OVERHAUL

NOTE: Special measuring tools are required for this procedure.

1. Support the master cylinder in a vise. Clamp the vise jaws to the bolt flange, not to the body of the cylinder.
2. If the fluid reservoir is attached to the cylinder, remove the reservoir retaining screw and gently lift the reservoir clear. If the reservoir is a remotely mounted one (1983–86 vehicles), remove the two screw clamps holding the nipples in place. Then disconnect the reservoir hoses from the nipples and from the reservoir itself.
3. Remove the small seals under the reservoir.
4. Use a long thin tool to press inward on the piston and remove the piston stopper bolt and gasket. The piston need only be pushed enough to take tension off the bolt.
5. Again pushing gently on the piston, use snapring pliers to remove the piston retaining clip at the end of the cylinder. Be ready to catch the piston as you release pressure on it.
6. Remove the primary piston assembly and set is aside. Remove the secondary piston assembly. If this is difficult, gradually apply a stream of compressed air from the outer port on the secondary end of the master cylinder.

   WARNING
   Do NOT disassemble the primary or secondary pistons!

7. Clean the components with either clean brake fluid or a special brake cleaning solvent. Do NOT use petroleum based solvents and do NOT use water.
8. Check the inner surface of the master cylinder for rust or pitting. Check the pistons for any signs of rust, scoring or wear.
9. Using a cylinder micrometer, measure the cylinder bore in six locations. The locations should be the approximate bottom, center and top of the bore and measurements should be taken of the height and width of the bore at each position. All six measurements should be virtually identical. Variations of more than 0.00098 in. (0.025mm) disqualify the unit from use. Record the bore diameter.
10. Measure each piston in two dimensions. Again, measurements must be virtually identical. Subtract the piston diameter from the bore diameter. If the difference is greater than 0.0059 in. (0.15mm), there is enough wear to warrant replacing both the cylinder and pistons. Always replace the cylinder and pistons as a unit; do not attempt to guess which component is worn and do not mix old and new parts.
11. If the cylinder measurements are proper, the inside may be cleaned with a piece of crocus cloth soaked in brake fluid. This will remove any glaze or very light scratches. Honing the master cylinder is specifically not recommended as it will affect the piston to wall clearance measured in Steps 9 and 10 above. If the cylinder bore is scratched or corroded enough to warrant honing, the unit should be replaced.

    Fig. 1: Mount the brake master cylinder in a vise so that only the mounting flange (and not the cylinder body) is in the jaws

    Fig. 2: Unfasten the reservoir mounting bolt, then remove the reservoir from the brake master cylinder

    Fig. 3: Remove the reservoir mounting seals — new seals will be needed for reassembly

    Fig. 4: While holding the piston assembly in, remove the piston stopper bolt

    Fig. 5: Using snapring pliers, remove the snapring from the piston hole in the master cylinder

    Fig. 6: Remove the primary piston assembly from the master cylinder — do not disassemble the piston assemblies themselves

    Fig. 7: Remove the secondary piston assembly — if the secondary piston is difficult to remove, apply compressed air to the secondary side outlet port

    Fig. 8: Make certain to note which components are mounted in which position

    Fig. 9: Check the clearance (A) between the brake booster pushrod and the primary piston — follow the special procedure

    Fig. 10: Using a Vernier caliper and straight scale (along the edge of the master cylinder), obtain dimension (B). Be sure to subtract the thickness of the scale

    Fig. 11: Measure dimension (C) from the master cylinder's brake booster installation surface to the edge

    Fig. 12: Measure dimension (D) from the brake booster's master cylinder installation surface to the end of the pushrod

    Fig. 13: If clearance is not within the standard value range, turn the pushrod screw to achieve desired length

12. Before reassembly, the piston assemblies and the cylinder bore should be liberally coated with clean fresh brake fluid.
13. Carefully insert the secondary piston into the bore, taking care not to gouge the bore or damage the seals. Insert the primary piston in similar fashion.
14. Push gently on the pistons to compress them and install the snapring.
15. Continue pushing on the pistons and install the piston stop bolt with a new O-ring (or gasket). Tighten the piston stopper bolt only to 1–2 ft. lbs. (1.5–3.0 Nm).
16. Install new reservoir seals on the cylinder and install either the fluid inlet ports or the reservoir. Tighten the reservoir retaining bolt to 2.2 ft. lbs. (3 Nm).
17. Before reinstallation, adjust the clearance between the brake booster pushrod and the the master cylinder primary piston. This dimension A is critical to the correct release of the master cylinder. Improper clearance will result in the brakes being partially applied under all conditions.
    1. Measure the distance between the master cylinder end face and the piston. This is most easily done by taking the measurement with a Vernier caliper and a straight scale (a piece of metallic bar stock whose thickness is uniform, such as a square) placed on the master cylinder end face. After measuring, subtract the thickness of the straight scale and record the measurement. Call this measurement B.
    2. Find the distance between the contact face of the master cylinder mounting flange and the end face of the master cylinder. Record this distance and call it dimension C.
    3. Measure the distance between the master cylinder mounting surface on the brake booster and the end of the booster pushrod. Record this measurement and call it dimension D.

       NOTE: Obtain dimension (D) by first placing a straight scale against the edge of the brake booster, then measuring and subtracting the thickness of the straight scale.

    4. Critical dimension A is found by using the equation A = B − C − D. The correct value for A is:
       • Montero: 0.004–0.020 in. (0.1–0.5mm)
       • 1983–88 2WD Pick-up: 0.015–0.031 in. (0.4–0.8mm)
       • 1987–88 4WD Pick-up: 0.028–0.043 in. (0.7–1.1mm)
       • 1989–91 Pick-up: 0.028–0.043 in. (0.7–1.1mm)
       • 1992–95 Pick-up: 0.004–0.020 in. (0.1–0.5mm)
18. If the free-play dimension is not within the acceptable range, adjust the length of the pushrod by carefully turning the adjustable (threaded) tip with pliers. After adjusting the rod, it will be necessary to re-measure dimension D and re-solve the equation.

    WARNING
    Insufficient clearance may cause excessive brake drag.

19. Reinstall the master cylinder.
20. Bleed the brake system.