Why Remove the Master Cylinder Cover When Doing a Brake Job?

Disc brakes use pads, rather than shoes, to apply pressure.

In some instances, the motions or methods exhibited by an auto mechanic might seem mysterious, or at least unfounded. The reasoning behind any particular practice may temporarily elude the technicians themselves, as long-standing routines become unquestioned habits. Application of some procedures and precautions often become automatic and universal to a busy mechanic, too keenly focused on tasks to devote much thought to motivation. Aside from brake fluid inspections, good reasons exist for removing the master cylinder reservoir cover during a brake job.

Fluid Movements

Disc brakes apply pressure by means of a piston, or pistons, incorporated into a caliper. Brake pedal force is transmitted hydraulically to the piston and ultimately to the pad. The caliper pistons do not retract completely when the brake pedal does, and as the pads wear, the pistons compensate for missing friction material by staying "out" further. Brake fluid occupies the vacated area of the piston bore, which is why low fluid levels in the master cylinder reservoir may indicate worn disc brake pads.

Backing Up

The caliper piston has to be pushed back into its bore to accommodate new, thicker brake pads. The fluid behind the piston is forced back to the master cylinder reservoir. The cover is removed beforehand to allow unrestricted fluid travel. The piston is more easily compressed as the fluid readily returns to the open reservoir. Fluid added to the reservoir to maintain the level between brake jobs now becomes superfluous. A savvy technician will spot this condition in advance, and remove some fluid to prevent spills.

Backing Down

Failure to remove the reservoir cover may cause air pockets between the fluid and the cover to become compressed. The caliper pistons resist efforts to push them back into their bores, and eruptions may occur if the cover is removed under these conditions. Brake fluid ruins the finish of painted or plastic surfaces on contact. It is also unwelcomed on skin and eyeballs, causing serious irritation to the affected area. However, some brake jobs may be performed with no concern for such matters.

Backing Plate

Drum brakes are operated hydraulically as well, but the pistons that apply force to the shoes are not meant to compensate for lost friction material. The pistons travel across a cylinder in reaction to brake pedal input, like disc brakes. However, once the pedal is released the pistons retract. Heavy return springs attached to the shoes and backing plate pull the shoes back in, forcing piston travel. The retraction forces fluid to return to the master cylinder and fluid levels remain unchanged. Inspections of fluid level and quality require cover removal, just the same.

References

• Automotive Suspensions Steering Alignment and Brakes; Walter E. Billiet and Walter Alley

Resources

• Popular Mechanics: Dealing With A Low Brake Pedal