Ford F-650 2001 Owner's Manual page 109

Driving
Hydraulic brake booster system (Hydromax)
The Hydromax systems receive fluid pressure from the power steering
pump to provide power assist during braking.
The Hydromax booster receives backup pressure from the reserve
system electric pump whenever the fluid in the power steering system is
not flowing. When the engine is OFF, the pump will turn on if the brake
pedal is applied, or if the ignition is turned to the ON position.
The sound of the pump operating may be heard by the driver, but this is
a normal characteristic of the system.
The reserve system provides reduced braking power, so the vehicle
should be operated under these conditions with caution, and only to seek
service repair and remove the vehicle from the roadway.
For Hydromax under normal operating conditions noise of the fluid
flowing through the booster may be heard whenever the brake is applied.
This condition is normal. Vehicle service is not required.
If braking performance or pedal response becomes very poor, even when
the pedal is strongly applied, this may indicate the presence of air in the
hydraulic system or fluid leakage. Stop the vehicle safely as soon as
possible and seek service immediately.
If the red BRAKE warning lamp in
the instrument cluster remains BRAKE
illuminated after engine start up,
!
this indicates a system failure in the
master cylinder of the brake system.
Stop the vehicle safely as soon as possible and seek service immediately.
If the yellow BRAKE RESERVE
BRAKE
(E-motor) warning light remains on
while the engine is running, this
indicates inadequate hydraulic
booster pressure or reserve pump
system failure. Stop the vehicle safely as soon as possible and seek
service immediately.
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