Thread: FC Models
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Old 04-21-2010
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rselin rselin is offline
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Join Date: Mar 2010
Location: Calgary
Posts: 796

The air brakes are a piggyback style with both air and spring actuation. Under normal use air pressure keeps the springs retracted, and air actuates the brakes normally. If you lose your air (down to between 20-45 psi), the spring actuators are no longer held back by air pressure and they will engage the brakes. If that happens, you aren't going anywhere. A low air pressure alarm will sound at 60 psi and that tells you to get off the road before the spring brakes engage. It's a failsafe system.

In addition to the main supply reservoir off the air dryer, there are separate air reservoirs for the front and rear axles. These are the secondary and primary systems. They are isolated by check valves so that if you lose one system, you don't bleed air from the other system. However, the air in one system will keep the spring actuators released, on brakes on both systems, in the event of a failure in the other system.

So there is adequate redundancy, both to ensure that the brakes will engage on a total loss of air, and to ensure you have time to get to the side of the road in case of loss of air.

This applies to FMVSS 121 brakes, which I think have been the standard since the early to mid-70s.
Richard Selin
Former owner 1992 SP36
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