Figure 7-58. - Stoplight switch.
stoplight circuit when the brakes are applied. When air pressure from the treadle valve enters the cavity on the one side of the diaphragm, the diaphragm changes position. This action overcomes the force of the spring and moves the contact plunger until the contacts close. This closes the stoplight electrical circuit causing the brake lights to come on. The switch is designed to close as soon as 5 psi is delivered to it. This means that the stoplight circuit closes immediately on brake application.
Servicing is the most important part of air brake maintenance. If the air brake system is kept clean. tight, and moisture-free, brake failures will be few and far between. Particular care must be taken to keep the air compressor intake filters clean and foreign material out of the lines.
The basic test made to an air brake system is the operational test. This test may be performed on the road or in the shop. During an operational test, the brakes are applied and released while observing for equal application, sluggish engagement or release, binding linkage, and exhaust of units.
To check the leakage of the overall system, fully charge the system. shut off the ignition, and observe the pressure drop on the gauge mounted on the vehicle dash. The maximum leakage will be expressed in pounds per a specific time.
Before making any leakage or pressure test. consult the manufacturer's specifications for correct pressure and maximum leakage.
To determine if leakage of various components is within permissible or authorized limits, use the soapsuds test. To make this test, use a thick mixture of soapsuds: do not use lye soap. This misture is applied to places in the system where leakage may occur. 7-44 While some places are authorized some amount of leakage, others are not. For example, castings and the tube in the governor should have no leakage. Points with authorized leakage will have a specified maximum in pounds per a specified time.
Soapsuds can also be used to check the internal condition of a component. By covering exhaust ports or casting openings, you can check the condition of the diaphragms and valves. For example, to check the condition of the treadle valve, release the brakes and cover the exhaust ports with soapsuds. Engage the brakes: any leakage indicates the valve is not sealing properly. If the diaphragm in the brake chamber is faulty, leakage will appear around the pushrod with the brakes applied.
As with the drum brake system the linings used with air brakes gradually wear from use and require periodic adjustment or replacement. Always consult the manufacturer's specifications before making any adjustments to the air brake system. This is to ensure that the correct adjustment is made and that any variations in procedure are followed.
Q1. What are the three common methods for driving the air compressor from the engine?
Q2. What is the purpose of the governor in an air brake system?
Q3. What component controls the compressor unloading mechanism?
Q4. On a type O-1 air compressor the pressure range may be adjusted by adding or removing shims from what location?
Q5. What valve controls the air pressure delivered to the brake chambers?
Q6. What is the function of the relay emergency valve?
Q7. What component is used to prevent dirt and moisture entering unused air lines?
Learning Objective: Describe the operation, terms, and component functions of an air-over-hydraulic brake system.Continue Reading