I just replaced the master cylinder on my '92 SSE */c for the 3rd time in 3 years, and I only drive 10,000 miles or so a year. Each master cylinder was a BRAND NEW one made by Fenco, from AutoZone...it'* a good thing that they were under warranty, so I only paid for the 1st one, but having to replace them this often is getting annoying. The first one lost internal pressure but didn't leak externally, and the last 2 didn't leak pressure but leaked fluid at the piston seal so fluid ran down the vacuum booster.
Have any of you had similar experiences with new master cylinders?

autozone may have been the problem....i have replaced with a Napa MC and have been fine for 4 years and 40K

Pat, do you happen to remember who the manufacturer was for the MC you bought?

The only reason for premature MC failure I know about is pushing dirty fluid back through the MC when brake pads are changed. This happens when the caliper piston is pushed home in the bore without opening the caliper bleeder screw. Doesn't seem likelyto explain all three failures.

The only advice I could offer is don't buy any more Fenco master cylinders, and thanks to your post, I won't be buying any Fencos, either. Quality of parts, and labor used to assemble are not the same, lifetime warranty notwithstanding.