Leave the maximum durability alone, and make the cost for low repair skill a cap on how much you can repair, so that you can repair that sword with Repair 1, but it'll only get up to 20% of its maximum, so it'll break sooner.
Something like this would have my vote. And I haven't actually put 5 points in the repair ability to see what that gets you currently, but someone who invests that many points should be rewarded with something a little cooler than just the ability to repair an item to its original 100% durability - like additional durability/damage/etc. over the 100% value.
