User's Manual

4
3/4"
Reservoir
Port
1/4"
Supply Port
1/4"
Control
Port
3/8"
Delivery
Ports
3/8"
Delivery Ports
TEV
3/4"
Reservoir
Port
1/4" Supply Port
1/4"
Control
Port
3/8"
Delivery
Ports
3/8"
Delivery Ports
STEV
FIGURE 3 - Spring Brake Control Valves
Brake Priority Options
In prior years, there were requirements for a protected
reservoir, separate from the main reservoir. The purpose
of the protected reservoir was to hold off the spring brakes
in the event of a failure of the service brake system.
In 1994, FMVSS-121 was revised to allow other approaches
to reservoir management. The protected reservoir
approach, although not required, is still acceptable.
Conventional trailers are designed for either:
• Spring Brake Priority or
• Service Brake Priority.
Spring Brake Priority–The advantage of spring brake
priority is that the parking brakes (spring brake) can be
released quickly to permit moving the trailer at start up.
However, spring brake priority systems have failure modes
under which the parking brakes can be released and the
vehicle operated without functional service brakes.
Service Brake Priority–The advantage of the service brake
priority system is that it assures that the service brakes
have adequate air pressure available to them before release
of the spring brakes is allowed. However, service brake
priority systems require more time to bring a vehicle up to
operational level.
The Bendix
®
A-18
™
trailer ABS system is compatible with
both Spring Brake Priority and Service Brake Priority
systems and does not require special installation
procedures. A number of spring brake control valves are
suitable for meeting current requirements. Bendix offers
spring brake valves suitable for a range of applications.