Forest, you have two air pressure gauges, a Supply and a Brake. The Brake has twin needles. The supply gauge shows the air pressure in the wet/auxiliary/main tank. This tank gets air from the air compressors, both the engine mounted compressor and the 120 volt compressor. From the wet/auxiliary/main air tank, air flows through a check valve into two brake tanks. The two needles on the Brake tank show the air pressure in each of the two brake air tanks. One of these tanks feeds air to the front brakes while the other tank feeds air to the rear brakes and tag brakes. Since it takes more air to activate the four chambers on the rear/tag brakes than the two on the front brakes, the rear gauge will typically drop a little faster. However, if the adjustment is off on the slack adjusters, that might not be the case as more air would have to be used to push the rod before some of the brakes start to activate.
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