Sorry, I know this question has been asked before, but I haven't seen an answer which applies to my specific situation.
I have a super budget built Surly Karate Monkey. It's got a used Avid BB7 disc brake set on it. The rear caliper looks like it might be some cheap thing which was taken off of a Walmart bike or something. It's very warped, and I can't get it adjusted right. Somebody on my local CL has a used 140mm caliper that can be had for only 10 dollars. The current size on the rear of my bike is 160mm. Can I buy this used 140mm caliper and make it work?