2013 F6b rear brake pedal rock hard

[BIGLRY]

Hello fellow f6B's,
My brakes are dragging. The rear pedal is rock hard. The brake lever is normal.
I've only owned the bike about 4 months. I decided after buying it to replace all fluids. So i replaced the clutch and brake fluid. When i went to check rear reservoir i noticed it was empty. I added fluid and bled it. They worked fine for 2 months until now. And the rear fluid is still normal level.
It had the rear brake recall done in 2016.
Any advice? I've heard people say check the calipers but not sure how to check them? Wondering if it could be air. Or seals in the rear master cylinder?
Or because it's linked could it be something else?

thanks,

ShadowAero

Has the factory SAFETY RECALL on the secondary master cylinder been done? Here is the Honda service bulletin
notice all about it. http://www.gwrra-wav.org/goldwingbulletins/1800SB23.pdf
Check for the punch mark on the secondary master cylinder as shown in the bulletin.