Chap70

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 70, the reader should be able to: Continued

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 70, the reader should be able to:

RESIDUAL CHECK VALVE ,[object Object],Continued Figure 70–1 Most residual check valves are located under the tubing seals in the master cylinder outlet ports. The check valve is located in the master cylinder at the outlet for the drum brakes. The ball and spring in the check valve permit all the brake fluid to return to the master cylinder until the designated pressure is reached.

Figure 70–2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system. Continued

[object Object],Figure 70–3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today.

[object Object],PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH) Figure 70–4 A red brake warning lamp. Continued The brake lines from both front and rear sections of the master cylinder are sent to this switch It lights the brake warning indicator lamp in the event of a “difference in pressure.” See Figure 70–5.

Figure 70–5 A leak in the hydraulic system causes unequal pressures between the two different brake circuits. This difference in pressures causes the plunger inside the pressure-differential switch to move, which completes the electrical circuit for the red brake warning lamp. (Courtesy of Ford Motor Company) ,[object Object],Continued ,[object Object],[object Object]

Figure 70–6 The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit. ,[object Object]

BRAKE FLUID LEVEL SENSOR SWITCH ,[object Object],[object Object],Continued

Figure 70–7 A movable contact brake fluid level switch. Figure 70–8 A magnetic brake fluid level switch. NOTE: Brake systems use either a pressure-differential switch or a low brake fluid switch to light the dash red “brake” lamp, but not both.

[object Object],DIAGNOSING A RED “BRAKE” DASH WARNING LAMP Continued 1. Parking Brake “ On .” The same dash warning lamp is used to warn the driver that the parking brake is on. 2. Low Brake Fluid . This lights the red dash warning lamp on vehicles equipped with a master cylinder reservoir brake fluid level switch. 3. Unequal Brake Pressure . The pressure-differential switch is used on most vehicles with a front/rear brake split system to warn the driver whenever there is low brake pressure to either the front or rear brakes.

[object Object],Continued Step #1 Check the Level of the Brake Fluid If low, carefully inspect entire hydraulic brake system for leaks; repair as necessary. Step #2 Disconnect the Wire from the Pressure - Differential Switch If the lamp is still “on,” the problem is the parking brake lever switch, or the wire going to the switch, shorted to ground. If the red brake warning lamp is “off” after being disconnected from the pressure-differential switch, the problem is due to a hydraulic failure (a low pressure in either front or rear system creating a pressure difference of at least 150 psi).

NOTE: Many Japanese vehicles energize the relay that turns off the red “ brake” warning lamp from the output terminal of the alternator. If a quick inspection of the brake system seems to indicate that everything is okay, check for correct charging voltage before continuing a more detailed brake system inspection.

PROPORTIONING VALVE ,[object Object],[object Object],Continued

Figure 70–9 Many proportioning valves are mounted directly to the master cylinder in the outlet to the rear brakes. Continued

[object Object],Continued ,[object Object],[object Object],A proportioning valve is used to compensate for these differences because it is easier to reduce pressure to the rear brakes than to increase pressure to the front brakes.

Figure 70–10 Typical proportioner valve pressure relationship. Note that, at low pressures, the pressure is the same to the rear brakes as is applied to the front brakes. After the split point, only a percentage (called the slope) of the master cylinder pressure is applied to the rear brakes. ,[object Object],During light or moderate braking, there is no rear wheel locking a problem. Before proportioning will begin, hydraulic pressure must reach a level called the split point . Continued

Figure 70–11 A Chrysler proportioning valve. Note that slope and split point are stamped on the housing. ,[object Object],Figure 70–12 These two proportioning valves are found under the vehicle on this Dodge minivan. Below the split point full system pressure is supplied to the rear brakes. Continued

[object Object],[object Object],Continued

Figure 70–13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes. Continued

Figure 70–14 At the split point, the proportioning valve piston closes the fluid passage through the valve. Continued

[object Object],Always Inspect Both Front and Rear Brakes A locked wheel has less traction with the road than a rotating wheel. As result, if the rear wheels become locked, the rear of the vehicle often “comes around” or “fishtails,” causing the vehicle to skid. Careful inspection of the entire braking system is required to be assured of a safe vehicle. Do not stop there—always inspect the front brakes, too. If the front brakes are rusted or corroded, they cannot operate efficiently and greater force must be exerted by the driver to stop the vehicle. Even if the proportioning valve is functioning correctly, the higher brake pedal pressure by the driver could easily cause the rear brakes to lock up.

