S. Kelli Kolf’s “ Stop It: Information You Need to Know About Automotive Braking Systems”, which appears in September’s issue of Hot Rod and Restoration is an excellent go-to source for diagnosing common brake problems and making effective upgrades. Whether you are a garage rat with years of brake repair and upgrade experience or you are working up the courage to take a first shot at your classic’s brakes, you should check out this article. In case you are looking for a sneak preview, we’ll use this post to cover some of the most important information found in the article below.
Choosing the Correct System For Your Application
Just because a brake system fits your car’s configuration doesn’t mean that it will work properly. Rather than simply looking for fit, it is important to consider how the vehicle will be used. Will you be racing your car (loop or drag racing), or does it fall into a street only category? Have you made modifications that have added or subtracted weight to the vehicle? Have you lowered the car’s ride, and if so, with spindles or suspension? Finally, you’ll need to consider wheel size diameter and backspacing. Once these variables have been considered, you can take a deeper dive into the world of brakes to find that perfect fit for your classic ride.
Manual v. Power Brakes and Pedal Feel
This decision is often dictated by application. Power brakes are great for daily drivers, but they require vacuum. High horsepower applications do not generate enough vacuum and often require a vacuum pump. When set up properly, manual brakes can be just as effective as power brakes—and you won’t need the leg strength of an Olympic power lifter to stop your car. The hard pedals that you often hear about can have a variety of causes (see our checklist on diagnosing and repairing a hard pedal) but after the usual issues have been eliminated, improper pedal feel often boils down to pedal ratio. Remember, your brake pedal functions as a basic lever that multiples force being applied in an amount proportionate to the pedal ratio.
Calculating Pedal Ratio:
- X = Distance between the pedal pivot and the middle of the foot pad
- Y = Distance between the pedal pivot and the master cylinder or brake booster pushrod attachment point
- X/Y= Pedal Ratio
The resulting ratios are generally between 4:1 and 7:1. A pedal with a 4:1 ratio generates 4 pounds of force for every pound of force applied by the driver. A 7:1 pedal will create 7 pounds of force for each pound of force applied. As a result, longer ratio pedals will have a longer stroke travel at the pedal pad relative to the stroke of the master cylinder piston.
The Scoop On Master Cylinders
Disc brake masters cylinders are different animals than drum brake masters because disc brakes require higher line pressure and an increased volume of brake fluid. The moral here is simple: use the correct part to avoid a spongy pedal and poor braking.
Bench bleed your master every time you service your brakes. In some circles, bench bleeding is considered an old-school step that isn’t really necessary these days. Failure to bench bleed a new (or dry) master means you will almost never get all the air out….resulting in a spongy pedal. While you may get away with skipping this step here and there, somewhere down the line, you’ll have issues and the only true fix is bench bleeding.
Brake bias is the distribution of braking power between the front and rear wheels. Applying the brakes transfers a certain amount of weight from the rear to the front of the car, which explains why the larger size and volume of front brakes. The percentage of braking power in front compared to the rear is sometimes referred to as brake bias. By way of illustration, a 3,000 lb car maintains 1,500 lbs of weight over each axle while at rest (50/50 weight distribution). When in motion, hitting the brakes, half of the weight centered over the rear axle is transferred forward, resulting in 2,250 pounds over the front axle with 750 lbs centered over the rear axle—a brake bias of 75/25. Brake bias can be changed by selecting different rotor and caliper sizes, or hydraulically by installing an adjustable proportioning valve on some vehicles. Another option to boost up-front braking power is to select a more aggressive pad for the front than the rear of the vehicle.
How to Select the Right Pad
Pad compound can make all the difference in your vehicle’s braking performance. Remember, pads clamping to the rotor, creating friction and heat. Different brake pad compounds are engineered for different amounts of heat based on application. Street pads are designed for low temperatures and will fail if they overheat…..and they will overheat during hard racing. Conversely, racing pads are most effective at high temperatures, so customers need to resist the temptation to play the cool card with racing pads that won’t stop their daily driver. Matching the right pads for your application will keep your car stopping properly…and more importantly, keep you and your passengers safe!
Choosing the Right Brake Fluid
All DOT 3, DOT 4, and DOT 5.1 fluids are fully miscible and can be mixed without consequence. With this said, you should never mix DOT 5 silicone based fluids in any system where DOT 3, 4 or 5.1 has been used. Also, there is also no harm in using DOT 4 or 5.1 in place of DOT 3 specified systems because the chemicals used to make all these fluid types are similar—no seal or system damage will result. For most hot rods, quality DOT 3 is will do. If your car is headed to the track, at a minimum, you’ll need a heavy duty or race grade DOT 3; DOT 4 or DOT 4 high temp racing brake fluid is your best bet.
This informative article closes with two things to avoid—stacking multiple fittings to adapt or connect and ignoring low pedal positions post-install. Avoiding multiple fittings doesn’t take much explanation: these scenarios are leak prone and make it hard to properly bleed the brakes. And while some may think a low pedal is normal, you should address this situation immediately since it can prevent proper braking during a panic situation.
At MP Brakes, we understand that upgrading your brakes can always take on an unanticipated wrinkle or twist since older vehicles are often modified over the years. We have years of experience and know brake systems inside and out, and we would be glad to share our knowledge with you. So whether you are sorting through options for an upgrade, or stuck on a problem during your install, give us a call at 1-800-397-2076. We are here to help!