Technical Bulletin
Bulletin No.: PRO-03-10 Effective Date: 6/13/77 Page: 1 of 7
Subject: Service Line Protection System (SP-1)
Ref: Attachment (6 pgs.)
Bendix HVSG developed a new tractor-trailer protection system, which not only retains
all of the efficiency of the present system, but also provides the capability of applying the
trailer spring brakes when the trailer service line has failed. This added braking ability is
achieved through the use of a new valve, the SP-1 Service Line Protection Valve, in a
standard tractor-trailer protection system. The driver is not required to reach for any
additional controls to activate this unique automatic braking feature. A self-action
decision to apply the trailer spring brakes is resolved by the pneumatic analysis
performed within this new valve.
With each brake valve application, the service line protection valve makes an
instantaneous pneumatic comparison to determine if the trailer service line has failed. If
a failure is detected, the valve then automatically causes the trailer emergency brakes to
be applied, simultaneously closing off the trailer service air.
This system which utilizes an SP-1, 288428, (see Fig. 1) has been installed on recently
delivered fleets of Mack’s, GMC’s and Peterbilt’s for U.P.S. The valve is usually
mounted at the rear of the cab, adjacent to the TP valve.
A system description and operation is attached with a diagrammatic of the piping
arrangement. Approximately 500 of these units are in the field at the present time.
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