BC Transit – Air Brake Course

The Modulator Valve (continued)
The modulator valve is never required to fulfil its function until the system suffers a primary air loss.
To understand its function in the system, let’s first look at normal operation. When the system is
charged and operating normally, primary air pressure is present at the modulator valve as indicated
by the green arrow. Air pressure from the two-way check valve (blended air) flows through modulator
valve and on to the spring brakes, holding them released (orange arrow.) Application air from the
front brakes arrives at the point indicated by the magenta arrow.
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