I need help troubleshooting brake problem

[Tommy]

I'm the fourth owner of a six year old E-M Cobra with 1987 Corvette suspension and Tilton brakes and clutch. A few weeks ago I replaced the clutch master cylinder after I noticed it leaking. There was significant wear in the bore of the master cylinder. Lately, I noticed that while sitting still with steady pressure on the brake pedal (cool brakes), after about 15-20 seconds, the pedal drops abruptly about a half inch. If I maintain steady pressure, it will continue to drop in little steps about every 5 - 8 seconds. Resevoirs on both brake master cylinders (its a dual system with adjustable brake bias) seem full and there are no apparent leaks. I suspect this phenomenon is caused by wear in one or both brake master cylinders. What do you guys think?

[WestCoastBill]

If there is no external leaks than it's internal. The brake fluid is bypassing the cups(seals) on the piston. A rebuild/replacement is in order. Bill