Is this light pullingonly first thing in the morning (or when the car is cold if you happen to work nights...)?
Although tricky to do, and requiring both luck and skill, from a cold start try to drive 2-3 miles without much using the brakes, GENTLY pull over and then carefully feel the wheels to see if one feels noticeably warmer than the other. (but not necessarily hot). next, run up to speed-30mph is ok-then gently brake almost to a stop. Take your foot off the pedal and let the car come to rest naturally on level ground. does the car stop smoothly, or does it give a (very) minor lurch as it comes to a stop?
Based on other postings in this thread, my gut feeling would be you have a brake caliper sticking ever so slightly. it doesnt pull under heavy braking so I dont think its a bad problem. Although front or back binding can cause pulling, IMHO for rear brakes to bind bad enough to start pulling, you would start noticing bad smells from the car. If you dont know the smell of burning brakes, then good for you! Although you never forget the smell, its not on the list of 50 things to do/smell before you die. I dont know what kind of calipers you have there, but it was the bane of my life on many old bikes of mine with a floating caliper front brakes, where you only have a piston on one side and it pulls the back side of the caliper against the disc. Kinda like a 'G' clamp. only unlike drum brakes there isnt actually any force available to retract the caliper. sure, the withdrawal of the master cylender piston when you take your foot of the pedal will (should!) marginally withdraw the slave pistons at all 4 wheels, but if one is sticking just slightly then it will tend to withdraw the other three just that little bit more. Also, the caliper might not be floating free from the back end, with the same results. If you dont have a floating caliper, ie you have pistons pushing the pads on both sides of the disc then I would look to see if a pad is stuck in slight contact with the disc. simply remove (Are you reading this Mr Haynes? I once had a good laugh when I read that in one of your books!) the pads, clean the edges up in an appropriate manner untill they are smooth and clean and also the corresponding area of the caliper, so the pads move smoothly. replace with just a lick of grease on the metal edges of the pad (for chrissakes make sure you use the proper high melting point stuff for the job, not the first pot that comes to hand) then put back together and road test. before you do, remember that brake dust can be dangerous to your health, so dont go sniffing it and dont forget to tighten ALL your wheelnuts.
Sorry for going on at some length, especially if this is all old stuff to you, but remember that other people read these posts and (hopefully) learn from them.
Cheers
Mike
BTW, forgot to add that if you have one pad noticeably thinner than the rest then odds are thats the culprit. if the problem turns out to be a sticking piston then you might have to spend some wonga. But get back to us first.