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All vehicles with air brakes have a pressure gauge connected to the tank. If the vehicle has a air brake system, there will be a gauge for each of the system (or a single gauge with needles). Dual systems will be discussed later. These gauges tell you how much is in the air tanks.
This gauge shows how much air pressure you are applying to the . (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are . You should slow down and use a gear. The need for increased pressure can also be caused by brakes out of , air or mechanical problems.
A low air pressure warning signal is required on vehicles with brakes. A warning signal you can see must come on the air pressure in the tanks falls below psi (or the compressor governor cut-out pressure on older vehicles). The warning is usually a light. A may also come on.
Another type of warning is the “wig wag.” This device drops a mechanical into your view when the pressure in the system drops psi. An automatic wig wag will rise out of your view when the pressure in the system goes 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 psi.
On large buses it is common for the low pressure warning devices to signal at to psi.
Drivers behind you must be 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 lights when you put on the air brakes.
Some older vehicles (made before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked “” and “”. When you put the control in the “slippery” position, the limiting valve cuts the “” air pressure to the front brakes by . Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping of the vehicle. Front-wheel braking is good under conditions. Tests have shown front-wheel skids from braking are not likely, even on . Make sure the control is in the “” position to have normal stopping power.
Many vehicles have automatic front-wheel limiting valves. They the air to the front brakes except when the brakes are put on very ( psi or more application pressure). These valves be controlled by the driver.
All trucks, truck tractors and buses must be equipped with brakes and brakes. They must be held on by mechanical force (because air pressure can eventually away). Spring brakes are usually used to meet these needs. When driving, powerful are held back by air pressure. If the air pressure is removed, the springs put on the . A parking brake control in the cab allows the driver to let the out of the spring brakes. This lets the put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the to put on the brakes. Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of to psi. Do not wait for the brakes to come on automatically. When the air pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away while you can still control the brakes. The braking power of spring brakes depends on the brakes being in . If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.
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