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This is due to the design of the wheel cylinder cup。 In order to prevent this from happening, older master cylinders were equipped with residual pressure check valves in the drum brake circuit s 。 These valves would close when hydraulic system pressure dropped to 3-9 psi。 This pressure kept the wheel cylinder cups expanded and sealed。 Newer models use cup expanders or the wheel cylinder spring to eliminate the need for these valves。 When the government mandated that vehicles be equipped with dual hydraulic systems in the 1960’s, they also directed manufacturers to install a warning system to alert drivers that a hydraulic system failure had occurred。

The warning system used by most manufacturers is triggered by the pressure differential valve。 This valve is installed between the front and rear hydraulic circuits and monitors pressure on each。 In the event of a leak in one half of the system, the valve shifts positions towards the low pressure area and activates a red warning light on the dash。 Some pressure differential valves have small springs at each end that will allow them to self-center when the system is repaired。 Others must be manually centered by bleeding the brakes。

Many cars now use a fluid level switch in the master cylinder to eliminate the need for the pressure differential valve。 If the system develops a leak, the fluid level in the reservoir will drop and a magnetic switch will close turning on the warning light。 On most vehicles, several of these control devices may be located in one part。 Known as a combination valve, this part may be located on a bracket near the master cylinder, on the frame under the drivers floorboard, or on the front cross member。

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