I have to say I got excellent service from CompBrake after speaking to David @ the Autosport show at the begining of the year.
I was after a dual master cylinder setup which bolted in place of the original ABS master cylinder which they did for many other cars off the shelf but not the Sierra.
He said no worries send me a ABS unit and I'll copy the bolt pattern and make you a custom one which he did for no more money than a stock item.
You can now buy a sierra one from CompBrake
And it's been faultless all season so much better than ABS setup.
Rich