Brakes

Most people concentrate on going fast in a C4 Corvette with little thought to something just as important, stopping.

Stopping is one of those little save your life details that gets overlooked in the pursuit of performance. I will cover as much as I can in this section as possible from how the brakes work to how to get more out of them. Of course the repair and maintenance will be covered too.

Lets start out with how exactly it all works. Knowing how something is supposed to work is always the first step to diagnosing and fixing it.

 

The Brake Booster

corvettebrakebooster

The vacuum assist brake booster is probably the most mysterious part of the braking system to most people. Sure you may know what it is and what its for but you actually know how it works?

The best way to explain how it works is to peel away the cover so to speak and see what is actually going on in there.

This is what is happening at rest. The motor is running but the brake pedal is not being pushed. With the pedal not applied the vacuum valve allows vacuum to both sides of the rubber diaphragm in the center. So the booster is in neutral so to speak with no force being applied by it.

brake-booster-resting

 

Now lets see what happens when you push the pedal down.

brake-booster-operation

When you press the pedal down the vacuum valve is closed and the air valve on the interior of the car side opens allowing air into that side of the booster’s chamber. With the vacuum valve closed the side of the diaphragm towards the brake master cylinder is being drawn to the master cylinder. The spring between the diaphragm and the master cylinder keeps the diaphragm from applying to much assist. Remember power brakes are a helper and are not intended to apply the brakes on their own. The stronger the spring the less assist and of course the weaker the spring the more assist. This is why you will find some cars have “easier” to press brakes than others.

Once you remove pressure from the pedal the air valve closes and the vacuum valve reopens. This removes the pressure the booster was applying.

One thing that most people I talk to have a hard to understanding is why their power brakes so hard to apply when the motor isn’t running. When the motor is applying vacuum to the booster the vacuum is assisting you over coming the string inside the booster. When the motor quits the vacuum assist part fails and now your fighting the spring inside the booster with no help from the motors vacuum.

This is also the only real difference between manual brakes and power assist brakes. Its a common misconception that the manual braking system has a different master cylinder design from the power assist type.

So in this simplified version of the booster we can actually diagnose a number of issues you might run into concerning it.

 

Lets look at a few problems you could run into with it.

You hear a hissing noise when the motor is started that stops when you apply the brake pedal.

This symptom is caused by a bad air valve allowing air into the booster when the pedal is not being pressed. When the pedal is pressed the air valve opens normally and the vacuum valve closes so no vacuum is on the cabin side of the booster. When the pedal is released the vacuum valve opens causing vacuum to be pulled against the cabin side of the booster and the air valve has to close to keep air from rushing into the booster from the cabin. If it doesn’t the booster will not work as well and you will hear the hissing noise. The booster has to be replaced to fix this.

 

You hear a loud hissing noise when the pedal is pressed with the motor running. Louder than normal.

When the pedal is pressed the vacuum valve closes and the air valve opens to allow air from inside the cabin. This causes a hissing sound when you press the pedal. Normally the motor is loud enough to cover this which is why you usually only notice the hissing when the brakes are applied with the motor not running. When you hear the hissing sound with the motor running and it is louder than normal it is usually from the vacuum valve failing and causing vacuum to keep being applied to the cabin side of the booster when the pedal is pressed. The booster has to be replaced to fix this.

 

When I apply the brakes with the motor running the pedal is very hard to press.

This can actually be caused from a few different things. If the vacuum hose from the motor to the booster has a leak and or vacuum isn’t reaching the booster for some reason the pedal will be hard to press. Repairing the vacuum leak will restore normal braking.

If the diaphragm inside the booster cracks or the outer casing of the booster cracks then the vacuum assist part will fail. The booster has to be replaced if this happens.

 

I replaced my brake booster and now the pedal is slightly harder to press than it was before or the brakes apply too easily now.

The new booster has a differently rated spring inside. Remember the stronger the spring the harder the pedal is to press and the weaker the spring the easier it is to apply the brakes. This isn’t adjustable or replaceable on its own so it can be the luck of the draw when replacing the booster. Sticking with one manufacturer helps to keep things the same.

 

The Master Cylinder

corvettemastercylinder

The master cylinders job is to apply fluid pressure to the brake lines thus applying pressure to the brake wheel cylinders. Simple enough in right? Well there is a bit more to it.

Lets see how the master cylinder works.

mastercylinder

As you can see there is actually a good bit going on inside that small housing. Basically the reservoir provides fluid to the cylinder body. When you press down on the brake pedal the plunger causes the primary and secondary areas of the brake cylinder to compress the fluid forcing it out the brake lines. That part is simple but what needs to be noted are the small fluid return ports and the equalization ports. The larger equalization ports allow fresh fluid into the cylinder but they also allow fluid to go back into the reservoir when you release the pedal or when the calipers piston retracts. The intake port allows for air bubbles to escape as the pedal is first depressed. The return spring is what pushes the piston back to the resting position when you release the brake pedal. If the pedal does not return fully to the resting position this can actually cause several problems. I will cover that in the diagnosing section below.

