SERVICE MANUAL DOUBLE CHECK VALVE DESCRIPTION Double  check  valves  or  two-way  check  valves  (Fig.  3) are used in an air brake system where it is necessary to automatically    direct    the    flow    of    air    pressure    into    a common line from either of two other lines. Fig. 3 Sectional View of Double Check Valve Shuttle Type 1 Delivery 4 Shuttle Valve 2 "O" Ring 5 Shuttle Guide 3 Supply OPERATION As  air  pressure  enters  either  end  of  the  double  .heck valve (inlet port) the movable shuttle responds to the air pressure  and  seals  the  line  on  opposite  inlet  port,  but permitting  air  pressure  to  ’low  out  the  delivery  (outlet) port.  The same action takes place if air pressure on one side of shuttle is higher than that on the other side.  It is not necessary  for  the  cavity  of  one  side  of  shuttle  to  be exhausted for valve to operate.  Double check valves are designed  so  it  is  impossible  for  shuttle  to  block  outlet port. MAINTENANCE Once  each  year  or  every  160,000  km  (100,000  miles) remove, disassemble, clean and inspect all parts.  Install new parts if signs of wear or deterioration are apparent. SERVICE CHECKS Due to the various applications of double check valves it is   best   to   bench   test   the   valve   using   two   separately controlled air supplies and connected to inlet ports. 1. Install an accurate test gauge in outlet port or in a line from outlet port. 2. Apply  and  release  air  to  one  inlet  port  and  note that gauge registers application and release. 3. Repeat  application  and  release  of  air  to  other inlet port. 4. Leakage check is performed at inlet ports by: a. Disconnecting line from one inlet port. b. Applying air to other inlet port.  If soap solution is used    to    check    leakage,    leakage    should    not exceed 25.4 mm (1 in.) bubble in 5 seconds. c. Repeat step "b" by applying air to other inlet port while checking opposite inlet port for leakage. 5. If  check  valve  does  not  meet  requirements,  it should  either  be  replaced  or  disassembled  and repaired using new parts. DISASSEMBLY 1. Remove end cap from valve. 2. Remove O-ring. 3. Remove shuttle (piston) and guide. CLEANING AND INSPECTION 1. Clean all metal parts in cleaning solvent. 2. Inspect all parts for signs of wear or deterioration.  Replace  all  parts  not  considered serviceable. 3. Replace all rubber parts. REASSEMBLY 1. Install shuttle guide and shuttle. 2. Position new O-ring in end cap. 3. Install end cap. 4. Perform  operating  and  leakage  as  described  in SERVICE CHECKS. PRESSURE PROTECTION VALVE DESCRIPTION The pressure protection valve is designed to close off air supply   to   any   air   operated   device   as   long   as   the   air supply is below a specified rating. The   rating   of   the   valve   used   in   the   FMVSS-121   air system is 448-517 kPa (65-75 psi). CTS-4079-CHAPTER III - PRINTED IN UNITED STATES OF AMERICA PAGE 4