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Air brakes - supply, control and the spring brake
Air brakes are built so that losing air applies the brakes rather than releasing them. Once you see that inversion - springs stop the truck, air holds them off - the whole system and every fault it can have starts to make sense.
Supply: build it, dry it, protect it
The compressor charges the system and the governor decides when it loads and unloads, typically cutting out around 130 psi and cutting back in near 100. A dryer removes moisture before it reaches the tanks.
Tanks are split and protected by check valves so a failure in one circuit cannot drain the others. That split is why a truck with a blown service line can still stop.
The spring brake fails safe
A powerful spring applies the parking brake mechanically, and air pressure compresses that spring to release it. Lose air and the spring wins - the brakes apply.
It is also why you must never cage or disassemble a spring brake chamber casually. That spring holds enough stored energy to kill you.
Leakage and stroke are what inspectors measure
Static leakage is checked with the engine off and the brakes released; applied leakage with the pedal held. Both have limits, and both are among the most common out-of-service items.
Pushrod stroke matters just as much. As linings wear the stroke grows, and past the adjustment limit the chamber can no longer generate rated force even though system pressure looks perfect.
How to test it
- Build and check the governorNote cut-out and cut-in pressures against spec.
- Static leak testEngine off, full pressure, brakes released. Watch the drop over one minute.
- Applied leak testHold the pedal down and repeat. The limit is higher, but still tight.
- Check warning and protectionWarning device by about 60 psi, and spring brakes should apply automatically as pressure falls further.
- Measure pushrod strokeCompare against the marked limit for that chamber size - a long stroke is a brake that cannot do its job.
Numbers to work to
| Governor cut-out | about 120-135 psi | check the chassis spec |
| Governor cut-in | about 100-110 psi | roughly 20-25 psi below cut-out |
| Low-pressure warning | by about 60 psi | light and buzzer |
| Spring brakes apply | about 20-45 psi | automatic as pressure decays |
| Static leakage, single vehicle | 2 psi per minute or less | 3 psi for a combination |
| Applied leakage, single vehicle | 3 psi per minute or less | 4 psi for a combination |
Common mistakes
- Servicing a spring brake chamber without caging it - the stored spring energy is lethal.
- Passing a truck on pressure alone while pushrod stroke is beyond the adjustment limit.
- Ignoring water in the tanks, which means the dryer is finished and corrosion is under way.
- Assuming manual slack adjusters self-adjust, or that automatic ones never need checking.
Drill this until it's automatic
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