Brake Basics for Street Rods
If You Have the Horsepower,
You Should Also Have the Brake Power!
There are a lot of street rod brake items in the market place today. As street rodder/builders we can get most everything we need already custom tailored to fit a street rod chassis, when designing a brake system. Here are some good idea you may want to incorporate in your current chassis project or file in your memory bank. After discussing brake tips and facts with some of the best brake experts in the street rod high performance business, the story begins to gel.
There are four methods of bleeding brakes - pressure bleeding, manual bleeding, gravity bleeding and vacuum bleeding. Bleeding is necessary to remove air bubbles from the system. Air can enter the brake system in a number of ways. A bleeder screw may not be tight, you may have a leak in the system and accidentally empty the reservoir, allowing the master cylinder to draw air into the system, removing a wheel cylinder or caliper, or opening the system for any reason will allow air in. Most systems have high spots where the air will accumulate if not bled. If a brake line has a loop in it or a section where the line is higher then the rest of the system, air will usually be trapped there. Realize if the high spot is three feet from the wheel you're bleeding, you could empty the line to that point to get the air out. Operating the bleeder valve can be accomplished using the proper size six point box-end wrench or a rubbing wrench. Do not use an open-end wrench, since they almost always slip off the bleeder valve. Complete fluid could be changed using the pressure bleeder method, just be sure it's DOT 3 or 4 you're changing to. The bleeding sequence on most street rods is usually the same for all methods of bleeding. Start with the wheel farthest from the master cylinder, usually the right rear, then the left rear, right front, and left front. Clear plastic surgical hose or aquarium air tubing will work fine to see the air bubbles coming out. Attach the wrench first and then push the clear hose over the bleeder.
requires a pressurized (air over fluid) tank with brake fluid and a special plate with neoprene gasket that locks over the top of the master cylinder. These two items are connected by a pressurized air hose that provides about 20psi to the system. This type of system should be used by a professional only. Available at mist brake shop, it simply pressurizes the entire system with brake fluid so one man can quickly bleed all four corners, without having to add fluid or have help from another person. Simply open the bleeder about 1/4 turn and allow all the air to come out then close the valve. This is the best method to bleed just about any brake system, although there are a few exceptions.
is the least expensive and requires another person and a couple of cans of brake fluid. Making sure the reservoir is topped off, one person pumps up and holds the pedal. While another opens the bleeder valve and lets the air out and tightens it. This process is repeated on the other wheels until all air is removed. This method has probably been used by every person that ever owned a car. It is one of the simplest ways to remove air from the system, however this method may not be 100 percent effective on a more complicated system like modern four-wheel discs which use a variety of combination valves.
is probably the slowest and least harmful to the system. As you probably already realize the air bubbles trapped in the system float to the highest point if you have a vented cap or a bad seal at the master cylinder lid. Vented caps are obsolete on today's American passenger cars. Diaphragms are usually found inside the lids of most master cylinders as part of the lid gasket and will hold air in the line just like putting your finger over a straw full of water. Depending on how you plumbed the system (high spots will hold bubbles), you may also have to power bleed and or manual bleed the system. For gravity bleeding the master cylinder must be higher then the calipers or wheel cylinders. Gravity bleeding is as simple as filling the reservoir (leave the lid off) and opening the brake bleeders. Do not let the reservoir go empty check it periodically or you will have air in the system again. You may want to put the wheel cylinders or calipers in a pan and make sure the bleeders are pointing up.
can be done with Neward Enterprises Mityvac vacuum pump designed to bleed brakes among many of its other capabilities. This vacuum pump can also be used for troubleshooting engine cylinder leak down, fuel pumps tank lines. Distributor vacuum advance, transmission modulator vacuum problems, brake boosters and bench bleeding new master cylinders, siphoning liquids, and much more. It's billed as a one man bleeder, since a vacuum is drawn on each caliper or wheel cylinder, one at a time (check the photos). It is recommended that 17 to 20 inches of vacuum be used on each caliper or wheel cylinder. This is about the equivalent to 10psi.
Changing Calipers or Wheel cylinders
If you have neoprene brake hose and are just changing wheel cylinders or calipers, a clamp can be used to keep the air from traveling up the line (check the photos). Good judgment and reasonable pressure should be used here. Use gentle pressure on the neoprene hose, just enough to stop the flow. This method should not be used on stainless braided lines, since they have a Teflon inner liner that can be collapsed. Disc brake bleeders should always be at the top of the caliper, so you can bleed them without taking them off the bracket. If they are on the bottom, remove them from the bracket and turn the bleeder up before bleeding. If this is the case you'll need to place an old rotor or piece of wood the same thickness, between the pads while the bleeder is pointed up.
