[Archived]

Thanks for the refresher. Everybody with airbrakes needs to know this.

[Peter]

I think you didn’t do the governor cut in test, the cut out test, or the spring brake effectiveness test. I always remember it as there are 7 parts to the air brake test. Did I just miss it?

Walt

[Dan]

Jon and I covered it all in the Video’s of Interest Topic.

Video’s of Interest

[Bill]

A pre-trip brake check is something that can be done by rote, or it can be a diagnostic tool.

We all know we rely on air pressure for stopping, but the pre-trip brake check if done correctly not only verifies the system functions correctly, but can be used to verify the system condition beyond making sure you will have brakes.

Here is a step by step and what information is available to assist you in maintaining a coach.

When you build air pressure and the emergency brake is applied you should have no leaks because the air is not going into the brake chambers that actually apply the braking force. If you have to build brake system pressure after the coach has been sitting something is leaking and it should be checked.

When the coach pressures are up to the max and the air dryer makes the huffing sound you should be very close to 120 PSI. Up to 130 is acceptable, and a little lower than 120 is OK, but your system should show both primary and secondary close to 120. It is adjustable on some coaches.

Before you turn off the engine, put it in gear and see if it will move with the parking brake on. If it doesn’t the parking brake is OK. Note that if you give it enough power you can overcome the parking brake.

When you turn off the engine and release the parking brake (making sure the coach will not move) you will see the pressure drop on both gauges and then there should be no further drop in pressure. DOT rules allow a loss of 2 or 3 PSI per minute. That is not acceptable and if you have an air loss find the leak and correct it. Leaks never fix themselves, and they are sure to get worse. It could be a hose ready to fail or a brake chamber diaphragm starting to tear or something else, but for sure that minor problem can turn into a bigger one quickly.

Continued in the next post.

[Dan]

Once you have verified the parking brakes hold, are released without a continuing air loss you want to check the service brakes.

Apply the service brakes with a fair amount of pedal pressure, looking for an initial pressure drop, and with your foot remaining on the brake pedal, not the pressure stays or leaks down. Again the DOT allows 2 or 3 PSI per minute, but you do not want any leak on either system, primary or secondary. For the same reasons as with the parking brake any leak of any size can be an indicator something is leaking and it is not going to get better.

If you have a leak in the primary or secondary air systems and it gets large such as a failed brake chamber idaphragm or hose, and the air compressor is unable to maintain pressure you have an emergency situation and if you do not get to a safe stopping spot NOW your parking brakes may automatically deploy. That is why verifying the service brake system is leak free.

As an aside just for information purposes, you can inch your bus a short distance if your service brakes have a substantial leak by applying the parking brake. That will repressurize the braking systems and unless the leak is upstream from the relay valves you can then release the praking brake and air pressures will hold. As soon as you apply the service brakes the pressures will again drop. If the failure is in a relay valve or worse, a failed air tank, if you have to you can move the coach with the emergency brake applied. That is a true emergency action, but might be required to get the coach off the road and out of harms way.

With the service and parking brakes checked the next step is to verify the safety devices are functioning. Turn the key on, but do not start the coach. Fan the brakes. Somewhere between 70 and 80 PSI or thereabouts you should get a red light and an audible alarm to show air system pressures are low. Continue fanning the brakes and somewhere in the vicintiy of 40 to 45 PSI the parking brake knob should automatically deploy, applying the parking or emergency brakes. You will not be able to release the parking brakes below this pressure.

Continued on the next post

[Archived]

You are now part way through the brake system check.

With the brake system at a low pressure, insure the aux air system pressure is also low. If it is not, do something to bleed air until it gets down to about 50 PSI. Once all pressure systems are at a low pressure, start the coach engine and observe the pressure gauges. Both the primary and secondary systems should build pressure and the aux air system should remain the same until the brake systems reach around 75 TO 85 psi. If that is what happens you have verified the protection valve designed to maintain brake system pressure is functioning correctly. Somewhere in that pressure range the red lights and audible alarms should also go off, typically before the protection valve opens to allow air into the aux system.If all the air pressure being built is solely to pressurize the air systems (and is not being used to raise the coach) the pressure should build from 85 PSI to 100 PSI in a minute or less. This verifies the compressor is functioning correclty.

At 120 PSI the compressor should huff and stop building pressure. If you wish you can again fan the brakes until the system pressures drop below 100 (90 in some cases) at which time the compressor should kick in and the pressure should again build to 120.

You have completed a check of your air systems and in doing so have verified the brakes function and are not leaking air, the warning indicators are working, the parking brake will automatically deploy, the protection valve is doing its job, and the compressor is strong enough and the pressures are set correctly.

Enjoy the drive.