Hard to stop ABS light on

Tiny
GREENHORNIT007
  • MEMBER
  • 1993 FORD F-350
  • 7.3L
  • V8
  • 2WD
  • AUTOMATIC
  • 300,000 MILES
Truck is hard to stop. Pedal stiff and ABS light is on.
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Tuesday, August 20th, 2019 AT 1:03 AM

1 Reply

Tiny
ASEMASTER6371
  • EXPERT
Good morning,

The ABS has nothing to do with this issue. It is basic brakes that is the issue.

Check the vacuum supplied to the booster. Remove the hose and see if you have a good supply of vacuum to the hose. If you do, then you need a booster. If you do not, you have a vacuum supply issue to the booster.

I attached a picture and procedure for replacing the vacuum pump. The diesel uses a belt-driven pump.

As far as the ABS, you will need to have the system checked for codes. The most common failure is a bad sensor.

Roy

The single and tandem vacuum boosters are self contained vacuum hydraulic power braking units. These are vacuum suspended units that use vacuum and atmospheric pressure for their power.
On gasoline engine vehicles, vacuum is supplied through a fitting in the intake manifold. On diesel engine vehicles, vacuum is supplied through a vacuum pump.
The three basic elements of the booster are the vacuum power chamber, mechanically actuated booster check valve and a hydraulic dual master cylinder which supplies hydraulic pressure to the brake system.
The vacuum power chamber consists of a front and rear shell, diaphragm, diaphragm plate, hydraulic pushrod and vacuum diaphragm return spring.
The mechanically actuated booster check valve controls degree of power brake application in accordance with foot pressure applied to valve operating rod through brake pedal linkage. This valve is integral with the vacuum power diaphragm.

Procedure

Make sure booster rubber reaction disc is properly installed as shown in Fig. 2, if the master cylinder pushrod is removed or accidentally pulled out. A dislodged disc may cause excessive pedal travel and extreme operation sensitivity. The disc is black compared to the silver colored valve plunger that will be exposed after pushrod and front seal are removed. The booster unit is serviced as an assembly and must be replaced if the reaction disc cannot be properly installed and aligned, or if it cannot be located within the unit itself.
1. Disconnect battery ground cable.
2. Disconnect stop lamp switch electrical connectors.
3. Support underside of master cylinder, then remove power brake unit-to-master cylinder attaching nuts.
4. Remove vacuum hose between manifold and power brake unit, or power brake unit check valve. Remove check valve, if equipped.
5. Separate power brake unit and master cylinder, leaving master cylinder supported far enough away to allow removal of power brake unit.
6. On models equipped with pushrod mounted stop lamp switch, remove retaining pin, then slide switch, pushrod, spacers and bushing off brake pedal pin.
7. On models equipped with brake pedal mounted stop lamp switch, remove attaching bolt, nut and plastic bushing, then disconnect power brake unit pushrod from brake pedal.
8. On all models, remove power brake unit attaching bolts and the power brake unit.
9. Reverse procedure to install.

REMOVAL
1. Loosen the hose clamp vacuum pump inlet fitting.
2. Remove the hose from the vacuum pump inlet fitting.
3. Remove drive belt by placing a 15mm box end wrench on the automatic belt tensioner pulley bolt and rotating it counterclockwise until belt can be removed.
4. Remove vacuum pump retaining bolts and remove vacuum pump.

NOTE: The vacuum pump is not to be disassembled. It is only serviced as a unit. The pulley is serviced as a separate item.

INSTALLATION
1. Position vacuum pump and install retaining bolt. Tighten bolts to 24-31 Nm (18-23 ft lb).
2. Connect the hose from the manifold vacuum outlet fitting to the vacuum pump and install the hose clamp.
3. Install belt by placing a 15mm box end wrench on the automatic belt tensioner pulley bolt and rotate the pulley counterclockwise. Position belt on pulley.
- The vacuum pump is driven by the back (flat) side of the belt.
- The BRAKE light will glow until vacuum builds up to the normal level.
- If new belt is installed, recheck belt tension after running vacuum pump for five minutes.
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Tuesday, August 20th, 2019 AT 3:46 AM

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