If you are using the original sierra setup like Gus & Adrian did, then you have to go from a sierra wheel cylinder to a smaller escort one.
Attached is a link to the page on Adrian's website which explains it all.
http://www.tamarisktechnicals.com/pages/iva.html
I think this only happens when using the larger rear drum setup from the 1.8 & 2 ltr cars.
I believe the 1.6 sierra had smaller rear drums and therefore smaller wheel cylinders.
HTH