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"Any tips on bleeding the smaller size or mismatched combo? " You might substitute another 1/4 inch bleeder to see if the present one is defective. It might be partialy clogged which is not uncommon. "Could it be that assembley correct in a 66 included a mis-matched rear pair?"Highly unlikely. The larger bleeder did not come into use on the rear wheel cylinders until at least the 1970's. The best of luck with your project. Bob