I don't think there is a parts store 15/16 master cylinder that you can buy that works correctly. That is why Doctor Diff has them. He did the leg work to find a vendor who would build a 15/16 master cylinder with the correct reservoir and pushrod retaining feature and stuff like that. Mopar Performance used to have them made then they got out of the business. Eberg had them made for some time and now I think Doctor Diff is the only source.