Problem: Wheel cylinder misdiagnosis
Cause: Not performing the necessary steps to determine proper function and attributing non-wheel cylinder symptoms to wheel cylinder conditions
Solution: Perform correct inspection on wheel cylinders and know how to interpret the results of the inspection.
Checking wheel cylinders for leakage:
1. On “soft design” dust boots pull each dust boot back to check the wheel cylinder for leaking (Figure 62.1). Soft dust boots are those without metal rings molded into them.
Figure 62.12. Some wetness is acceptable. If the bore and piston have a sheen to them that is normal. Check for brake fluid accumulating at the bottom of the bore or for fluid dripping out of bore as boot is pulled back. If either is found wheel cylinder replacement is in order. While not necessary, it is generally accepted that if you replace one wheel cylinder you should change both.
3. Press on/in dust boot design wheel cylinders require a slightly different approach. Use a dull pick or screw driver to pull the boot down from around the wheel cylinder output rod as shown in Figure 62.2 to check for leaking. If fluid drips out wheel cylinder replacement is in order.
Figure 62.24. Fluid accumulation under the dust boots as described above is not going to have an effect on the brake pedal. This type of leak occurs when the brake pedal is released. It is a one-way leak with a fluid going out over a long period of time but no air entering the system. The cup seals are at their weakest seal upon brake release. If the vehicle has a low brake pedal replacing the wheel cylinders will not fix the problem. The low brake pedal requires performing a diagnostic procedure to determine the cause.
Check for wheel cylinder function:
Check for wheel cylinder function:
Wheel cylinders should be checked for function as part of a brake inspection. The method of checking will depend on the type of rear drum brake.
Duo-Servo Drum Brakes (adjuster at bottom)
1. use a screw driver or other suitable tool to physically push each wheel cylinder piston in while noting the amount of resistance (See Figure 62.3). Do not push the piston in too far or seal damage could occur. If any of the pistons does not move or moves requiring considerably more force the cause should be investigated.
Figure 62.3Non-Servo Drum Brakes (anchor at bottom)
The same technique can be used on this design or an alternate method can be used. With no anchor pin at the top the brake shoes can be used to push the wheel cylinder pistons in both directions. Take care not to push the pistons too far into the bore or seal damage could occur. Use the shoes to move both the front and back piston while noting the amount of resistance.
