My shimano 105 front brake won't release after engaging. The problem seems to be the arm spring connects through a plastic sleeve. The housing is broken: on the functioning brake, the spring/sleeve/brake moving as a group. On the broken brake, when you pull the brake lever, the spring just slides through the sleeve and gets stuck. Does anyone know how or even if I can replace the spring/housing? Or is there a work around?
if i'm understanding your post correctly i've had this exact problem before... i couldn't figure out how to fix it and luckily scored a used one at kraynicks... not that much help but if we had the same problem i tried for a long time to fix it and failed, you might ask around here for a replacement.
You can use a flat head screwdriver and pop the spring out and replace the broken plastic piece. But that would mean you have a spare brake with that part in which case you'd just put that brake on.
Dunno what to tell you beyond that.
update: after visiting several bike stores, one place gave me the number for shimano. So I called really begged for the part. Turns out even they don't have it. The guy told me I'd have to buy new brakes or find a used set to scavenge from 
Having this problem right now with the original front brakes on a 1990 Trek 1420 I just bought. They are "105 SC Super SLR." Great condition bike overall, but the left caliper on the front brake is not fully releasing after the brake cable goes slack. Not a centering problem, and it's not the plastic spring retainer the OP describes. Probably need to disassemble and clean. I'll post if I learn something useful.
Oh p.s. if in the intervening 2 years an expert on late 80's/early 90's shimano components has signed up to this messageboard, LI'L HALP PLZ 
Those brakes are great until they go wrong.. I have some I've attempted to service but have died.
I'd recommend washing out the mechanism in nasty solvent and then re-lubing. The pivots may be seized up with corrosion. There are two pivits on the brake one in the center one on the right arm. See if you can get them moving.
I've never had this problem with Shimano parts, but I did with old Campy calipers, and would add this to what Ted said: use waterproof marine grease like the Bel-Ray or Maxima brand (sold at motorcycle shops) on the pivots as it will last longer than white lithium or garden variety wheel bearing grease, especially if you ride in the rain.
Thanks Ted & Jacob! 'Bout to go to the hardware store next door and get a bottle of something gross.
LIQUID WRENCH ftw! Put it in the pivot points and it's releasing fully and getting smoother. This after both WD40 and Tri-Flow didn't touch the rust.
Oh yeah PB Blaster that shit will fix anything. Especially styrofoam coffee cups.
