What is the reason for pressure leakage after replacing the brake master cylinder?
2 Answers
If there is still air leakage after replacing the brake master cylinder, it may be due to a fault in the vacuum booster tube or the vacuum booster itself, and it has nothing to do with the brake master cylinder. You need to go to a repair shop or 4S store for inspection. The main function of the brake master cylinder is to provide brake assistance, which only takes effect after the vehicle's engine is started. The symptoms of a faulty brake master cylinder are as follows: 1. Oil leakage at the connection between the brake master cylinder and the vacuum booster or at the limit screw. 2. Poor or ineffective braking even when the brake pedal is fully depressed. 3. The brake pedal feels soft when pressed. 4. The vehicle pulls to one side after braking. 5. Sudden brake failure. 6. The brake pedal does not return or returns slowly.
Having worked on cars for so many years, I've often seen cases where the brake master cylinder is replaced but still loses pressure. The biggest reason is incomplete bleeding. Air bubbles in the system cause pressure to drop when braking, making the pedal feel soft. Another common issue is leakage at the wheel cylinders or calipers, especially when the seals are aged or damaged. The brake lines might also have tiny cracks, or the fittings might not be tightened properly during installation, leading to seepage. Don't rush to replace parts—first, perform a thorough bleed: start with the wheel farthest from the master cylinder and use a proper tool to ensure no air remains. If that doesn’t solve it, inspect all line connections for signs of fluid leakage—replacing the flexible brake hoses is often a good idea. Additionally, while new master cylinders rarely have defects, improper installation (like misaligned seals) can cause similar issues. Remember to change the brake fluid regularly, as old fluid contains contaminants that affect sealing. Never cut corners—pressure loss in the system increases stopping distance, so always fix it before highway driving.
