The wheel cylinder seals are prone to leakage from deterioration. This condition may be checked on the truck without disassembly. After the truck is elevated and supported safely, remove the rear wheels. Remove the brake drum on each side.
Carefully lift the edge of each boot away from the body of the cylinder. Inspect the inside of the boot and the edge/end of the cylinder. You can expect a very slight (normal) moistness in the area-usually covered in dust-but any sign of wet fluid is cause for immediate repair. A leak, no matter how slight, can significantly reduce the braking effort on that wheel. It can also admit air into the brake system, causing poor pedal feel.
REMOVAL & INSTALLATION
See Figures 1, 2, 3 and 4
- Remove the brake drums and shoes as described previously. Although some vehicles will allow the cylinder to be removed with the brake shoes in place, this is not recommended. Chances are good that the brake shoes will become soaked with brake fluid. Contaminated brake shoes will need replacement so you'll have to take them off anyway.
- Place a drain pan under the work area and disconnect the brake line at its connection to the wheel cylinder (behind the backing plate). Plug the open end of the brake line.
Work carefully and use a line wrench on the fitting. This fitting is usually rusted or otherwise difficult to turn. Don't damage the brake line.
- Remove the two bolts that fasten the wheel cylinder to the backing plate and remove the wheel cylinder. Again, these bolts may be harder than usual to turn.
- Install the cylinder and tighten the mounting bolts to 13 ft. lbs. (18 Nm). This is just enough to hold the cylinder in place; overtightening will strip the bolt holes.
- Connect the brake line to the cylinder.
- Install the shoes and drums correctly. Make sure the self-adjusters have taken up play so the brakes actuate normally.
- Bleed the system thoroughly before operating the vehicle.
This procedure requires a special piston cup installing tool MB990623-01 for 2-wheel drive Pick-ups and all Monteros, 4-wheel drive Pick-ups need MB 990621-01. A generous supply of DOT 3 brake fluid, alcohol or aerosol brake cleaner should be available for cleaning purposes.
- Remove the wheel cylinder.
- On either side, work the rubber boot off with a soft, blunt instrument such as a rounded piece of wood and then remove the piston.
- Inspect the piston and cylinder walls for corrosion or scoring and replace parts that are defective. Check the clearance between the piston and cylinder wall. (Subtract the outer diameter of the piston from the inner diameter of the bore.) The limit is 0.006 in. (0.15mm). If clearance exceeds that value, replace the parts. Use inside and outside micrometers to make the measurements.
Clean all the parts with either brake cleaning spray or clean brake fluid. Clean the pistons and cylinder wall with DOT 3 brake fluid. If the piston/cylinder can be reused, replace the piston cup as follows:
- Work the cup off the end of the piston without causing damage.
- Apply the special grease included in the rebuild kit to the new piston cup and the special tool. If no grease is provided, use clean brake fluid.
- Set the piston on the workbench with one end up. Mount the special tool on top of the piston. Put the piston cup over the special tool with its lip facing upward.
- Slide the cup slowly and steadily down the special tool and into the groove of the piston. Be careful not to stop partway down! The idea is one smooth motion without stretching or ripping the cup.
- Repeat the operation for the other piston.
- Coat the wheel cylinder bore and and piston cups either with the corrosion preventive agent contained in the repair kit or with clean brake fluid.
- Carefully install the pistons into the wheel cylinder.
- Apply the grease included in the repair kit to both of the piston ends and then install new rubber boots on both ends. Do not use other greases; the seals may be damaged.
- Install the wheel cylinder.