| 摘要 |
A valve for a tractor air brake system for changing the pressure delivered to air brakes on the rear axle depending on whether the trailor is attached or whether the tractor is operating in the "bob tail" mode. According to the invention, a rear axle ratio control valve provides an automatic selection of input to output ratio in accordance with the presence or absence of trailor emergency pressure. In the ratio control valve, service brake control pressure always acts against a first effective area of a piston for providing a first ratio of input to output. A second effective area of the piston receives pressure with the presence of the trailer emergency so that the ratio of pressure delivered from the valve will be changed. Further in accordance with the invention, a blend-back valve operates at a higher, predetermined pressure range, even when the pressure communication is closed by the absence of the emergency pressure to enable the valve to deliver the maximum pressure at high input pressure.
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