Silicone verses regular brake fluid
Probably two of the most likeable differences about silicone (DOT 5) is the fact that it won't eat up paint if spilled and it acts as a (non-corrosive) lubricant for all brake parts. Regular brake fluid (DOT 3 and 4) will soften the paint before you can wipe it off or sometimes you will wipe off the paint too! Using a plant sprayer to lay a fine mist of water on the painted surfaces first will float any DOT 3 or DOT 4 fluid and keep it from attacking the paint, since oil and water don't mix.
Brake Fluid Temperature Ranges
Both types of regular brake fluid (DOT 3 and 4) and silicone (DOT 5) all have different boiling points. A dry boiling point refers to the time of manufacturing. A wet boiling point refers to a time after the can has been opened. Do not leave the lid off no matter what type of brake fluid you use. If you find it has been stored this way (lid off) throw it out! They will absorb moisture although silicone seems to be least affected. Regular brake fluid is designed to absorb the moisture of assimilate it, so water pockets will not occur in the system.
To give you an idea of the temperature ranges, DOT 3 has a dry boiling point of 401 degrees and a wet boiling point of 284 degrees. DOT 4 has a dry boiling point of 446 degrees and a wet boiling point of 311 degrees. And DOT 5 has a dry boiling point of 500 degrees and a wet boiling point of 500 degrees because it does not absorb moisture. Bleeding bubble from silicone is more difficult then regular brake fluid and it will at times generate foaming internally even when pressure bleeding, and generally does not give a hard pedal like regular fluid.
Master Cylinders Lower Than Calipers
This should not be a problem. If you do experience problems you feel are due to a low reservoir, check the master cylinder lid. It should not be vented and should have a neoprene diaphragm/seal. If it has a poor seal or is vented, fluid could drain back to the reservoir. If the cap is vented get rid of it and get a newer master that is sealed and has dual chambers. You can also have bad seals in the caliper or wheel cylinder that need to be replaced (air could be drawn on around the seals). There are two-pound check valves that can be placed in the line for disc brake setups, and 10 pound check valves for drum brakes to keep the fluid from flowing backward in the line. These are used to keep from loosing the pedal.
Camshafts and Vacuum
They don't always go together to benefit your brake booster. Your high lift cam can suck away a lot of vacuum from your power brake booster. You must have a vacuum check valve between the engine and canister/booster. There are remedies available like electric vacuum pumps that come on only when necessary, dual-diaphragm vacuum boosters, or additional volume can be achieved by adding an extra reservoir canister. This will give more storage capacity, but it will not increase the overall level of vacuum. This should stabilize the level of vacuum if you have other vacuum operated accessories.
Rule of Thumb
Your brake pedal ratio should be about 5 to 1. We're talking about the length of the brake pedal arm her (check the drawing). To figure your pedal ratio you'll need to measure in a straight line from the centerline of the pivot pin to the centerline of the pedal, we'll call this value X. Then measure, in a straight line, the distance from the centerline of the push rod to the centerline of the pivot pin, we'll call this value Y. Simply divide Y into X. Example; if X=10 inches and Y=2 inches then the answer should be 5, or 5:1. If you're putting a brake system together for your street rod and you're not sure what diameter master cylinder bore to use, find out what the manufacturer recommended or used with your setup, both front and rear. This should give you a good idea on diameter. Remember when bore size goes up line pressure goes down and pedal travel decreases, and when bore size goes down line pressure increases and pedal travel increases.
Seasoning the Brake Lining
This is a must! New brake lining or pads should never be driven hard right out the box Metallic pad should have 15 to 20 soft stops (two or three miles). Don't come to a complete stop if you don't have to. Organic pads and shoes should have about 10 easy stops or slow downs. If you do have to come to a complete stop, don't hold your foot on the brake. The idea is here is to avoid getting the new pads or lining hot, let them cool completely.
Safety means your brake system works correctly! If you don't know or are not sure about the bleeding or installation of new parts call one of the street rod sources listed at the end of this article. Get some help if you're not sure, it could mean saving your car and a lot of bodywork or the back of the garage!
Breaking in New Metallic Lining or Pads
Metallic takes twice as ling to seat-in as compared to organic, but metallic also lasts twice as long as organic. The organic break-in period takes about half the time and the lining lasts about half as long. Always arc the shoes to the drums.
Make approximately 20 complete stops (10 for organic) from 30mph, or 20 "slow downs" from 50mph to 20mph, with light to moderate pedal pressure.
Allow at least 30 seconds between brake applications for the brake shoes or pads to cool down
You can do this as part of your normal driving time during the first 100 miles (50 for organic). Try to avoid hard stops during this time.