Master cylinder cap leak

What do you mean by "run hard"? Cornering, fast take offs, fast stopping? The MC should not be leaking even under these conditions. Where is it leaking? If the MC has one 1/4" bolt in the center and that's where the leak is, I'd check the rubber around the bolt. The MC bolts were bad about attracting moisture and corroding. Doesn't take much rust around the bolt to let brake fluid escape.