Hmm, I dunno what to say, but my c-pack has been very good waterproof. I certainly cant't blow of the cap, and sucking air into it is out of the question. Even worse: when I use primairies for longer times, the rubber boot will blow up a bit, but there is no leak. I've been swimming with the c-pack for several times, all together I guess over 4 or 5 hours. Never had a leak.
The o-rings in the c aren't moving, opposed to the PD's. The PD-sealing ring is moved a bit with every click. But I also must admid I regularly take the c-pack apart, clean and use a bit of Krytox 260 (less liquid as the 50/50 from the shoppe) to lube the inside ring a bit. I guess this helps a bit as well for keeping waterresistant in more or less shallow water.