You can use an adjustable bias adjuster but it needs to be locked(to the satifaction of the tester) so that it cannot be adjusted. This usually means pining the bias bar. The alternative is to fit a brake proportioning valve in the circuit for the rear brakes, the Sierra is fitted with one as is the Fiat Uno range, these reduce the pressure to the back brakes but are not adjustable. The prototype is fitted with Standard Sierra front discs and rear drums and didn't need a proportioning valve, the brakes are excellent - do you need rear discs? It's worth considering.

If you go with rear discs I'd recommend making an adjustable proportioning valve as shown on this page by Dave Andrews.

http://www.members.aol.com/dvandrews/bpvalve.htm

This would need setting up OFF ROAD by turning up the valve until the fronts lock up first in all situations. As this is adjustable, but not from the drivers seat (it's usually under the car or in the rear suspension area) it's ok for the SVA.

Cheers

Chris