BC Transit – Air Brake Course

The Modulator Valve (primary air loss)
When the primary system is affected by an air loss, the rear service brakes will not work.
Remembering that the spring brakes contain potential energy and the ability to take the
place of the rear service brakes, let’s take a look at how the modulator valve facilitates
this function. First, we will open the drain cock on the primary reservoir to simulate a loss
in the primary system. The one-way check valve at the secondary reservoir closes. The
two-way check valve moved down to protect the secondary reservoir. Secondary air only
now holds the park brake released. …next slide...
Left click for animation
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