how to test everything on air brake systems - Back Up!

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How to test everything on bus airbrake systems…

Not really.  But this is as much as I can manage to figure out so far on how to test bus air brake systems with DD3’s.  This is relevant to my bus – a 1980 MCI MC-5C.  I developed this so that I would have a way of checking way beyond the MTO/DOT mandated tests, check the various systems actually pretty deeply, and test some parts of the system that virtually never get used, so you never know if they are even still there…

On my bus, the dash air pressure gauge reads from the service tank.  I use that connection to check a number of sub-systems.  If your bus reads the pressure on the dash gauge from the accessory tank, there are a number of things that you can’t check.  My bus has check valves between the various tanks, and has a pressure protection valve between the service tank, and the parking brake/accessory tanks.  It has an air dryer with a check valve between the compressor and the wet tank.  It has a shuttle valve that forms part of an emergency brake system with DD3 rear brake canisters.  Your bus may have all of these, some, none, or a more complex system.  My hope is that these tests may be relevant to many buses, and may help people think about and understand how their equipment works, and how to ensure that it is working properly and safely.

Anyway, this is the list of checks that I do on my bus, and what they test.  A lot of things get tested multiple times, like low air warnings.  Some get tested when draining the tanks.  I have never done all of these tests in one go, it would take about two days to do right.  But I have done them all at some point, and some I do every day that I drive the bus.


Air brake tests – pressure going up.  Bus is chocked.  DOT tests are part of the daily/monthly inspection requirement for commercial operators.

Start with all tanks at zero.  Gauge reads zero.
Start engine.  Observe gauge reading start to climb.  Gauge reads service tank pressure.
Tested – compressor and gauge work and bus runs.

Low air audible and visual alarm active.
Tested – low air alarm.  DOT test

Note – the gauge reads immediately if the gauge is connected to the service tank.  If the gauge is connected to the accessory tank AND there is a pressure protection valve between the service tank and the accessory tank, the gauge should not read above zero until the service tank pressure is above the PPV pressure set point (usually 65 psi)

Observe the pressure building to 65 psi.  The pressure increase should stall at 65 psi for a minute or two while the accessory and parking brake tanks are equalized to 65 psi through the PPV.
Tested – PPV valve operation and PPV valve pressure setting

Observe pressure building to 85 psi – low air warning should go out.  Generator should start to charge, also, and some other non-brake related things might happen.
Tested – low air warning off.  DOT test

Observe pressure building to 120 PSI.  Governor should cut-out, and air dryer should purge.
Tested – Governor cut-out pressure, signal line from wet tank to governor, and signal line from governor to air dryer.  Air dryer purge mechanism. DOT test.

Turn off engine

Apply service brakes fully.  Hold for one minute and observe no audible leakage and less than 3 psi air pressure loss.
Tested – service brake leakage – lines, quick release valves, brake cans, R-8 valve, treadle valve, all sorts of stuff.  Basically the entire service brake system for leakage on service application.  DOT test.

Release DD3 parking brake (bus is chocked)

Fan brakes down to between 90 and 100 psi.  Apply service brakes and hold, measure all the brake can extensions (takes a while, under the bus, use all the safety precautions, etc).  For commercial buses, this is required every 30 days or 12,000 Km’s in Ontario, your mileage may vary.  You have to be a licensed under-coach technician, by the rules.  We need to have this done regularly.  The pressure is important if you get pulled over on a road-side check.  With a DOT guy checking with a micrometer and poor maintenance on your part, 90 psi (which is legal) and 120 psi (which the tech may ask you for) may be the difference between calling a licensed tech (the DOT guy ain’t gonna let you crawl under there to adjust your slacks) and a $500 fine, vs a cheery wave and on your way…
Tested – slack adjustment, brake canister pushrod extension. DOT test.

Fan the brakes down until the low air pressure warning comes on, between 55 and 60 psi
Tested – low air warning on pressure.  DOT test.

Start the bus, observe the air pressure building on the gauge.  Set engine speed to 800 – 1000 rpm (high idle is fine). Measure the time it takes for the pressure to build from 85 psi to 100 psi.  This is the compressor recovery time.  It must be less than 2 minutes, it should be less than 45 seconds, it should remain consistent.  If it changes, it indicates a change or progressive failure in the compressor.
Tested – compressor recovery time.  DOT test.

Observe air pressure building to cut out.  Note the cut out pressure.  With the engine running, fan the brakes to lower the air pressure to 25 psi below the cut out pressure for your vehicle.  Observe the pressure building, indicating that the compressor has cut in.  Cut in pressure must be within 25 psi of the cut out pressure.
Tested – governor cut in pressure setting within 25 psi of cut out pressure.  DOT test.

This pretty much completes the normal daily testing of the air brake system, and covers most of the DOT required tests that commercial operators must do.  Now, I want to test a number of other systems using tank draining sequences to show me what various pressure protection valves and check valves are doing for me.

Run bus up to cut-out pressure, observe cut-out and air dryer purging.  Bus off, note gauge pressure reading.  With manual pressure gauge, read pressure at the fill valve on the Ping tank near the compressor (Ping tank, also called Discharge Muffler on MCI buses).  Pressure should read zero.  This tests the check valve between the air dryer and the wet tank.
Tested – Air dryer check valve.

Drain the wet tank.  Observe the pressure on the dash gauge should not change.  This tests the check valve between the wet tank and the service tank.  The longer you wait, the better the test – but leaks elsewhere in the system can also reduce the pressure in the service tank.  The initial reading is the major test of the check valve.
Tested – check valve between wet tank and service tank.

