What is the reason for air constantly getting into the system after replacing the clutch master cylinder?
3 Answers
Air constantly getting into the system after replacing the clutch master cylinder is due to poor sealing. Here are the relevant explanations: 1. Function: The clutch master cylinder is the component connected to the clutch pedal and linked to the clutch booster via hydraulic lines. Its role is to collect pedal travel information and use the booster's function to achieve clutch disengagement. 2. Overview: When the driver depresses the clutch pedal, the push rod moves the master cylinder piston to increase hydraulic pressure, which travels through the hose to the slave cylinder, forcing the slave cylinder rod to push the release fork and move the release bearing forward. When the driver releases the clutch pedal, the hydraulic pressure is relieved, and the release fork gradually returns to its original position under the action of the return spring, putting the clutch back into the engaged state.
I recently helped a friend with a similar issue. After replacing the clutch master cylinder, there was always air in the system, most likely due to incomplete bleeding during installation. It's like changing the brake fluid on a bicycle—if you're not careful, bubbles can remain. Also, pay attention to the inlet pipe fitting of the new master cylinder. If the rubber sealing ring isn't properly seated or has scratches, air can seep in like a leaking tire. The bleeder screw on the slave cylinder should also be checked. Some mechanics overtighten it, which can deform the sealing cone surface. Another possibility is an improper routing of the hydraulic line, creating air pockets. I remember the last car I worked on—we had to bleed the system three times before realizing the clutch pedal travel was adjusted too long, preventing the pushrod from fully returning. In such cases, you need to adjust the pedal free play first before bleeding the system effectively.
I've encountered quite a few cases of this issue. The main reason is incomplete bleeding of the hydraulic system, which appears simple but actually requires proper technique. When installing a new master cylinder, if the fluid level in the reservoir is too low or the pedal pumping rhythm is incorrect, air can enter the lines just like bubbles getting sucked through a straw. Pay special attention to the metal hard lines between the master cylinder and wheel cylinders - I once found a case where excessive tubing curvature created a U-shaped air trap. Be cautious with aftermarket parts too, as some have poor casting quality with porous sealing surfaces on pistons that leak air. For bleeding, I recommend a two-person operation: one rapidly pumps the pedal from the driver seat while the other repeatedly loosens and tightens the bleeder screws at each wheel cylinder until bubble-free fluid emerges. The process must be continuous - when topping up fluid midway, use a syringe for slow injection to avoid bubble agitation.
