Drum to disc brake conversion

Tiny
MARK TILGNER
  • MEMBER
  • 1996 CHEVROLET TAHOE
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 328,000 MILES
I recently had my rear drum brakes converted to discs. The pedal felt a little soft, but brakes worked okay.
When I mentioned that to mechanic, he said I may have to replace the master cylinder to one for all wheel disc brakes. I also had all of the brake lines replaced with stainless steel lines.
SO does the master brake cylinder really have to be replaced as well?
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Monday, September 13th, 2021 AT 6:39 PM

2 Replies

Tiny
CARADIODOC
  • EXPERT
Absolutely correct. Master cylinders for drum brakes have a "residual check valve" in the port where the steel line is attached That valve maintains 10 pounds of fluid pressure on the wheel cylinders to prevent the rubber lip seals from falling down, (unlikely to happen as there's a spring in there for that), and to prevent the entry of air when barometric pressure goes up overnight. That would, in effect, put the hydraulic system into a vacuum and pull air in past the lip seals.

The shoe return springs are unaffected by that 10 psi of fluid pressure. They're very much stronger and will easily return the shoes to the anchor pins. That's not the case with disc brakes. There can't be any pressure held in their hydraulic systems as that will cause them to drag and overheat.

If it will help, there's a link to an article on replacing the master cylinder:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Your vehicle wasn't available with rear disc brakes from the factory, but there are aftermarket kits. If they used GM parts from another model, most of them had the parking brake built into the calipers. If that is the case, you'll see a lever on the back of each caliper. Unlike front calipers that are always self-adjusting, GM's rear design has to be adjusted by applying the parking brake. If that is not done, you'll have a low brake pedal. If the parking brake cables are rusted tight, you can do the same adjustment by using a large Channel Lock pliers to work each lever by hand a few times. That will run the pistons out until they adjust.

Also be aware with this design, the pistons have to be manually filled with brake fluid before they're installed. If that isn't done, air will be trapped in the pistons and you'll have a low, soft brake pedal. Once installed, there's no way to bleed that air out.

None of this applies if you have the rear caliper that uses a smaller internal drum brake inside the rotor for the parking brake. With that design, you won't have that lever on the back of the caliper, and you treat them the same as any standard front caliper.

I serviced a lot of vehicles with the parking brake calipers, but after searching for a photo to show you, it appears they weren't as widely-used as I remember. This photo is for a 2010 Cobalt, but it's of a similar design.
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Monday, September 13th, 2021 AT 7:00 PM
Tiny
SQM
  • EXPERT
Hello,

Soft pedal typically due to an issue with the master cylinder. It can also be due to having air in the brake system. As when you push the brake the air will compress but the fluid will not, so the softness of the pedal is due to the air compressing.

It wouldn't hurt to bleed the brakes just to be sure.

As for needing a different master cylinder, it might be necessary due to having different brake components between the drum and disk brakes. As the drum brakes have a check valve that keeps constant pressure. Which could be the difference between a firm and soft pedal feel.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please let me know of any questions.
Thank you.
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Monday, September 13th, 2021 AT 7:06 PM

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