mopowers said

Not sure if this is the same issue you're having, butt this is what I did. I used a piece of 1/8" aluminum and sandwiched it between the MC and the firewall. There seemed to be a small void on the top part of the new disk brake master cylinder where light was shining through. You can see this in the picture. I didn't like that. It's much better with the homemade "washer" to close things up and seal everything. I used the master cylinder for a template.

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