How to Replace Your Classic Corvette's Brake Vacuum Booster

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Do You Need to Replace Your Classic Corvette's Power Brake Vacuum Booster?

Restored Corvette Engine Bay
This is a great example of a fresh booster in a nice Corvette engine bay. The booster is the golden ball at the bottom right of the picture. That gold tone is cadmium plating. This restored Corvette was sold at auction by Mecum. Photo courtesy of Mecum Auctions

Corvettes have been using vacuum boosters for power brakes since the 1963 introduction of the C2 design. The systems have grown more complex over the years, but the basic idea is the same. Vacuum from the intake manifold creates suction through a hose connected to a round plenum placed between the firewall and the brake master cylinder. This plenum contains an airtight diaphragm that separates the firewall and brake pedal side from the brake master cylinder side.

The brake booster works by using the engine's natural vacuum in the intake manifold to exert suction on the master cylinder side of the diaphragm when you step on the brakes. This assists your foot pressure on the brake pedal to give you additional braking force. When you release the brakes, the pressure equalizes on both sides of the booster.

But the diaphragm in the booster breaks down eventually - especially if your brake master cylinder is leaky and deposits brake fluid in the booster body. When the diaphragm finally rips or develops a hole, you lose the vacuum boost to your braking, but there's also a more insidious problem - when the diaphragm no longer holds vacuum, every time you step on your brakes you're allowing air to rush into your intake manifold, changing the fuel-air mixture your engine needs. What's worse is that in Chevy small block designs, all the vacuum used by the brake booster is drawn from the #1 cylinder runner. This means that every time you step on the brakes, you're creating a super-lean running condition in that cylinder, and that will shortly lead to detonation (pinging) and potentially damage to the #1 cylinder that will require an engine rebuild or replacement.

You can tell when your brake booster is dead because your brake feel will change. You may also hear a "whoosh" sound when you step on the brake pedal. You can do an easy test to make sure the booster is working by stepping on the brake with the engine off. The pedal should feel firm. Now start the engine and if the pedal drops an inch or so as the engine starts, your booster is in good shape! But if your booster isn't boosting any more, replacing it is easy. Just follow the steps in this article.

The following photos and instructions are correct for a 1977 Corvette, but you should always use a proper repair manual for your year and model of Corvette.

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Loosen Your Corvette's Brake Master Cylinder

Corvette Brake Vacuum Booster
Here's the old brake booster that's leaking vacuum because its diaphragm is torn. You can see that we've removed the nuts that hold the brake master cylinder to the booster, and we're moving the master cylinder out of the way. Photo by Jeff Zurschmeide

Begin the replacement by loosening and moving your Corvette's brake master cylinder. This is held in place with just two nuts at the junction between the booster and the master cylinder. You don't need to disconnect the brake lines, so don't! Just move the master cylinder out of the way.

However, if you find brake fluid in your booster when you remove it, you may want to replace your brake master cylinder at this time.

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Remove Your Corvette's Brake Vacuum Booster

Corvette Brake Vacuum Booster
You can see the big center hole where the brake pedal clevis goes, and the four holes for the studs on the booster to penetrate the firewall. Installing the new booster is the reverse of removal. Photo by Jeff Zurschmeide

Now to remove the old vacuum booster, you have to dive underneath your dash on the driver's side. There are four nuts on the inside of the firewall that hold the booster to the firewall. Plus, you need to undo the clevis pin that holds the top end of the brake pedal arm to the booster. These nuts are way up high - you might have to remove your driver's seat to get access to them. Take your time and you'll get it done.

There is also a grommet on the engine side of the booster, and a plastic elbow that connects the vacuum hose to the engine. You can usually pull this fitting right out of the booster, but you may need to unclamp and remove the vacuum hose. Inspect the grommet, elbow, and hose carefully for cracks to see if they need replacing too!

Once the booster is completely disconnected from the interior, you can pull the booster away from the firewall. Take it away from your corvette and turn it so that any fluid in the master cylinder side can drain out. If you have fluid, you should replace your master cylinder now, too.

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Install The New Corvette Brake Booster

Corvette Brake Vacuum Booster
This is a used brake booster we bought for the project - it's good, but we recommend you get a new or remanufactured one to be sure it will hold vacuum. You can see the clevis and the four mounting bolts. Photo by Jeff Zurschmeide

Installing the new booster is the reverse of the removal process. Place the booster against the firewall and install the four nuts where the studs on the booster penetrate the firewall, then connect the brake pedal to the clevis, install the vacuum line to the engine, and finally reconnect the brake master cylinder. That's all there is to it!

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Test The New Brake Vacuum Booster

Corvette Brake Vacuum Booster
The new booster is installed and ready to use! It tested out fine in our 1977 Project Corvette. Photo by Jeff Zurschmeide

The test for the new vacuum booster on your Corvette's brakes is the same as the test you used to determine that the old one was bad - Step on the brake with the engine off. The pedal should feel firm. Now start the engine and if the pedal drops an inch or so as the engine starts, your booster replacement is good and your job is done!