Master cylinder and brake lever issue

I got a new, to me, brake lever and master cylinder for an '89 zx750f and I just went to put brake fluid in it to set up my front brake system. the master cylinder doesn't leak at all but it won't let any fluid into the line. I can't find why not.

Any thoughts?

This is my first time setting up a hydraulic brake system so its definitely a possibility I am just doing it all wrong

Piston is probably stuck in a pushed in position. Sometimes you can use a rod pushed into the outlet to push the piston back and get it working again, but most likely you will have to rebuild it.

Thank, it was the piston. I finally got it working