How to check a master cylinder?

Answer 1

Start the vehicle and drive foreword a few feet, and then stop and hold pressure on the brake pedal. If while holding firm pressure on the brake pedal you feel the pedal, very slowly, falls downward toward the floor the Master Cylinder needs replacing ASAP. Most master cylinder failures are caused by leaks developing around the piston seals. As the piston seals wear or the cylinder bore becomes scuffed or pitted, piston seal leaks cause a falling or spongy feel in the brake pedal. The falling pedal symptom is caused by fluid leaking from the brake hydraulic system into the atmosphere or by fluid leaking internally around the piston seals.

A spongy brake pedal can occasionally be attributed to a master cylinder with a worn rear seal. In these cases, air enters the cylinder as the pedal is released and is sometimes pumped into the hydraulic system. Because air compresses, the most noticeable symptom is a spongy brake pedal. Theoretically, the ingested air should harmlessly bleed through the breather or compensation ports into the master cylinder reservoir. In reality, when combined with system failures that cause excess pedal travel, the ingested air can migrate into the main hydraulic system. A master cylinder mounted at an upward angle is also very conducive to allowing ingested air to migrate into the brake hydraulics.

A worn rear seal will also allow brake fluid to mysteriously disappear from the master cylinder reservoir into the vacuum brake booster and ultimately into the engine intake itself. One indicator of this condition, of course, is fluid consumption without evidence of external leakage. Another indicator is a wet area forming around the mounting flanges of the master cylinder. When you suspect a defective Master Cylinder, replace it. Good functioning brakes are way too important to take any chances.