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Know Your Air Brakes

16th September 1966, Page 104
16th September 1966
Page 104
Page 104, 16th September 1966 — Know Your Air Brakes
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Which of the following most accurately describes the problem?

Part 26—Three-line Braking Fault Finding

exhaust would confirm two things: an air leak indicating trouble at the relay piston or ball valve; and excessive oil loss, indicating a compressor worn enough at rings or bore to pass oil rather than compress air. If all is correct at this point, open the input to the compressor straight to atmosphere without filters, and then check pressure build-up and pumping time, as very often the input filter can be the cause of the slow build-up.

With these points cleared and no fault found, it is fairly obvious that a leakage exists between compressor, reservoir or brake valve. So clear the obvious points of leakage first, drain cock, pipe joints, .hose or warning device.

With the engine running, it is hard to detect an air leak by sound, yet on stopping the engine no sound may be heard if the fault is between relay valve and pump, and the surest test can still be soapy water applied with a plastic spray bottle. The compressormounted safety valve has fooled most of us one time or another by its occasional habit of leaking when the engine is running and causing vibration, so give it a squirt also.

"Loss of pressure when stationary": This is very often the non-return valve section of the unloader, but can be a fault in a brake valve or a loose or cracked joint. "Slow brake application": Apart from trouble in the hub section—i.e., linings, drums or adjustment, which would show up when either system was operated—and eliminating the possibility of air loss at brake pipes or chambers, two other points mainly contribute to this fault: low line pressure, which will mean that the brake valve unit requires adjustment; or—quite a common feature—brake pipe lines that are partially closed and reduced in I.D. by grease and dirt. Here it is often necessary to remove and clean them thoroughly, and with steel pipes, warming them is often the best way to loosen the glue-like coating.

"Slow release": With the drum mechanism in order, this fault usually lies in blocked or restricted inlet or exhaust valves, particularly the disc type seating badly.

"Binding brakes": If the shoe and expander mechanism is in order, this fault is usually due to a defective brake valve, and air loss will be audible on removing pipes to brake chambers. In the case of sudden and severe retardation on a brake valve being opened slightly, this valve is seriously out of adjustment, and should have very early attention.

To finalize this series on your air brakes, I would like to state that it is intended to help make up for an obvious shortage of data on, or near, the shop floor. And as equipment is now becoming more and more of a standard pattern, it is a good plan to keep in close touch with the equipment manufacturers, who release first-class manuals. I find that good photostat copies can be made from these, and the main items can be given a coat of clear varnish that will give long service in foreman's office or on the notice board.

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