It's great that it works, but it's also a good opportunity to clarify things in case someone having same issues read this later.
First of all, it is Master Password, it has nothing to do with Steemit, and that's good because Steemit is known for stealing user funds.
So, your Master Password starts with P5 and it's purpose is to provide a single Password that user can save and when needed "derive" their individual Private Keys from it.
It's done automatically via https://wallet.hive.blog, but under the hood, it just do:

• what's the Private Posting Key for @stephie.spicer, given her Master Password is P5****

when you click to reveal your Private Active Key, it do similar thing:

• what's the Private Active Key for @stephie.spicer, given her Master Password is P5****

But when you log in using your Private Posting Key instead of Master Password, you can't reveal your Active Key, because your browser can't tell how does it look like.
And that's a good thing!
For this reason for day to day casual Hive browsing, posting, upvoting, etc. you should use your Private Posting Key, while having your Master Password and Private Active / Owner Keys securely stored in safe place unless you really need to use them (for example use Private Active Key to vote for Hive Witnesses or Hive Proposals, or to transfer your funds).