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Corvette Master Cylinders - Brake Fluid Keeps Falling On My Floor

Replacing The Master Cylinder

Richard F. Newton Dec 1, 2002
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Once every few weeks I kept having to stop at the local Ford dealer to replenish my brake fluid supply. (You are using Ford brake fluid aren't you?) Right away I knew that this was not a good sign. The other bad sign was that I kept finding peeling paint on my garage floor where the brake fluid was leaking onto the nicely enameled finish.

The first thing I did was buy two wrenches for $70.00. Well, not really. Actually the first thing I bought was a can of brake fluid. Next, I bought a new pressure differential switch for my old brake master cylinder. Then I bought the two Snap-On wrenches. I can explain all of this. Really, I can.

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The first thing is to make sure that all the bleeder screws and master cylinder lines can actually be loosened. If any of these items are going to give you a problem you want to know this now-not half way through the job.

It all started when the shiny enamel paint on the garage floor started peeling. I figured that since it was under the area of the brake master cylinder I should look at what was going on. Sure enough, the underside of the master cylinder was covered in brake fluid. The interesting thing was that it was leaking out of the pressure differential switch. I've never seen that before, but then again I haven't seen a lot of things.

Master cylinders usually leak out of the rear seal. This means that your brake booster is covered in brake fluid. The other failure is when an internal seal breaks down and your brake light goes on. Sometimes you can lose your rear brakes, in which case the rear rotors will get rusty. These are all normal and easy to find.

I went into denial about a failing master cylinder and simply replaced the pressure differential switch. Within the week I was back at my Ford dealer to buy more brake fluid. The next call was to Zip for a new master cylinder. I've always had good luck with Zip when I want OEM parts. I really didn't want to install an off-shore master cylinder in my car. I knew that with Zip I was going to get an AC Delco unit.

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It takes two different line wrenches to break the lines lose from the master cylinder. You're going to need a 12mm and a 14mm flare nut wrench if you have a C4. If you have an earlier Corvette you can use standard wrenches. Which ever you use remember that you want a wrench that won't spread apart as you turn the fitting. The cheap wrenches actually flex if the fitting is difficult to remove. Once you round the fitting you're in serious trouble. You're better off to spend the money on a quality wrench than to spend money fabricating new brake lines.

Master cylinders are easy to replace. In the C4, and all the early Corvettes, it should only take you a half hour to install a new unit. Most shops will charge you for an hour of labor, which is totally reasonable. They may also charge you additional time to bleed all four calipers, which is also a reasonable item.

There are two problems in replacing the master cylinder. First, you can have trouble when you try to break the old brake lines lose from the master cylinder. If you round the corners on these fittings you're in a huge amount of hurt. You can simply start replacing lines half way through the car.

I've installed possibly a hundred master cylinders in my lifetime and the only line wrenches I trust are Snap-On. Normally I'm a Craftsmen type of guy, but when it comes to line wrenches I get the best I can. These things really work. There are a lot of places to save money, but line wrenches are not the best place to balance your budget.

The next place to look for trouble is when you check the bleeder screws on the calipers. Make sure that all of your bleeder screws can be loosened. If you have a bleeder screw that won't loosen, or worse yet-one that breaks off-start checking the balance on your VISA card. A frozen, or broken, bleeder screw means you're going to purchase a new caliper. I've heard of all the various ways of removing obstinate bleeder screw and I don't like any of them. Just go out and get a new caliper.

I've now got $70.00 worth of wrenches to install a $119.00 genuine-GM master cylinder. It takes the average Corvette shop less than a 30 minutes to install a master cylinder. And as I said, they'll bill you for an hour of labor. Labor charges are now around $70.00 an hour for Corvettes so I'm already in the hole on this job. Then again, this is supposed to be a hobby-right?

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The master cylinder design changed during the production of the C4, but the basic things remained the same. After all, a master brake cylinder is a master brake cylinder. This particular unit is on an '85 Corvette. If you have an ABS system don't get overly concerned. The master cylinder is upstream of the ABS system and should cause no problems.

Our esteemed leader here at Team Vette suggest that the money I've saved by doing this job myself just purchased two really nice wrenches. He calls this a good deal. Then again he's also thesame individual who just purchased a Greenwood race car to drive on the street. I'm not sure how much I want to listen to his advice.

