Problem: Red brake warning light on after service on vehicles equipped with pressure differential switch.

Cause: Pressure differential switch piston is offset causing pressure differential switch to remain in grounded position.

Solution: The solution applied will depend on what type of pressure differential switch the vehicle is equipped with. Most pressure differential switches have only one function – turn the red warning light on if a hydraulic failure occurs in either the primary or secondary circuits. Some others have an additional function of limiting fluid loss to the rear brake circuit in the event of a rear circuit failure.

Conventional pressure differential switch (fluid flow to rear brake):

Bleed the system using manufacturer’s sequence until all air is removed. If pressure differential switch does not re-center itself during bleeding create a pressure loss opposite of the original hydraulic failure. For example, if the original failure were a broken rear brake line you would open either front bleeder to create this difference. Once open, spike the brake pedal a couple of times while watching the red warning light (key on) using caution not to exceed ¾ pedal travel to prevent master cylinder damage. The difference in pressure should push the piston back to center.

Special purpose pressure differential switch (restricted fluid flow to rear brakes)


This type of pressure differential switch will not allow the system to be bled until the piston is re-centered. To re-center this type bleed the system from the master cylinder down to the valve’s inlet until no air is seen. This is best accomplished by cracking the line fitting at both the master outlet and valve inlet. Once bled spike the brake pedal a couple of times while watching the red warning light (key on) using caution not to exceed ¾ pedal travel to prevent master cylinder damage. The difference in pressure should push the piston back to center. Once re-centered complete the rep air by bleeding the rear brakes.

More Info: To better understand the fix an understanding of the pressure differential switch’s function is in order. Figure 51.1 is a cross sectional view of a stand-alone pressure differential switch. The pressure differential switch consists of a piston exposed to both primary and secondary circuit pressures and a normally open switch. The switch consists of the switch body,a spring- loaded plunger and a contact pin.

Screen Shot 2016-04-12 at 8.34.18 AM  Screen Shot 2016-04-12 at 8.35.13 AM Figure 51.1

The piston has what is called a detent in the center of it. The detent is a beveled indentation where the switch plunger rests. The switch plunger is held in place by the spring tension of the switch. Each end of the piston is fitted with o-rings to form a seal from both the primary and secondary circuit pressures. One end of the piston is exposed to primary circuit pressures while the opposite end is exposed to secondary circuit pressures.
Normal system operation produces near equal pressures in both the primary and secondary hydraulic circuits. The spring loaded plunder on the switch keeps the piston from moving under minor pressure differences. Typically it will prevent movement until the pressure difference reaches approximately 150 psi.
If a hydraulic failure occurs in either the primary or secondary circuits a pressure difference will be created. The high pressure on one side of the piston will push the piston towards the failed side or low pressure circuit (See Figure 51.2). The movement of the piston will cause the open switch to go to closed position which completes the ground to the red warning light. The light will remain on until the piston is re-centered.

Screen Shot 2016-04-12 at 8.40.27 AM Figure 51.2

Most vehicles will never experience a hydraulic failure and therefore the pressure differential piston will never move. This lack of movement is responsible for the piston “sticking” when it is offset due to a hydraulic failure. The piston bore corrodes and causes an interference fit.

Special Purpose Pressure Differential Switches

There is a group of specialized pressure differential switches that have an additional function in the event of a rear circuit failure. In addition to turning the red warning light on they also restrict fluid flow to the rear brakes in the vent of a rear circuit hydraulic failure. A typical method is shown in Figure 51.3. Figure 51.3 shows the piston in the neutral position. Fluid flow to the rear brakes is through the passages marked as such. Figure 51.4 shows what happens to theses passages when a rear circuit hydraulic failure
occurs. The piston offsets and covers the holes. This restricts fluid flow to the rear circuit which limits fluid loss.

Screen Shot 2016-04-12 at 8.43.29 AM Figure 51.3  Screen Shot 2016-04-12 at 8.44.16 AM Figure 51.4

If a rear circuit hydraulic failure occurs and the piston does not re-center the rear brakes will not be able to be bled until the piston is re-centered. See procedure under “Solution”.

Brake Job Done Well

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