Lesson 2/Learning Event 2
Several different applications of airoverhydraulic brake systems
are used on trailers, but they can be broken down into two general
typesbrake systems with automatic breakaway protection, and brake
systems that do not have breakaway protection.
Trailer airoverhydraulic brake systems that do not have breakaway
protection have only one air hose coupling which must be connected to
the service trailer coupling on the truck. When either the brake
foot pedal or the hand control valve on the truck is applied, brake
controlled air pressure flows through the service air line to the
On the trailer, the air line carries the controlled air to an air
filter and then to the brake air chamber. The brake air chamber
changes the air pressure to the mechanical motion needed to work the
hydraulic master cylinder. As the air chamber pushrod moves the
master cylinder piston, brake fluid is forced into the wheel
cylinders to apply the wheel brakes.
When the brakes are released by the driver, air pressure in the
trailer brake air chamber flows back to the truck through the service
line and is exhausted through the hand control valve or the air
Trailers with automatic breakaway protection have an emergency air
hose coupling, emergency relay valve, and an air reservoir plus all
the parts used on trailer brake systems without breakaway protection.
The action of this brake system is controlled by the emergency relay
valve. Let's discuss its operation.
With the air hose couplings connected to the truck and the air
shutoff cocks open, compressed air from the truck reservoirs flows
through the emergency line to the supply air inlet opening of the
emergency relay valve. The air flows through the check valve in the
center of the emergency valve diaphragm, through the air supply
passage, and out the reservoir line opening to the trailer reservoir.
The airflow continues until the pressure in the trailer reservoir is
equal to the pressure in the truck reservoir. Under these conditions
the air pressure above and below the emergency diaphragm is equal, so
the emergency diaphragm spring holds the diaphragm in the raised