Brake Master Cylinder: Testing and Inspection

INSPECTION
Inspect for Leakage

NOTE: The master-cylinder should be cleaned thoroughly prior to inspection (several days preferably). Spilled brake fluid can be easily confused with leakage.

Piston Seals
- Leakage around the piston seals will seep out from the end of the cylinder and leak down the outside of the brake booster.
- Inspect for signs of seepage or bubbled paint on the brake booster.
- If any leakage is detected the master-cylinder should be replaced or overhauled

Brake Line Fittings
- While someone is applying pressure to the brake pedal, inspect the brake line fittings on the master-cylinder for leakage.
- If the fittings are leaking they should be disassembled and inspected.

Reservoir to Cylinder Gasket
- Inspect the gasket for signs of leakage or hardening and cracking.
- Overhaul or replace the master-cylinder if any leakage is indicated.

Inspect for Oil Contamination.
- Inspect the rubber gasket on the inside of the master-cylinder lid. If the gasket is swollen or bloated, the brake fluid is contaminated with oil.
- Siphon or scoop a small amount of brake fluid out of the master-cylinder and place into a styrofoam cup filled 2/3 of the way with water. Brake fluid will mix with the water while oil contaminants will float on the surface and dissolve the styrofoam.

NOTE: If any oil contamination is present ALL brake components containing rubber seals will need to be replaced or overhauled. This includes the master-cylinder, both front calipers, both rear wheel cylinders, and all flexible brake hoses.

Verify Hold-Down Bolts Are Tight
- While someone is applying and releasing pressure to the brake pedal, verify the the master-cylinder is tightly secured to the brake booster.
- Retighten hold-down bolts if necessary.

Functional Checks

WARNING: Always bleed the entire brake system and verify the rear brakes are properly adjusted prior to diagnosing a master-cylinder as defective. For additional information on master-cylinder / brake pedal relationship see Fundamentals and Basics. Fundamentals and Basics

Check Related Systems
- Check for proper brake booster operation (NON-ABS ONLY)
1. With the engine "OFF", depress and release the brake pedal 4-5 times (this bleeds off the vacuum reserve in the booster).
2. Depress the brake pedal firmly and start the engine.
If the pedal goes down slightly the brake booster is assisting in brake operation.
If the pedal does not go down slightly the brake booster is not assisting in brake operation and should be diagnosed for problems.

- Check and adjust rear brakes.
- Check all brake lines and brake assemblies for leakage.

Repair all deficiencies with related systems prior to diagnosing the master-cylinder as defective.

Check Brake Pedal Travel (Non-Antilock Brake Systems Only)
- Fully depress the brake pedal and check for the following:
- The pedal should have a 75% reserve.

Pedal Reserve Checks:

- The pedal should be firm and not leak down.

Prior to replacing a master-cylinder for poor pedal travel or feel verify the following:
- There were no problems with any of the related systems.
- The entire brake system has been bled.
- Rear brakes are properly adjusted.