Quite often this is a blocked compensating port in the master cylinder. The brake pedal needs to come back far enough for this; it's worth checking - you need some freeplay there.
Otherwise like kyteler says "Just pull apart your calipers, check the condition of the seals, use a little emery paper on the bores, they might come up sweetbix without spending any money at all."
I had the same problem on my old car and I have done this^ recently with 6 calipers; I was lucky with the seals, I used rust killer on the calipers and pistons to rid any corrosion. I found that the problem was most likely the pistons catching the bores between the seal and the dust seal. The 2 that were on the car are back there now and seem to be fine. I guess out of the 6, 2 are in very good condition, 2 are quite good (on the car) and 2 are average spares probly not useable.