I've also been using DOT 5 for 25+ years and had White Post rebuild mine 5 years ago. Last spring when I took it out of storage, there was a puddle of brake fluid on the floor of the garage. Turned out to be from a leak past the rear o-ring.
More details:
http://www.saacforum.com/index.php?topic=10452.msg89915#msg89915This year, everything is still dry so maybe my fix with known EPDM and slightly thicker o-rings worked. Looking back now, part of the testing I did last year did show some minor swelling of Nitrile o-rings in DOT 3 when I tested against DOT 5. Conventional wisdom has been that swelling is bad however for leaks past the rear o-ring, I would argue that swelling is good. I had also found signs of moisture in my valve when I opened it and now wonder if freezing temperatures caused that to expand just enough to push past the o-ring.
If it ever leaks again, I'll consider switching over to Castrol SRF which claims:
"It is miscible with all conventional fluids meeting the US Federal Standards FMVSS 116 DOT 3, DOT 4 and DOT 5.1, ISO 4925 and current SAE J1703. This means that changing the brake system to Castrol SRF is easy. Drain and flush out the conventional brake fluid with Castrol SRF then top up the system with Castrol React SRF Racing. Although Castrol React SRF Racing is miscible with conventional brake fluids, topping up Castrol React SRF Racing with conventional brake fluids will reduce the benefits of the product."
https://www.moreheadspeedworks.com/product/castrol-srf-1l/Keep in mind that you can not drain and flush K/H calipers unless you remove them, turn them upside down and let the old fluid drain out. The bleeder only purges air and fluid at the top of the caliper.