I am stuck with 42 slots and gotten several useless grenade mods instead. I tried the trick of crashing the game a couple times on this last effort to turn in a repair kit, but still only grenade mods. Is 42 the effective cap? I also have all 3 DLC, but i think i got a grenade in knox as well..
42 is not the effective cap, as Corv1nus said it can be up to 57 without downloadable content: http://borderlands.wikia.com/wiki/Storage_Deck_Upgrade.
There is a chance of getting a Backpack SDU instead of a grenade mod in Playthrough 2. In terms of rarity, "The closer to being full a character's inventory is, the more likely the reward of a Backpack SDU.", quoted from the wiki: http://borderlands.wikia.com/wiki/Claptrap_Rescue_Missions.
The crashing trick I think you're using (also on the wiki page) should work if you attempt it enough times with a full backpack. In summary, get the repair kit and save your game. Then go to the claptrap and if you get a grenade mod kill your Borderlands game (there are ways to do this on both PC and console, see the same wiki page under "SDU Reward Tips For Playthrough 2"). Then reload it to try to get a Backpack SDU again.
Also, although I know nothing of the game's random number generator, it may be worth killing some enemies randomly on each reload in order to cycle the game's random numbers, otherwise you may get the same outcome from a rescue mission.
It is a maximum of 42 for the first playthrough and there are five opportunities in playthrough two. They are in the first of the following quests (the first 5 claptraps basically):
These only offer a chance though, nothing is guaranteed. I'm at 48 slots currently on mine.