1978 750F. About to rebuild rear caliper, and was wondering how crucial the dust seal is in that process. Apparently the last rebuild did not include the dust seal (missing??). Obviously it will keep some crud out of the piston area, but don't they tend to make the brakes drag if not perfect? Also, should I put some white grease on the piston to keep corrosion to a minimum? Thank you for any/all responses.