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Components of the supply circuit of a CMVSS 121 air brake system


The following are the major components of the supply system.

The compressor

The compressor is driven by the engine and provides a flow of compressed air to the reservoirs. The compressor must have sufficient capacity to build air pressure from 345 kPa (50 PSI) to 621 kPa (90 PSI) in three minutes or less with the engine running at 1,200 RPM. This is known as the compressor build-up time and is used to measure the compressor's performance.

The governor

The governor limits the maximum air pressure that can be reached in the system. Maximum air pressure on vehicles manufactured after 1975 is 828 to 897 kPa (120 to 130 PSI). The governor stops the air flow from the compressor when the maximum pressure is reached. Maximum air pressure is often referred to as cut-out pressure, the point when the governor cuts-out the compressor, preventing further air pressure to build-up. As air is drawn from the reservoirs, the air pressure in the system drops. When system pressure drops approximately 173 kPa (25 PSI), the governor causes the compressor to cut-in and supply air to the reservoirs, which increases the system pressure.

The supply reservoir

The supply reservoir is a place for the incoming air to cool. This causes any moisture vapour in the air to condense. The supply reservoir is a gathering place for other contamination, such as oil. All reservoirs should be drained daily to prevent contamination from entering the system and interfering with its operation. The drain cocks should be opened fully to allow all the air to escape.

 


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