TESTING YOUR D'S AIR BRAKES AND SLACK ADJUSTERSAuto slack adjusters ( this is what is on the D's) are used to maintain proper brake chamber stroke and lining to drum clearance during normal operation.   The entire slack adjuster operates as a unit rotating with the brake camshaft as brakes are applied or released.   THE MOST EFFICIENT BRAKING OCCURS WHEN PUSH ROD TRAVEL IS HELD TO A MINIMUM.
Auto Slack Adjusters adjust themselves automatically during full brake applications to accomodate brake lining and drum wear.   The key words here are FULL BRAKE APPLICATIONS.   Have you ever engaged your brakes full.  .  .  .   pedal to the floor.  .  .   probably not many have.   But there is way to do this properly to ensure you get your auto adjusters to work for you.   If you have never fully depressed this pedal it is likely that your auto adjust system has not been fully activating its full adjustment potential.   DO NOT RUN OUT AND DO IT NOW.  .  .  .  DO THE FOLLOWING PROCEDURE TO ENSURE THE SAFETY OF YOUR BRAKING SYSTEM AND THAT YOU DO NOT DAMAGE IT.
Following is a procedure that should be done every day before you start your drive.   Certainly, it should be done if your unit has been sitting for a time without use.   This will tell you if your air brake system is functioning satisfactorily and will also adjust your slack adjusters.
1.   Start engine and let air pressure build up.
2.   Block wheels (use chocks that can be purchased from an RV supply store and chock the outside dual on each side.   Be sure to insert a chock in front and behind the tire to ensure no vehicle movement.   Parking on level ground when you do this is a big help.   After this is done and you are sure your Discovery can not move RELEASE YOUR PARK BRAKE.
3.   Your brake pressure should be at max.   Now fan your brakes to 85psi.   NOW ENGAGE YOUR PARK BRAKE AGAIN.   Compressor should cut in and watch to ensure a return to full pressure.
4.   When you have reached max pressure and the compressor has cut out RELEASE YOUR PARK BRAKE AGAIN AND SHUT OFF THE ENGINE.
5.   MAKE FULL BRAKE APPLICATION ( BE SURE you have released your park brake before you do this or you will damage the system.  ) Hold the the pedal all the way to the bottom for one minuete.   Watch your air guages as they should not move ( allowable loss in this mode for 60 seconds is 3 psi.   Listen for audible leaks as you wait for the
60 seconds to expire.   After 60 seconds release the pedal.   (YOUR AUTO SLACK ADJUSTERS WILL SET UP AUTOMATICALLY DURING THIS PROCEDURE.  )
6.   Turn on key.  .  .   fan brakes to 80psi.   LOW AIR WARNING SHOULD START.   continue to fan down to 20psi.  .   by now your park brake will have engaged automatically although you will not have any indication of this.
7.   Start engine and run at 1000rpm using your cruise setting.   Time 3 minuets.   In this time your air pressure should be up to between 50 and 90psi.   This will tell you your compressor is functioning properly.
8.   Apply park brake.
End of Test
-----Submitted by Bob Cook, with thanks to Ron Nagle