JM,
I'm assuming that the valve is adjustable, and that despite having that puppy cranked all the way open, you were STIL caught in an uncontrollable ascent?
And that no matter what position you have it in (eg: highest possible position) that is refuses to purge properly?
Bearing the above in mind, take that suit back to the shop IMMEDIATELY and have them send it back to the mfg for testing and/or replacement of the valves.
IMHO, regardless of where the valve is located, you should be able to purge it properly.
Background: my current suit is a DUI 350TLS trilam. The dump valve is located on the deltoid. It IS adjustable. The only time I've had a problem is when I put on my BC the valve got caught & inadvertedly closed the valve. But a test at the surface showed the problem & I fixed it.
My first suit was a Wardell with both purge and the inflator on the chest. It was a manual, non-adjustable purge: I would have to rear up so that my chest was as high as possible & then hit the purge. Otherwise it wouldn't vent properly.
What I'm getting at no matter where the valve is located, it should be able to work properly. And it sounds like yours isn't.
~SubMariner~