1985 Chevrolet Silverado Master cylinder

Tiny
HENRYR79
  • MEMBER
  • 1985 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
What would cause three master cylinders to go out within approximately three years on this truck? It is driven only about 6 to 8 months out of the year and these times are usually short trips around town. Two for sure have gone bad leaking fluid out of the rear into the brake booster. I have had to change this as a result once within the past three years.
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Friday, September 26th, 2014 AT 6:06 PM

7 Replies

Tiny
CARADIODOC
  • EXPERT
Are these used or rebuilt master cylinders?
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Friday, September 26th, 2014 AT 6:11 PM
Tiny
HENRYR79
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I am not sure. Haven't found the receipt, but they are from O'Reilly Auto.
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Friday, September 26th, 2014 AT 6:29 PM
Tiny
CARADIODOC
  • EXPERT
Dandy. That means they're professionally-rebuilt, not used ones from a salvage yard.

The most common cause of repeat failure is improper bleeding procedures, namely pushing the brake pedal all the way to the floor, but that results in different symptoms, and it doesn't cause leaking from the rear seal. This also doesn't apply to master cylinders less than about one year old. On older ones, crud and corrosion build up in the bottom halves of the two bores where the pistons don't normally travel. Pushing the brake pedal all the way to the floor runs the lip seals over that crud and can rip them. Often that takes a while to show up, but it results in the pedal slowly sinking to the floor under light foot pressure.

The next common thing is brake fluid contaminated with a petroleum product like engine oil, transmission fluid, or power steering fluid. We used to see that after someone repacked wheel bearings by hand, then wiped them off to pop the rubber bladder seal back into the reservoir cap. The residue on their fingers was more than enough to contaminate the brake fluid. Most professionals even wash their hands with soap and water before handling any parts that will contact brake fluid so they don't get fingerprint grease in there.

The repair for this gets real expensive because the only proper way to fix it is to remove every part with rubber in the hydraulic system that contacts brake fluid, flush and dry the steel lines, then install all new parts. If any part is not replaced, the contamination will leach out of the rubber parts and recontaminate the new brake fluid. That means replacing the master cylinder and reservoir cap, three or four rubber flex hoses, calipers, wheel cylinders, height-sensing proportioning valve, and combination valve.

If your fluid is contaminated, it is going to damage the next master cylinder you install, but you should have noticed other symptoms first. The first complaint almost always is brakes that don't release, or they self-apply, and they get so hot they start smoking. The rubber lip seals in the master cylinder grow past the fluid return ports and block them. The clue is the stuck brake will release when you loosen the lines at the master cylinder.

The rear seal in the master cylinder can also get soft and mushy and start to leak, but if contamination is the cause, expect to see other symptoms.

The last thing to look for pertains to trucks, diesels mostly, with the hydro-boost brake power boosters. If those leak power steering fluid in front, it can get onto the push rod for the master cylinder and find its way into the brake fluid. If you have a standard vacuum booster, you don't have to worry about that.
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Friday, September 26th, 2014 AT 6:53 PM
Tiny
HENRYR79
  • MEMBER
As far as I know I haven't been doing anything to contaminate the seals or the fluid upon installation of these m/c. I did change both rear wheel cylinders about three years ago. Last year I never could get the brake light on the dash to go off so I let a professional mechanic work on the truck and they claimed the reason why I never could get the light to go off was because I had not gotten all the air out of the line and I think they might have even changed the m/c. The truck is not mine, I am doing the work for a family member. It is hard to keep up with the repairs done to this truck and my own vehicles also.
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Friday, September 26th, 2014 AT 7:20 PM
Tiny
CARADIODOC
  • EXPERT
If there's air in a line it can make the red warning light turn on, but you would have a low, mushy brake pedal too. The switch responsible for that is the pressure-differential switch located inside the combination valve. That sits on the frame right under the master cylinder. The two lines from the master cylinder run to it. To verify that is the reason for the light being on, just unplug that single wire in the middle of it on top.

Those switches are REAL frustrating to reset on Fords, but on GMs and Chryslers, they're spring-loaded and will reset automatically when you release the brakes. Often the valve sticks and the switch stays on. A good, quick jab on the brake pedal usually pops them free.

The red warning light also is turned on by the parking brake and the low-fluid level switch on the master cylinder, if there is one. You can unplug those too to see which one is keeping the light on.

Next time you replace a master cylinder, here's a trick to keep you from having to bleed at the wheels. This works best when the master cylinder has only two steel lines and they're on the same side. Loosen the nuts for those lines just a little so you don't have to tug on them later. Unbolt the master cylinder and pull it forward off the mounting studs. Use it as a handle to bend the steel lines upward a little. Unbolt the lines to remove the master cylinder. The fluid won't be able to run out of the lines now.

After bench-bleeding the new master cylinder, start the lines by hand, then use the master cylinder as a handle again to bend the lines back down. Snug the line nuts, then have a helper push the brake pedal down very slowly to about halfway to the floor. That should take about ten seconds. When he does that, loosen one of the line nuts a little. You'll see air bubbles come out. Be sure to tighten that nut before the helper releases the pedal, otherwise air will get sucked back in. Do that a second or third time if necessary until no more air bubbles come out, then do the other line the same way.

The secret is in pushing the pedal down slowly, and allowing it to spring back rapidly. When it is pressed, the fluid will go down toward the wheels but the air bubbles will have time to float back up. When the pedal is released quickly, the fluid rushing back into the reservoir will wash any last air bubbles with it.
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Friday, September 26th, 2014 AT 7:53 PM
Tiny
HENRYR79
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I found out that the wrong master cylinders were being but on the truck. They were for the wrong size rear brake drums.
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Saturday, January 21st, 2017 AT 1:36 PM
Tiny
CARADIODOC
  • EXPERT
I'm happy you found that, but it doesn't explain the repeat failures. The wrong diameter master cylinder, calipers, or wheel cylinders will cause some symptoms like the brake pedal traveling too far, easy rear-wheel lock-up, or the rear brakes not doing their share of the stopping.
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Saturday, January 21st, 2017 AT 2:04 PM

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