Brakes on snow

Tiny
SHORTBO
  • MEMBER
  • 1979 CHEVROLET IMPALA
  • V8
  • 2WD
  • AUTOMATIC
  • 90,000 MILES
I own a 1979 Chevy Impala 4 door. I use studded tires on all four tires. This has been working fine for the past 5 years. This year I am sliding on the snow. I cannot seem to get grip. I put it into neutral so I can slow down and stop. I had all front and rear brakes replaced, rear brake drums turned, master cylinder replaced. All back about 8 months ago. I put my car into drive and the back wheels seem like they want to spin, even though I have my foot on the brake.
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Friday, January 7th, 2011 AT 3:03 AM

3 Replies

Tiny
CARADIODOC
  • EXPERT
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.

Caradiodoc
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Friday, January 7th, 2011 AT 5:58 AM
Tiny
SHORTBO
  • MEMBER
Thank you, so do you think I should have my rear brakes checked out? After the rear brakes were installed, I was told to push down and then release the parking brake a few times to help adjust the rear brakes. To be honest, I rarely use the parking brake.
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Friday, January 7th, 2011 AT 7:26 AM
Tiny
CARADIODOC
  • EXPERT
Your brakes do not adjust with the parking brake. Many import front-wheel-drive cars work that way. Yours adjust when backing up and applying the regular brakes.

One other thing to check that is specific only to GM is the backing plates the wheel cylinders are clipped to. Instead of bolting on, they are clipped to an hour glass-shaped hole. That hole rusts out and lets the wheel cylinder turn so the pistons don't push on the shoes. There are replacement backing plates that use the same design bolt-on wheel cylinders every other manufacturer uses but you have to replace the wheel cylinders too. It's a big job because both of the axle shafts have to be pulled out. When it gets bad enough, one of the pistons will pop out and the red warning light will turn on and the pedal will go down further than normal.

Caradiodoc
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Friday, January 7th, 2011 AT 8:02 AM

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