hard to say if it is the booster or not, but I don't recall that they let air into the system when they fail. I think the deal is that the MK2a just ceases to boost, although I've heard that they fail and you get no brakes at all.
Myself I've had three. One completely seized, one fully functional and one maybe/maybe not on my 68 sedan. I was dealing with a non-functional brake system over last weekend and I ended up swapping in my known good booster, and installing a 1969+ 1800-style brake MC. At first I got no brakes at all, then after using my home made power bleeder, they started working, and now seem almost normal. The car's been sitting for 22 years so you never know... I need to chase out a couple other systems before I'm ready to trust it.

Anyway, my experience with the 1968 12/6 oriented master cylinder had been twice poor, so my experience tells me to go with a common single-circuit shaefer or to go split circuit (like you already have) and splurge on the conversion ring and 1800-style cylinder...

I have two gas caps on hand. One is a chrome locking pentagonal WASO cap with the key busted off inside and the little keyhole cover missing. I'd send you that for the cost of postage. The other is a completely original stock, light blue, non-locking cover with the beaded chain on it. I have a good locking waso cap on both my sedan and wagon, so these I've described are just taking up space in my parts boxes. E-mail me off list at repairmanjackatsbcglobaldotnet (edit as needed) if you want either or both.

Let me know if you'd like to know how to make you own power bleeder. You can do so for ~$5.00 mostly with spare stuff you likely have on hand.