Abs Light Transit?

My Abs light is on and whilst pressing the brake i am hearing a springing noise... The light does go off at times but will reappear and stay on... The last time it was the abs ring on the front driver side. But this time i am stuck as one mechanic could not find the fault. He suggested the shoes and the back Abs sensor as well as possibly the rear driver side abs ring... So i went to ford to purchase a ring only to find that they don't supply it any more, i would have to buy a the hub just for the ring costing £200 as well as a sensor costing £179 as well as Fitting £100, thats half the price of the van...

Any suggestions as to what it may be??? I am going to try and change just the shoes on the back and see if that helps