I bought a 1986 635csi auto about 3 months ago, 90,000 miles.I don't think it has been driven much in the last few years.

Despite prowling the chat rooms I have something I can't fix.The car is leaking clear brake fluid from the small rectangular hole at the junction of the master cylinder and the brake booster.I have twice removed both and replaced the O ring with the correct size ( 3mm x 36mm ) at the plastic cup.I am confident that I put it all back together correctly including the 4" pushrod direction .It will drip about 1 drop every 3 minutes or so.The pedal does not go to the floor, it stays hard when properly bled.

Does this mean a new brake master cylinder ( not hard to do ), it's currently an ATE brand, possibly original.
Is the leaking at the seal where the 4" pushrod goes through do you think?
How do I troubleshoot this, I have a brake pressure bleeder ( Motive ) that I can use.

Thanks to all,