Noticed that I am constantly having to add brake fluid to the rear brake reservoir on my 78 FJ40. Have gone through the lines with a fine tooth comb and can't find any leaks or have not noticed fluid in the drums. Had my son pump the brakes and noticed a great deal of brake fluid level fluctuation in the rear reservoir. Also that the top of the cover is wet, compared to the front one which is dry. Will replacing the rear cover help or is there something else causing this?