Rear wheel cylinders

So I replaced the rear drum brakes on my '72 Duster340. Exhausted, I cleaned up, leaving the car on stands, in the garage overnight. Next morning, I see that my new cylinders have leaked a little (4" pool's on both sides of the garage floor) around the seals. Now I've never seen this before, and I opened the bleeders to let off any pressure that might have been applied when I hooked up all the new springs. Did I buy defective cylinders? Or does this happen sometimes. I've yet to bleed them. As I don't want to make a bad situation worse. Did I stupidly over look something reinstalling everything? The last time I changed the cylinders was around 20 years ago, so maybe I forgot something? :banghead: