There isn't anything in the hydraulic system, like height-sensing proportioning valves, that would keep the rear brakes from working, but if they aren't applying hard enough, the wheels might spin on snow and ice. One thing that can cause that, especially since you were smart enough to include lots of related details, (thank you), is the rear shoes could have been installed incorrectly. Many beginning mechanics make this mistake and will argue when you try to correct them.
At issue is the length of the brake lining material. On each side, one lining is much longer than the other one. Logic would dictate the longer lining goes toward the front since most stopping power is needed when going forward, but that is wrong. The shorter lining goes toward the front. It's job is to grab onto the rotating drum and try to rotate with it. As it does, it pushes on the star wheel adjuster link which pushes the bottom of the rear shoe into the drum. At the same time the rear piston in the wheel cylinder pushes the top of the rear shoe into the drum. Two forces are acting on the rear shoe so it's called a "duo-servo" brake. The rear shoe does the stopping when the car is going forward. That's why it always has the larger lining. When the short and long linings are switched around, the rear brakes will likely still lock up on snow and ice if you press harder than normal on the pedal, but they won't be as effective on dry roads. The goal however, isn't to lock the brakes. The goal is to have the front brakes, with most of the car's weight, and the rear brakes, balanced, so they both lock up at the same time, if you choose to push that hard on the pedal. That balance is lost when the rear shoes are reversed.
Another potential problem has to do with sticking parking brake cables. That's a common problem in states where they throw a ton of salt on an ounce of snow. When a parking brake is partially applied due to a sticking cable, the self-adjuster mechanism stops functioning normally. As the shoes wear and don't adjust up over time, the brake pedal slowly goes closer to the floor. That's hard to notice because it occurs over such a long time period and because rear wheel cylinders use very little brake fluid compared to the front disc brake calipers. The rear shoes will still lock up on snow, but you'll have to push the brake pedal further than normal, and when they do finally lock, the fronts have already been locked up and skidding for a while.
Friday, January 7th, 2011 AT 5:58 AM