288428
SP-1
Tractor-Trailer Service Line Protection System
System Description
The service line protection valve is used in conjunction with a standard tractor-trailer
protection system. When a brake valve application is made and the trailer service line is
intact, the system operates in a normal manner. When a brake valve application is
made and the trailer service line has failed, the new system simultaneously vents the
trailer supply line (causing the trailer spring brakes to apply) and also shuts off the
tractor service air to the trailer (preserving the tractor air pressure). A subsequent
release of the trailer spring brakes can be accomplished by releasing the brake valve,
thereby recharging the supply line and releasing the trailer brakes. Five or six repeated
full applications and releases are possible before the system pressure is depleted to a
point where the trailer supply valve (PP-7) trips.
Operation
The typical system is piped as shown in Figure 2. A trailer supply valve, in the “on”
position, allows air pressure to pass through the tractor protection valve to supply the
trailer. Application of sufficient trailer supply air pressure across the tractor protection
valve plunger causes the service inlet valve to open.
A. Normal Operation (With Trailer Service Line Intact):
1. When the brake valve treadle is depressed, service air is delivered through the
double check valve to the tractor service port of the tractor protection valve.
2. The inlet valve, being held open by the force of trailer supply air across the
plunger, allows the service air to continue, through the trailer service port and
service line, to the trailer.
3. During every brake application the service pressure is also being monitored at
two sources.
a. One monitor line (#1) originates at the trailer service port side of the tractor
protection valve and is piped to the top control port (to the topside of piston
Item #1 Fig. 3) of the service line protection valve (a normally open valve).
The pressure of this delivered air is equal to the service air pressure being
transmitted to the trailer.
b. The other monitor line (#2) originates at a brake valve primary delivery port
and is piped to the bottom control port (leading to the underside of piston Item
#2 Fig 3.) of the same service line protection valve.
c. When the brake valve delivers more than 50 psi air pressure, a valve (Item 3
Fig. 3 immediately downstream of the bottom control port) opens and allows
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