At any rate let's check out all of our bleeder screws and get started on this job.

You might need to replace the master brake cylinder in your Corvette if:
Your brake pedal slowly sinks to the floor while you wait for the light to turn green
Your brakes work better after you pump the pedal a couple of times
You have to open the hood to add brake fluid more than every few months
You find brake fluid running down your brake booster

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This is the pressure differential switch. This unit is totally separate on the earlier cars, which means you could have a great master cylinder and just a bad switch. Fluid shouldn't be leaking out of the spot you find this switch located. Leaks like this aren't very common but you should check everything once you have to start adding brake fluid on a weekly basis. A leak at this point with the C4 Corvettes mean that you have an internal seal problem. The replacement C4 master cylinder comes with a brand-new switch already installed so you needn't concern yourself with this item. On the earlier cars it's a separate purchase.

Brake Fluid
I'm not even sure that you can purchase a rebuilt master cylinder for a C4. I know you get them for the old C3 Sharks These rebuilt units are something you really want to avoid. There's no good reason to use a rebuilt master cylinder. Purchasing one of these units is simply asking for trouble. Most good shops won't even consider putting a rebuilt unit on your Corvette. Rebuilt units are for bottom feeders who purchase solely on the basis of price.

The problem is that in the rebuilding process they hone the bore of the master cylinder out. This removes the rust and pits that have built up over the years. While you end up with a nice shiny, perfect bore, the process also enlarges it.

The new seals that are installed are the same as the factory originally inserted. This means the little rubber seal has to fill a great amount of space. It can generally do that for a short period-usually a very short period. As the seal wears you've got a leak. Now you're back to replacing the master cylinder. Even if the you get a free replacement, because of the lifetime warranty, you've still got a lot of work to do. It just isn't worth the hassle.

With the C4 we have an even greater problem because of the aluminum construction. Aluminum master cylinders do not like to be rebored. You can never get all the fine aluminum out of the bore. These microscopic aluminum particles then create wear on the seals and you have a leaking unit before you know it.

You only want to replace your master cylinder once a decade, if that often. The master cylinder in the "X-X" Corvette lasted almost 18 years. Don't get cheap and buy something made in China. Go for the name brands such as AC Delco. If a part comes in a white box with no name, I'll usually return it. If a company takes so little pride in a product that they're afraid to place their name on the box then I don't want to use it. It's that simple. Anything worth doing is worth doing to X-S.

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After you've made sure that all the lines are loose you can go move ahead to the real task. Leave the mounting bolts that hold the master cylinder to the vacuum booster connected at this stage. Also, don't remove the hydraulic lines, simply leave them loose. We want to minimize the amount of brake fluid that drips out of the master cylinder while the new unit is prepared.

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Now that you've gotten all the lines loose we can turn our attention to bench bleeding the new master cylinder. Clamp one of the mounting ears in a vise and attach the bleeder kit. You can get a nice metric kit from Pep Boys or AutoZone for just a few dollars. The idea is to pump fluid through the master cylinder and remove any air from the internal passages. I use a big Phillips screwdriver to pump the master cylinder piston. Once you have nice clear fluid with no air bubbles coming out of the little plastic hoses you're ready to begin the great master cylinder swap.

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Now that you have the new master cylinder in place you want to bleed the final amount of air out of the unit. There's a trick here that your local Corvette shop never showed you. Get your buddy to sit in the car while you should loosen one of the master cylinder line fittings so that a little fluid leaks out. Now have him push the brake pedal slowly to the floor. Now repeat this with the brake line. Any air in the master cylinder will be expelled before it gets to the brake. Do this with both the front and rear brake lines about three times. Just remember to close the brake line before your buddy releases the brake pedal. If you do this correctly you won't have to bleed the individual calipers.

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The trick is to swap the new cylinder for the old while getting as little fluid as possible on your Corvette. The good guys won't let a single drop get on the car. We won't even think about what the bad guys can do with brake fluid. It's best that you spread a big towel under the area beneath the master cylinder.

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The final step is to fill the master cylinder reservoirs to the full line and start the car. Make sure you have a firm brake pedal before you drive out of the garage. If you press the pedal several times and the dash brake light goes on you've got to bleed the air out of the individual calipers. If the light stays out take the car for about a10-mile drive using the brakes very carefully.


Zip Products
Mechanicsville, VA 23111



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