Drain the accessory tank.  This drains the tank and the lines between the accessory tank and the parking brake tank, and the 65 psi pressure protection valve that separates the service tank from the accessory tank and the parking brake tank.  Observe the dash pressure gauge.  It should now read 65 psi.  This tests the Pressure Protection Valve and tells you what the PPV is set to.
Tested – Pressure Protection Valve.

Drain the parking brake tank.  If there is air retained in the parking brake tank, then the check valve leading into the PBT is good.  If you had a gauge on the PBT, it should have read the same pressure as the dash gauge prior to draining the accessory tank (basically cut out pressure), proving that the check valve is retaining air inside the PBT when the feed to it falls below it's internal pressure.  This check valve is critical to the emergency brake function.
Tested – parking brake tank check valve.

Drain the service tank.  Dash gauge should fall to zero.  Doesn’t test anything, but you should do it anyway…

Note that by draining the tanks in this sequence, or in other sequences, and waiting between stages you can isolate air leaks to various systems.  For example, if you drain the accessory tank and the gauge stays at 50 - 65 psi steady, or leaks down very slowly,  for a few days, you know that your leak isn’t downstream of the PPV.  My bus leaks down to 50 psi in 8 hours, then sits there for days, slowly leaking down from that point.  I know my big leaks are therefore in my accessory system, since my PBT holds pressure pretty good for several days.

This sequence allowed us to check the operation of all of the critical check valves in the air brake system and allowed us the check the operation and pressure setting of the pressure protection valve.

Parking and emergency brake tests – DD3 system.

This sequence allows me to check the operation of the parking brake and emergency brake systems on my bus.

Start the bus, build air pressure to cut out.  Apply the parking brake by pulling the push/pull  valve up.  Test that parking brakes have applied by trying to move the bus, bus should not move.  Do a full (120 psi) service brake application, test again, bus should still not move.  
Tested – DD3 parking brake action works.

Turn bus off.  Fan the brakes to release air pressure to below 85 psi (75 psi is appropriate).  Attempt to release parking brake by pushing in push/pull valve and applying service brakes fully. Start engine and try to move bus.  Parking brake should NOT release.
Tested – parking brake 85 psi pressure regulator.

Bus is running, allow pressure to build to 100 psi, and apply service brakes.  Parking brake should now release.
Tested – parking brake pressure regulator setting and DD3 brake can parking brake action, also the inversion valves etc.

Based on a comment, I now think the emergency brake section was wrong.  This is the revised version.  Please check it and verify it, it you could.

Emergency brake shuttle valve test.

Build up air pressure to cut out.   Turn the bus off.  Drain the service tank to 60 psi below the initial starting pressure.  Apply the service brakes, observe the rear brake can push rods should activate normally.  Drain the service tank to zero, apply the brake pedal, observe the rear brake can push rods should activate normally.  The shuttle valve is routing air from the parking brake tank (now acting as an emergency brake air supply) to the emergency brake diaghrams in the DD3 brake canisters at the rear (drive) wheels.

Start the bus, build up pressure to cut out, apply the service brakes.  The shuttle valve should return to normal and close the emergency brake air line.  Open the drain on the service tank and allow it to drop (engine running) to 60 psi below cut out, and to zero again, this time moving the bus and verifying that you have full brake action on the rear emergency brakes.  You may have to do this with a helper, since the compressor is going to be filling the service tank at the same time as you are trying to test the brake action with it drained.  This tests the action of the shuttle valve, which is supposed to switch to using the parking brake tank to supply air to the rear emergency brake section of the drive wheel DD3 brake canisters when the pressure of the service tank falls over 40 psi below that of the parking brake tank.  Caution is required, because to test it you have to move the bus, and if it doesn’t work, you have no brakes and you can’t use the engine stop because by draining the service tank you also drain the accessory tank ( no check valve) and the engine stop valve will not work.  The emergency stop solenoid, it it’s hooked up, would stop the engine.
Tested – emergency brake shuttle valve,  DD3 emergency brake function, and your nerves.

With bus running, close the drain cock on the service brake tank and build pressure to cut out.  With bus off, drain the parking brake tank.  Test the brake action again, observing the front brake pushrods, to make sure the shuttle valve returns to normal and is using the service brake tank to supply air to the service brakes.  If the shuttle valve stuck in the emergency position, there would be no ability to apply the parking brake and the front and rear service brakes would not work.  Only the rear emergency brake diaghram works on the emergency tank.
Tested – shuttle valve return to normal.

come on, guys, 33 reads and not a single comment! give me some love here   :o


Brian, I will do it.

Wow.......Hows that?

I did read most of it, but I think that I fell off of the wagon ;D. Pretty good though, good job.

God bless,



You forgot about the foam on the ice ramps and small hydrogen leak. :-\

That/Those tests would take 5 people 3 days to perform "if" they even understood
the instructions. I note that many of these tests require someone "under the bus" and test's that would require additional plumbing and fittings to be installed in order to make pressure tests. And a Pit or Wheel lifts.

My brain hurts now... Thanks 8) 8)


This is great! I will have to read this thru a few times to and think about the specifics. At the beginning when building air do you have the engine on fast idle? I assume so but thought I would ask.

Also, I assume it is possible to have a DOT inspection completed on my bus? At what cost and how often should this be done?

I an going to print this and keep it in the technical manual. I will follow this post to see if there are any changes.




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