PREVOST PNEUMATIC SYSTEM IMPROVEMENT, Сервисный бюллетень

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PROCEDURE
Warning:
Park vehicle safely, apply parking brake, stop engine and set battery master switch(es) to the
OFF position prior to working on the vehicle.
1. Safely support vehicle at the axle jacking points.
2. Jack vehicle until the tire clears the ground, and place safety supports underneath body.
Caution:
Only the recommended jacking points must be used as outlined in Maintenance Manual Section
18,
″
Body
″
or in Operator’s Manual.
3. Exhaust compressed air from accessories and wet air tank by opening drain cock under reservoir. Close
air tank drain cocks.
Note:
The wet tank is located under the vehicle before the drive axle. The accessories air tank is located
under the vehicle near the front axle. For more information, refer to maintenance manual, Section 12:
″
Brake
″
, Section 16:
″
Suspension
″
, and to vehicle's pneumatic diagram.
STEP 1 : TAG AXLE UNLOADING AND RETRACTING LOGIC
INVERSION
After this modification, air spring valve pilots must be under pressure to unload the tag axle. When the air
spring valve pilots are under pressure the air exhausts from springs. The lifting air cylinder is activated and
the tag axle raised. At this time, tag axle brakes are not applied, because the tag axle interlocking brake
valve controls (R-12) has no pressure on the controls. In fact, when pressure is applied to air spring valve
pilots to lifting the air cylinder, air flow is reversed by an air valve (inversion valve) and no pressure activates
the tag axle brake valve pilots and consequently, brakes do not apply.