Linsmc, if it IS PTT, you need support under the arch so that tendon doesn't have to work all the time (it's involved in maintaining the arch of the foot, all the time). I've ended up using half-length Scholl orthotics, just to give some support (when I'm not running barefoot or in Five Fingers). Those or some sort of little cushion under the arch might help for the moment. Good luck.
The brace was pretty restrictive - rigid supports on both sides, plus the cushion underneath. I had to wear shoes a size or more too big (two or three sizes at first while the ankle was swollen) and I was only walking e.g. round the house or while volunteering at my parkrun. I certainly wasn't able to run with it on. And it was painful, because my nervous system translates constant pressure on my ankle bones into pain (and the rigid tube to the air cushion went straight over the injured and tender tendon) - so I spent as much time as possible sitting, with the brace OFF!
I tore my posterior tibial tendon end September 2013 (in a cycling accident, when it was improving after I'd nursed it through five 50-milers). I ended up in a PTT brace (with an inflated cushion under the foot to totally rest the tendon) for five months (three would probably have been enough). Then I had to return to running very carefully and gradually to allow the tendon to adapt to being used again, but I did return to marathon and ultramarathon running. No experience of sinus tarsi syndrome, although I have had peroneal tendonitis on the outside of the foot.
I did a 15-minute jog on grass yesterday - my longest so far post stress fracture. Felt fine - the only bad part was having to slow to a walk after my 15 minutes was up. So that's me well on the way to qualifying...