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Parts Of An Air-Brake System: Part 4

5.1.9 – Supply Pressure Gauges

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.

5.1.10 – Application Pressure Gauge

This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.

5.1.11 – Low Air Pressure Warning

A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer may also come on.

Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops below 60 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 60 psi. The manual reset type must be placed in the “out of view” position manually. It will not stay in place until the pressure in the system is above 60 psi.

On large buses, it is common for the low-pressure warning devices to signal at 80-85 psi.

5.1.12 – Stop Light Switch

Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.

5.1.13 – Front Brake Limiting Valve

Some older vehicles (made before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked “normal” and “slippery.” When you put the control in the “slippery” position, the limiting valve cuts the “normal” air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front-wheel braking is good under all conditions. Tests have shown front-wheel skids from braking are not likely, even on ice. Make sure the control is in the “normal” position to have normal stopping power.

Many vehicles have automatic front-wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure). These valves cannot be controlled by the driver.

Multiple-Choice Questions:

Question #252 (1 of 6)

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In an air brake system, what is a wig-wag?

  • It indicates your brakes linings have worn thin and need replacement
  • It adjusts the brakes on the trailer by sliding the adjustment arm back and forth
  • It's a device that helps keep the gladhands in place when connecting the air lines to the trailer
  • It drops a mechanical arm into your view when the pressure in the system drops below 60 psi.

Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops below 60 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 60 psi. The manual reset type must be placed in the “out of view” position manually. It will not stay in place until the pressure in the system is above 60 psi.

You will never see one of these in a modern tractor-trailer, but they may ask about it on the test. I remember our school buses having these when I was a kid (in the 1970's)
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Question #253 (2 of 6)

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What is a stop light switch?

  • It's a switch inside the tractor's fuel tanks that shuts off the fuel pumps when the tanks are nearly full
  • An electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.
  • A switch that activates a fuel increase for pulling away from a dead stop, like at stop lights or stop signs.
  • A switch that turns off the running lights when the brake lights are activated

5.1.12 – Stop Light Switch

Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.

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Question #249 (3 of 6)

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In an air brake system, what is the difference between the supply pressure gauge and the application pressure gauge?

  • The application pressure gauge tells you how much pressure is in the air tanks. The supply pressure gauge tells you how much air pressure you are applying to the brakes.
  • The supply pressure gauge tells you what volume of air is in the air tanks. The application pressure gauge tells you what volume of air you are applying to the brakes.
  • None of these are correct
  • The supply pressure gauge tells you how much pressure is in the air tanks. The application pressure gauge tells you how much air pressure you are applying to the brakes.

5.1.9 – Supply Pressure Gauges

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.

5.1.10 – Application Pressure Gauge

This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.

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Question #255 (4 of 6)

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The need for increased brake pressure can be caused by:

  • Mechanical problems
  • The brakes being out of adjustment
  • Air leaks
  • All of these are correct
Increasing application pressure to hold the same speed means the brakes are fading. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.
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Question #254 (5 of 6)

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Increasing brake application pressure to hold the same speed can mean what?

  • The brakes are properly cooled off
  • All of these are correct
  • The brakes are fading
  • The brakes are working properly
Increasing application pressure to hold the same speed means the brakes are fading. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.
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Question #250 (6 of 6)

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In an air brake system, the low air pressure warning signal:

  • Is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 200 psi (or one-half the compressor governor cutout pressure on older vehicles)
  • Is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles)
  • Is optional on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles)
  • None of these are true

A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer may also come on.

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