Master Cylinder Site Glass Replacement

Over many years, the OEM site glass can get cloudy, soft and may unnoticeable start leaking. The solution is to replace the site glass with a durable watch crystal.

Use BLACK RTV (or Epoxy) It's the only one I know that will stand up to brake fluid.

Using towels or turkey baster, remove all brake fluid from the MC and dry.

Remove existing lense by poking with a sharp instrument and pulling/pushing it out.

Remove any glue/debris and dry the area the sight glass covered. You want a smooth clean surface.

Clean the new sight glass and the MC sight glass area with alcohol.

Take a piece of masking tape and wrap it around your finger with the sticky side out. You will use this to hold the sight glass.

Take a toothpick and apply a smooth, thin bead of rtv around the sight glass area.

If I remember correctly, there is a groove machined into the MC that works as a good guideline. The bead needs to be really thin, but make sure you don't leave any area uncovered.

Smoothly and gently apply the sight glass to the MC. Once applied, shift the sight glass as little as possible.

Using a flashlight, inspect the edges of the sight glass for spaces where the rtv does not seal. Shift the sight glass if necessary.

The bead will not form a perfect circle around the sight glass, but it should not interfere with looking through the metal slit in the MC. If it does, remove the sight glass, clean and start over.

Another bead can be run around the outside edge of the site glass as a precaution

Leave alone to cure for 48 hours or according to the RTV instructions.

Fill the MC with fluid and check for leaks.

If none, gently squeeze the lever until no bubbles appear. Burp the banjo and you should be done. No more bleeding should be necessary.

Before

After

Watch Glass Source (there are others):www.ofrei.com http://www.ofrei.com/page_176.html

Size:

16.7 mm lens of either glass or mineral glass

0.8 mm thickness recommended

(Pictures courtesy of Jason Grennell)

Site Glass sizes may vary but some have also suggesting using gas resistant epoxy and a dime if it's a tight fit.

Roughly measured a dime is 18mm or .70 inches and a penny is 19mm or 3/4 inch.