How to Replace the Small Oil Window on the Brake Master Cylinder?

[JayceRose]

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.

[Kenny]

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.

[OZane]

Don't force it when replacing the small oil window—details are key. First, remove the upper pump screws to detach the handle assembly. When using needle-nose pliers to extract the C-clip, it's best to cover it with a cloth to prevent it from flying off. Thoroughly clean any remnants of the old seal, and use a cotton swab dipped in alcohol to wipe away grease from the oil groove. Choose an oil window made of oil-resistant material. Here's a tip for installing the new seal: drip two drops of brake oil into the groove before inserting the seal—this both lubricates and enhances the seal. When reinstalling the clip, listen for a 'click' to confirm it's properly seated. Finally, top up the oil to the midpoint of the window. Important reminder: don't rush to drive after installation. Pump the brakes several times in place until the pedal feels firm. If you notice any oil stains around the window, you'll need to redo the job.

[StCooper]

Having the right tools makes the job easier. For replacing the oil window, you'll need: a set of T-type hex wrenches, circlip pliers (straight-tip recommended), brake fluid, and carburetor cleaner. During disassembly, first remove the residual oil from the upper pump, then take out the spring and push rod and set them aside. Align the circlip's opening with the plier tips and rotate 15 degrees counterclockwise to release it. Before installing the new oil window, lightly sand the sealing surface with 800-grit sandpaper to remove burrs, paying special attention to ensure the oil window's scale marks face upward. During testing, squeeze the brake to check if small bubbles appear in the oil window with the action—this is a practical method to test the seal. The entire process can be completed in half an hour, but beginners are advised to allow an hour for safety.

[Remington]

Sealing is more crucial than installation! Last time when helping a fellow car enthusiast replace the oil sight glass, it started leaking immediately after installation—turns out old seal residue wasn’t cleaned properly. You must use a plastic scraper to thoroughly remove hardened adhesive from the pump body groove, then rinse with precision electronic cleaner. The new seal should be made of nitrile rubber and soaked in DOT4 brake fluid for 5 minutes to activate before installation. A slight exhaust sound when pressing the sight glass indicates proper seating, and the retaining clip must fully snap into the second groove. After completion, mark the edge of the sight glass with a marker pen and check for displacement after two days. Don’t forget to dispose of waste brake fluid properly—it’s highly corrosive.