1990 CHEVROLET TRUCK MASTER CYLINDER

  • Tiny
  • joe1953
  • 1990 Chevrolet Truck
  • 5.7L
  • V8
  • 2WD
  • manual
  • 194,000 miles

If somebody put transmission fluid in the master cylinder will it damage the master cylinder? How do I bleed the master cylinder while on the truck? When pumping the brake no fluid come out the rear brake line but when pumping the brake with the top of the master cylinder fluid shoot up from it do I need to replace the master cylinder?

Sunday, February 9th, 2014 AT 9:43 PM

7 Answers

  • Tiny
  • caradiodoc
  • Expert
  • 24,890 posts

No. This is going to come as a shock, but the ONLY proper repair is to replace every part that has rubber that contacts the brake fluid. This is a really serious problem and there is no shortcut. Experienced mechanics will wash their hands with soap and water before using their fingertips to push the rubber bladder seal back into the master cylinder reservoir cap. They don't want fingerprint grease or any oil residue on their fingers to get in the fluid.

The parts that must be replaced are the master cylinder, front calipers, rear wheel cylinders, three rubber flex hoses, and the combination valve under the master cylinder. Many trucks also have a height-sensing proportioning valve attached to the rear axle. That has rubber o-rings so it has to be replaced too. Those parts must be removed first, then all the steel lines must be flushed and dried, then the new parts can be installed. If any part with rubber is not replaced, the contamination is going to leach out of it and recontaminate the entire system again.

If you look at that bladder seal under the reservoir cap, you'll see it is blown up and mushy. That is what has happened to all the other rubber seals and hoses. The lip seals in the master cylinder grow past the fluid return ports and block them. The typical symptom is dragging front brakes that get real hot. As the brake fluid heats up, it expands but it is blocked from flowing back up to the reservoir. The more it expands, the more the brakes self-apply, and the hotter they get.

Please believe me that you can not leave any rubber part in the system. People have tried and it always results in a brake system that doesn't work properly.

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Sunday, February 9th, 2014 AT 10:30 PM
  • Tiny
  • joe1953
  • Member

Once I replace all the new parts can I bleed the master cylinder with it on the truck do I need to replace the master cylinder reservoir to? Do it have rubber seal between the reservoir and the master cylinder?

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Monday, February 10th, 2014 AT 7:16 AM
  • Tiny
  • caradiodoc
  • Expert
  • 24,890 posts

Most of the time rebuilt master cylinders come with a new reservoir. You're going to need the rubber bladder seal under the cap. If all you can find is a replacement without the reservoir, it still will have two round rubber seals that the reservoir plugs into. You can reuse the reservoir if you wash and dry it thoroughly, then grab a used bladder seal from a salvage yard.

The new master cylinder should be bench-bled first to get all of the air out of the cylinder. On a lot of GM products the master cylinder sits at an angle with the front up real high. Air can get stuck in that high spot and never bleed out. Bench-bleeding will take care of that. Most master cylinders come with a pair of hoses and the plastic fittings for bench-bleeding. They will pack instructions too on how to do that.

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Monday, February 10th, 2014 AT 3:16 PM
  • Tiny
  • joe1953
  • Member

I am having trouble loosen the steel lines to the combination valve seem like the lines are frozen what can I use to loosen the lines?

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Monday, March 3rd, 2014 AT 7:22 PM
  • Tiny
  • caradiodoc
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  • 24,890 posts

Sorry for the delay. I had a major house fire, so my replies are going to be sporadic.

Normally I wouldn't say this but it's not going to cause any further damage to use penetrating oil on those nuts. The Chrysler dealer has a real good product called Rust Penetrant that will do in 20 minutes what WD-40 does in a weekend. PB Blaster is good stuff too.

You have to be careful too once the nuts work loose from the valve. They can still be rusted to the steel lines. That will result in twisting them off when you turn the nuts. Work them back and forth to work them loose. Heating the nuts with a propane torch may help too. Use flare nut wrenches on those nuts because they're real soft and will round off easily. Be sure to wash off that penetrating oil with brake parts cleaner before you connect them to the replacement valve.

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Wednesday, March 5th, 2014 AT 12:21 PM
  • Tiny
  • joe1953
  • Member

I am so sorry to hear about your house fire. I did get the lines loosen and I replace the combination valve so now how do I bleed the combination valve?

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Wednesday, March 5th, 2014 AT 7:27 PM
  • Tiny
  • caradiodoc
  • Expert
  • 24,890 posts

Thank you.

You don't have to do anything with that valve. Brake fluid will flow freely through it both ways. There are some service manuals for Ford products that tell you to pull the little button on the front for the front brakes when bleeding, but I've never had to do that. That is the "metering" valve. It's also called a "hold-off" valve. It's purpose is to delay the application of the front brakes until you push hard enough on the brake pedal to cause the rear shoes to move out far enough to contact the brake drums.

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Monday, March 10th, 2014 AT 11:44 AM

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