM PNEUMATIC SCHEMATIC EXAMPLE (WITH OPTIONAL GSAT)

15 16

13

20

8

15 16
6

17

15 16

9

9

7
10

14

21

11
12

12

18 19

3

14

5

1

4
5

2

8

9

15 16

15 16

9
15 16

COLOR CODING OF BRAKE SYSTEMS PER TMC RECOMMENDED PRACTICE #423
Charging

Primary

Secondary

Trailer (Supply)

Trailer (Control)

Parking (Control)

Accessories

Figure 5 – GSBC System Pneumatic Schematic Example* (With Optional GSAT)
*This schematic is representative only and may differ from your vehicle. Verify your vehicle schematic with your OEM.
NOTE: In most applications of GSBC, the primary circuit is connected to the steer axle, and the secondary circuit is connected to the drive
axle(s) unlike most conventional U.S. plumbing configurations.
ITEM

COMPONENT

ITEM

COMPONENT