How to Replace the Small Oil Window on the Brake Master Cylinder?
2 Answers
After removing the top cover and sealing gasket, use a syringe to extract the brake fluid from the oil reservoir, then cover the inside of the reservoir with plastic film. Use a flat-head screwdriver (or an 8mm steel rod) to tap out the old window and remove the old O-ring. Clean the mounting hole thoroughly, insert the new O-ring, and press in (requires a pressing tool) or tap in the new window (interference fit installation, which is quite tight). Remove the plastic film, refill with brake fluid, and reinstall the sealing gasket and top cover.
Replacing the small oil window on the brake master cylinder requires some finesse. Remember to prepare a dedicated circlip pliers and rubber sealing ring beforehand. During the operation, first unscrew the two bolts on the brake lever, then carefully pry open the metal circlip securing the oil window—this circlip is particularly prone to flying off, so it's recommended to work inside a plastic bag. When installing the new transparent oil window, pay attention to its orientation; usually the side with the raised edge faces outward. Applying a thin layer of brake fluid on the rubber sealing ring before installation can prevent leaks. During final testing, pump the lever a few times to check for any air bubbles seeping from the oil window. Last time I replaced mine, I found the circlip pliers were the wrong size and ended up struggling for half an hour—it's advisable to confirm tool compatibility in advance. After replacement, remember to top up the brake fluid and bleed the air from the system. The entire process demands patience and steady hands.