HEIGHT-SENSING PROPORTIONING VALVES ,[object Object],[object Object],Continued

Figure 70–15 A height-sensing proportioning valve provides the vehicle with variable brake balance. The valve allows higher pressure to be applied to the rear brakes when the vehicle is heavily loaded and less pressure when the vehicle is lightly loaded. Continued

Figure 70–16 A stepped cam is used to alter the split point of this height-sensing proportioning valve. CAUTION: Some vehicle manufacturers warn that service techs should never install replacement air lift shock absorbers or springs that result in a vehicle height difference than specified by the manufacturer.

PROPORTIONING VALVE ADJUSTMENT ,[object Object],Continued

[object Object],PROPORTIONING VALVE DIAGNOSIS AND TESTING Continued

Figure 70–17 A proportioning valve pressure test can be performed using two pressure gauges—one to register the pressure from the master cylinder and the other gauge to read the pressure being applied to the rear brakes. This test has to be repeated in order to read the pressure to each rear wheel. Continued

ELECTRONIC BRAKE PROPORTIONING ,[object Object],Continued

METERING VALVE (HOLD-OFF) OPERATION ,[object Object],Continued

Figure 70–18 A metering valve when the brakes are not applied. Notice the brake fluid can flow through the metering valve to compensate for brake fluid expansion and contraction that occurs with changes in temperature. Continued

Figure 70–19 A metering valve under light brake pedal application. Continued

Figure 70–20 A metering valve during a normal brake application. Continued

[object Object],No Valves Can Cause a Pull Just remember that if a vehicle pulls during braking that the problem has to be due to an individual wheel brake or brake line. The master cylinder and all the valves control front or rear brakes together or diagonal brakes and cannot cause a pull if not functioning correctly. NOTE: Neither the metering valve nor the proportioning valve can cause a pull to one side if defective. The metering valve controls both front brakes, and the proportioning valve controls both rear brakes. A defective master cylinder cannot cause a pull either. Therefore, if a vehicle pulls to one side during a stop, look for problems in the individual wheel brakes, hoses, or suspension.

SYSTEMS WITHOUT METERING VALVES ,[object Object],Continued ,[object Object],[object Object],[object Object]

METERING VALVE DIAGNOSIS AND TESTING ,[object Object],Continued If the front brakes lock up during these conditions, the front wheels cannot be steered. Inspect the metering valve for these conditions: ,[object Object],[object Object],More accurate testing of the metering valve can be accomplished using pressure gauges.

[object Object],NOTE: Braking systems that are diagonal split, such as those found on most front-wheel-drive vehicles, do not use a metering valve. A metering valve is only used on front/rear split braking systems such as those found on most rear-wheel-drive vehicles. When pressing the brake pedal, both gauges should read the same until 3 to 30 psi (20 to 200 kPa) when the metering valve shuts, delaying operation of the front brakes. The master cylinder gauge should show an increase in as the brake pedal is pressed further. Once 75 to 300 psi is reached, the gauge showing pressure to the front brakes should match the pressure from the master cylinder. If the pressures do not match these ranges, the metering valve assembly should be replaced.

[object Object],Push - In or Pull - Out Metering Valve? To bypass the metering valve, the service technician has to push or pull a small button located on the metering valve. An easy way to remember whether to push in or to pull out is to inspect the button itself. If the button is rubber coated, then you push in. If the button is steel, then pull out. Special tools allow the metering valve to be held in the bypass position. Failure to remove the tool after bleeding the brakes can result in premature application of the front brakes before the rear drum brakes have enough pressure to operate.

COMBINATION VALVE ,[object Object],Continued Figure 70–2 Typical two-function combination valves. ,[object Object],[object Object],[object Object],On a rear-wheel-drive vehicle, a typical combination valve consists of these components, all in one replaceable unit:

[object Object],Figure 70–22 Combination valve containing metering, pressure-differential (warning switch), and proportioning valves all in one unit. This style is often called a “pistol grip” design because the valve section resembles the grip section of a hand gun. Continued

[object Object],BRAKE LIGHT SWITCH Figure 70–23 Typical brake light switches. CAUTION: Check service information for procedures to follow when replacing and/or adjusting a brake switch to ensure proper operation. Mechanical switches operating directly off the brake pedal arm are most often used. Brake light switches are normally open; when brakes apply, the switch closes, completing the light circuit.

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],( cont. )

Chap70

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