Figure 3. EMERGENCY APPLICATION (Trailer Air System charged to normal operating pressure) Venting the trailer supply line to atmosphere will cause the emergency portion of the Relay Emergency Valve to apply full trailer reservoir pressure to the trailer air chambers if the trailer supply line pressure is reduced to approximately 20 psi due to leakage or conditions other than above, a graduated trailer air chamber application will occur.  The rate of this brake application will depend upon the rate of pressure loss in the supply line If the trailer is not equipped with parking actuators (such as spring brakes), the trailer wheels should be blocked to prevent trailer movement, in the event reservoir pressure would be depleted. TO RELEASE AN EMERGENCY APPLICATION Recharge trailer air system or: A. For trailers equipped with standard brake chambers, block wheels and drain trailer reservoir. B. For trailers equipped with spring brake chambers, block wheels and mechanically release spring brake chambers via the mechanical release mechanism (for specific instructions, refer to manufacturers service literature.) PREVENTIVE MAINTENANCE Every 12 months, 100, 000 miles or 3600 operating hours disassemble, clean metal parts in mineral spirits. Replace all rubber parts and any part worn or damaged. Check for proper operation before placing vehicle in service.  To prevent entrance of foreign matter, it is recommended that filters be installed in the trailer supply and service hose couplings.  Check periodically, and clean or replace as necessary. OPERATING AND LEAKAGE TEST Check tractor dash gauge against a test gauge known to be accurate prior to performing these tests Connect tractor air lines to the trailer on which the valve is to be tested.  Block wheels or otherwise hold both vehicles by a means other than air brakes during these tests. 1. Start these tests with no air pressure in the tractor or trailer system.  Make certain trailer connection cut- out cocks are open if present Build tractor air system to at least 50 P.S.I.  Move tractor protection control valve to normal position As trailer system starts to charge, trailer brakes should apply and then completely release as trailer supply line pressure reaches 60-65 P.S.I 2. Fully charge tractor and trailer air brake system Make several service brake applications and check for prompt braking response at all trailer wheels. With brakes released and system reservoir pressure stabilized at 100-120 psi with engine stopped, a two minute check should result in no more than a six pound pressure drop for the combination vehicle system.  If this check indicates possible excessive leakage of valve, soap suds should be applied to the exhaust port to detect possible leakage of inlet valve and emergency piston O-Ring leakage.  Leakage of a one inch soap bubble in not less than five seconds is permissible.  No leakage is permissible at pipe plugs or fittings. 3. Make and hold a full service brake application with system reservoir pressure stabilized at 100-120 psi with engine stopped.  A two minute check should result in no more than an eight (8) psi drop for the combination vehicle system.  If this check indicates possible excessive leakage of valve, soap suds should be applied to cover to detect body O-Ring leakage and to exhaust port to detect exhaust valve and O-Ring leakage.  A combined leakage as indicated by a one inch bubble in not less than three seconds is permissible.  Release the service brake application. 4. With tractor and trailer air brake system fully charged, place  the  tractor  protection  control  valve in  the "emergency" position or close the emergency line cut-out cock on the tractor and uncouple the trailer emergency line coupling.  Note that trailer brakes apply promptly.  Check at the supply (emergency) line coupling at the front of the trailer for leakage.  No leakage permitted.  A leak detected at the supply (emergency) line coupling would indicate leaking check valve or piston O-Rings in the valve Check at service line coupling at the front of the trailer.  A leak at the service line coupling would indicate leaking relay piston O-Ring. Reconnect service and emergency lines and recharge tractor and trailer air brake system, noting that trailer 8-6