Rebuilding the master cylinder is actually very easy as long as the cylinder bore itself isn’t damaged. But even then it can be bored to correct it. Click on the picture below for a larger view of the various parts.

master_cylinder

 

One thing of note that is different with the C4 Corvette over the standard master cylinder such as the diagram I used at the start of this section to show how a master cylinder works is the proportioning valve.

In the older Corvettes for instance the proportioning valve was external. So the brakes lines would go from the master cylinder to a proportioning valve and then from the proportioning valve to the wheel cylinders. The C4 Corvette’s master cylinder has this valve made onto the master cylinder. Its the bulge you see on the side of the master cylinder in the picture above. The purpose of this valve is to apply more brakes to the front of the car than the rear. As a car stops the rear end naturally becomes lighter as the weight is shifted to the front of the car. If the brakes are applied evenly between the front and the back then the back tires would lock up as the weight was removed from them in hard braking actions.

You can change the spring inside the proportioning valve to change the bias between the front and back brakes. I will cover that upgrade further into this article.

 

Now lets go over some of the basic symptoms you can experience involving the master cylinder and how to go about fixing them.

 

My brake pedal feels mushy or spongy, I don’t have a firm brake pedal, the pedal goes almost to the floor.

This usually is caused by air trapped in the system. You will need to bleed the brakes to restore proper operation. (Brake bleeding instructions are provided later on in this article)

After bleeding the brakes at each wheel you should have a firm pedal again. A faulty master cylinder can cause this effect too and the only real way to know is to try bleeding the brakes first. If you cannot get the pedal back to the normal firmness after bleeding the brakes then more than likely the master cylinder is the problem. You will need to replace or rebuild the master cylinder to fix this.

 

The brake pedal becomes very hard to press after driving for awhile. Stop and go traffic causes the brake pedal to have no free travel and is very hard to press down. My brakes start dragging after I drive for awhile.

This involves the master cylinder not being allowed to fully return to the rest position. A faulty master cylinder can cause this but most of the time its an incorrectly adjusted brake pedal or the pedal adjustment rod is adjusted wrong or the wrong rod was used on the brake booster. To fix this you have to find a way to allow the rod coming out of the brake booster to fully extend when the brakes are not being applied. I know this is a bit vague but as I mentioned before there are several possible causes for this. The main point is the rod must be fully extended when not applying the brakes. If the rod remains pressed enough it causes the return port to remain covered in the master cylinder. When you drive your car the brakes naturally get hot from friction of the pads on the rotors. This coupled with the natural heat generated each time you stop the car causes any air in the system to expand. The fluid itself will expand also if the temperature of the fluid is raised enough. If the rod in the master cylinder remains pushed down to far the pressure that builds up in the brake system cannot vent back into the reservoir. This causes the brakes to become very hard to press. It also will cause the brakes to start dragging.

This can also be caused by a faulty anti lock brake module. So this particular problem is a bit harder to diagnose. Start with making sure the brake rod from the booster is fully extended first.

 

Brake Lines

I imagine most people will consider this section a waste of space but you would be surprised how important it is to make sure the brake lines are in good shape. Normally brake lines are considered a leaking or not leaking kind of problem. So if the brake fluid level isn’t dropping and there are no puddles of brake fluid found your all good to go. WRONG.

The brake lines are made up of two different sections. The first section which is the largest part is made up of the metal lines running from the master cylinder to each wheel. These lines can corrode over time from road salt or become damaged from running over objects. If you live in an area that uses salt to cover the roads in the winter you need to inspect your metal brake lines yearly for damage.

Next are the rubber lines at each brake caliper. These lines connect from the metal lines to the caliper. They have to be flexible since the wheels move as the car travels over the road and in the case of the front wheels they have to flex to handle steering changes.

The rubber lines will deteriorate over time and eventually rupture so they need to be checked from time to time for cracks or swelling. Also when these lines swell from age even a small amount you will loose pressure when you apply your brakes. This can result in longer stopping distances and pulling to one side during braking. Also if the inside of the line comes apart from age they can cause a valve like effect thus not allowing the fluid to return from the caliper. This can result in the brake dragging or even locking up. You can replace these lines with braided steel lines to achieve a firmer brake pedal feel and increased stopping power. I will cover that later in the upgrade section of this article.

