Break Master Cylinder question

[xrally]

I recently boughta 1992 240sx and I just noticed that it took a lot more pressure to use the breaks than it should. I saw that the break master cylinder barely had any fluid, so I refilled it, only to have it all leak after about a week.
The leak is coming from the middle hole and I honestly know nothing about what is to go there. Any ideas? Thank you in advance!

[catnipple]

Does your s13 have ABS? I believe if it does, the center should be plugged. If it does not have ABS, there should be a third hard line to fit into the center.
Possible BMC swap by previous owner? Maybe threw in a BMC from a non ABS equipped s13 that had all three lines and failed to plug the middle.

[xrally]

The car does not have ABS, so I'll just plug it up. I just was not sure what it was for, so thank you very much!

[Hijacker]

Is it leaking from the middle fitting or the pressed in bearing plug? If it's the latter, you should consider getting a new BMC.