How to reset the Master Password without current password on OS X? This is not the user or admin login password. Running OS X Yosemite 10.10.5. 

To set or change the Master Password first click on the gear (1) and a new dialog box will open (2)
The Master Password is more a safety net than a security measure.

Without any user password you may reset or delete the Master Password by booting to your Recovery HD, starting Terminal, mounting your main volume, navigating to /Library/Keychains and deleting the files FileVaultMaster.cer and FileVaultMaster.keychain.

If your main volume is encrypted you won't be able to mount it without a proper FileVault password.