 

Brake Caliper

caliper_front_84-87

The C4 Corvette actually has very good brakes from the factory for the most part. They will deliver excellent braking for the needs of the average Corvette owner. When pushed to extremes while racing they can be pushed passed their limits though. I will cover improvements that are available later on in this article.

 

Lets first cover how the calipers work.

Modern disc brakes are made up of a disc rotor which is attached to the wheel and a brake caliper which is mounted to the steering knuckle.

brakecaliper

As you can see from the diagram above the caliper is very simple in design. It only has one moving part internally which is the brake caliper piston. This piston is forced outwards from the cylinder by brake fluid pressure. The caliper housing floats on guide pins to allow the caliper to achieve a clamping action on the brake rotor. The housing that the guide pins float on is attached to the steering knuckle of the car itself.

One of the most overlooked areas of disc brakes are the guide pins. If the caliper doesn’t float on the guide pins the only pad that is pressed against the rotor will be the pad that piston itself is pressing against. So when you change your brake pads you need to pay attention to the condition of these guide pins. If one brake pad is worn a good bit more than the other then the guide pins are the cause. Corroded guide pins will greatly shorten the life of your brake pads and reduce the ability of the brake to stop the car.

The clamping action of the brake caliper causes heat as the pads clamp down against the rotor. This heat will cause the pads to have a harder time slowing the car down as they heat up. Early designs of brake rotors were of a solid one piece design which worked fine except the rotor could quickly overheat with hard braking or braking over long periods. This was solved to an extent by dividing the rotor thickness in half and adding a webbing between the rotor halves that acted as a cooling fan to an extent. These are called vented rotors. But even with this improvement the brakes can still over heat. I will cover more on this in the brake upgrade section later on in this article.

The front and back brake calipers are not the same. The reason for this is because of the parking brake mechanism. The parking brake’s purpose is to hold the car in place when set. Simple enough but it has to work even if the primary braking system fails. On the C4 Corvette this was done in two different ways depending on the year of the car.

The first design uses a smaller drum style brake located inside the normal disc brake rotor like this :

86rearbrakecylinder2

 

Later models use a ratcheting mechanism on the caliper itself like this :

90rearbrakecylinder

Each has its unique issues when servicing the brake pads. The original design requires the drum brake shoes to be changed  when they wear out. But they do last a long time. The disc pads on this setup are serviced the same as the front brakes.

The later model design requires no extra parts as the disc pads are used for the parking brake too but the piston is driven by a lever action which presses against a collar that is attached to the caliper piston.

The brake caliper can be rebuilt but normally people just replace the caliper.

Lets looks at why you might need to rebuild or replace a brake caliper.

The piston will not go back into the caliper during a brake pad change, The caliper piston is leaking brake fluid, One of my brakes is dragging all the time, The car pulls to one side when I apply the brakes.

All of these symptoms can be caused by a failed brake caliper. Anytime the piston in the brake caliper sticks inside the cylinder bore it can cause the piston to not retract (brake drag) or it can cause the piston to not extend outward (brake isn’t applied at that wheel). The only fix for this is to either rebuild the caliper or replace it. Again the normal choice is to replace the caliper.

 

Bleeding the Brakes

Whenever air is introduced into the brake system it has to be removed for the brakes to function correctly. Basically air can be compressed while brake fluid cannot. Its the core of how the brakes work. When you press the pedal to apply the brakes your attempting to compress the fluid with the master cylinder which it can’t do. So the fluid it forced from the master cylinder down the lines to the caliper which then pushes the caliper piston outward forcing the pads into the disc rotor. Since the fluid cannot be compressed its a very direct effect. Air on the other hand can be compressed. So in place of the fluid being forced downward immediately you have to compress the air until it moves the fluid. This will always have a spongy effect as the air is compressed before an effect can occur. Bleeding the brake system solves this.

Also if the brake fluid ever boils it can cause air bubbles into the system. So the brakes must be bled for this too.

To bleed the brakes you normally need an assistant, a wrench that fits the brake caliper bleed port and a section of hose for the fluid as its bled from the caliper. The hose should be slipped over the brake port bleeder nipple but you need to stop before the part where you will use the wrench to turn the bleeder. One person must pump the brake pedal until pressure is built up and then they must hold the pedal down on the last stroke. Another person then opens the brake bleeder port on the caliper allowing fluid and air to shoot out the port. Then the port is closed and the brake pedal is released. Repeat the process over and over until only fluid comes out the bleeder port. Check the fluid level in the brake reservoir master cylinder from time to time to make sure it doesn’t run dry. If it does you have to do the process over again. When bleeding the brakes you have to do all four brake calipers one at a time to properly bleed the system.

 

Next up Brake Mods – Brake Bias

 

Brake Bias

 

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