Air Brake Check

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Pre-Trip Procedure

Start engine and let air pressure build up. 

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. 

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. 

When you have reached max pressure and the compressor has cut out RELEASE YOUR PARK BRAKE AGAIN AND SHUT OFF THE ENGINE. 

MAKE FULL BRAKE APPLICATION ( be sure you have released your park brake before you do this or you will damage the system.) Hold the pedal all the way to the bottom for one minute. Watch your air gauges 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.) 

Turn on key... fan brakes to 80psi. LOW AIR WARNING SHOULD START. continue to fan down to 20 psi by now your park brake will have engaged automatically although you will not have any indication of this. 

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 90 psi. This will tell you your compressor is functioning properly. 

Apply park brake. 


 

Subject: Air Brakes Part 1
Categories: Chassis
From: Ron Nagel on Tuesday, July 28, 1998 3:23pm 

As promised I will post a few articles on air brakes . The information I am using here is taken from the Air Brake manual published by our Saskatchewan Highways and Transportation study guide used for the testing and endorsing of our licenses. In Saskatchewan you must have this endorsement on our drivers in order to drive a vehicle with Air Brakes.

Basic System Components

There are five main system components that make up Air Brakes. 


Compressor -- builds up and maintains air pressure. 

Reservoirs -- stores compressed air. 

Foot Valve -- draws compressed air from the reservoirs when it is needed for braking. 

Brake Chambers -- transfers the force of compressed air to mechanical linkages. 

Brake Shoes and Drums -- creates the fiction needed to sop the vehicle. 


Compressor 


The function of the air compressor is to build up and maintain air pressure required to operate the brakes. Air compressors are either gear driven or belt driven. The compressors intake system draws air form either its own air filter or from the engines air intake. All compressors run continuously while the engine is running, but the air compression is controlled and limited by the GOVERNOR which loads or unloads the compressor. In the loaded stage air is pumped into the RESERVOIRS and in the unloaded mode air is pumped back and forth between the two cylinders of the air compressor without supplying the reservoirs. 

The GOVERNOR must take the compressor out of its loaded position or pumping stage when the air pressure reaches 115-135 p.s.i. and in turn put the compressor back into the loaded position when the air pressure drops to 85 p.s.i. 

Reservoirs 


The tanks are pressure rated holding tanks where air is stored until needed for braking. Each reservoir is equipped with a drain cock and should be drained regularly. Our Discovery units drain automatically, but it does not hurt to give the string a pull and manually drain them before storage at least. 

Foot Valve


The foot valve applies air to operate the brakes. The amount of air delivered to the brakes is regulated by the driver according to the distance the treadle, or brake pedal is depressed. Releasing it exhausts the air in the service brakes. 

Brake Chambers


Brake chamber convert compressed air pressure energy into mechanical force and movement, which apply the vehicles brakes. When the driver applies pressure to the foot valve, air pressure enters the pressure side of the brake chamber through the inlet port and forces against the diaphragm, which moves the push rod assembly forward. When air pressure is released from the brake chamber, the return spring returns the diaphragm and push rod to their released positions. 

There are many other components involved here but for our purposes I need not bore you with a lot of theory and components that we will never actually see operate or have access to . The one area however that we do need to pay attention to is the area of brake adjustments. 

Our Discovery units have auto brake slack adjusters and we should not have to worry about that. Well, it is just a mechanical device and YOUR LIFE DEPENDS UPON YOUR ABILITY TO STOP... SO we must know how it works and how to check it to make sure they are working. 

Section 2 will deal with this and section 3 will actually give you the procedure to check your brakes for proper operation each time before you head onto the roads. Return to Top Return to Index 

Subject: Air Brakes Part 2
Categories: Chassis
From: Ron Nagel on Thursday, July 30, 1998 10:04am 
This time we will deal with checking our brakes to ensure proper adjustments are taking place.

The most common cause of loss of braking in air brakes is POOR BRAKE ADJUSTMENT. Now I know our Discovery's have auto adjusters but you still need to know if these little wonders are doing their job. Freightliner tells me that these devices cause very little problem and work well, and I do not check my system every time, but occasionally I will check for push rod travel. 

A correctly adjusted brake will have one half inch or less of slack, leaving two inches of reserve chamber stroke. Three quarters of an inch can be deadly and this is why. 



At an 80 psi application, a brake chamber with three quarters inch of slack will stroke one and three quarters inch due to component stretch. This reduces the reserve chamber stroke to three quarters of an inch. 

Cast iron expands when heated. On the hot brake drum this can cause the chamber to stroke a further one half inch reducing the reserve stroke to one quarter inch. 

At high temps, brake linings wear rapidly. Ten thousands of and inch of lining wear (the thickness of a sheet of paper) causes the chamber to stroke a further one quarter inch, resulting in the chamber bottoming out and a probable brake failure. 

Even with cold drums, a vehicle with poorly adjusted brakes will have up to a 75 per cent longer stopping distance than normal. 


KEEP THIS IN MIND: Under normal light braking conditions even grossly maladjusted brakes seem to respond satisfactorily. It is only under moderate to heavy braking that poorly adjusted brakes will come to light. 

Slack Adjusters


Auto 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 accommodate 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 and their is way to do this properly to ensure your 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. This will be covered in the last section to show you how to do this without doing damage to your braking system. DO NOT RUN OUT AND DO IT NOW....THERE IS A PROCEDURE TO FOLLOW TO ENSURE THE SAFETY OF YOUR BRAKING SYSTEM. 

It is important that you check your push rod travel ONE INCH WHEN MANUALLY PULLED AND TWO INCHES WHEN YOU APPLY BRAKES. You must have your park brakes off to run this test so you must block your bus well to make sure there is not movement. If the rod travel is not up to par, you can not adjust them yourself, but will have to have a qualified brake mechanic do this for you. 

Obviously, this is a cumbersome thing to do as you have to crawl around on the ground etc. so I have my mechanic do this when I get the unit serviced...Brakes today are much better than they used to be and the auto slack system has proven to be a good system....and if you perform my pre trip test each time you drive the system should be adjusted to max ability all the time.