I recently won an auction on eBay for a dirt cheap Esterbrook J in great shape except for a slightly funky nib and a dead bladder. I bought it to use with a new nib I'd just gotten, a 2314-M medium oblique nib that is one of my favorite writers. Now, I'd never replaced a sac, but from what I read it seemed pretty simple. I got a #16 sac, a jar of shellac and some pure talc, and it was very easy and straightforward, except there were some bulges on the latex where it was seated on the end of the section. I thought they might be air bubbles or even shellac bubbles, but when I pressed on them they just wouldn't budge. Could the inner surface of the latex have somehow bunched up?

Anyway, the shellac held and the sac/section assembly seems airtight and waterproof, and I inked the pen and it works fine, but I'm still wondering about those bulges. I've attached a photo. Perhaps some knowledgeable person could let me know what caused them?

Thanks!