While it’s not unusual for car owners to talk about how much horsepower is under the hood of their haulers, seldom does anyone brag about brakes. And while most of us appreciate the feeling of being pushed back in the seat when the throttle pedal is mashed, nothing matches the relief of being thrown against the seatbelt in a panic stop.
How well your cars comes to a stop is basically dependent on how much friction is created between the brake shoes and the discs (or drums). Today most cars are being equipped with a combination of front disc and rear drums or disc brakes all around that are more than adequate to do their job. But a key factor in how well they work is the pressure in hydraulic system that applies them, which can range from 800-1,500 psi or more.
There are factors to consider in creating the required system pressure: hydraulics and mechanical advantage. The main principle of hydraulic theory is that a liquid does not compress and that pressure applied to a liquid in a closed system is transmitted equally to every other part of the system. With 100 pounds of pressure applied to a 1-square-inch piston in the master cylinder the result is 100 psi in the entire hydraulic system. As the fluid passes through the lines to all the wheel cylinders (or calipers), with 1-square-inch pistons they will all push with 100 pounds of force.
Now consider the following scenario: 100 pounds of pressure is applied to the same master cylinder with a 1-square-inch piston. The system pressure is still 100 psi however now the pistons at the wheels are increased to 3 square inches. Because the pressure in the system is 100 psi and the wheel cylinders (or calipers) have 3 square inches of surface area, the result is the piston pushes with 300 pounds of force. Increasing the surface area of the pistons at the wheels increases the force applied to the friction surfaces, and, all things considered, the more pressure applied to the friction surfaces the more stopping power.
One of the most notable changes in braking systems occurred in 1967 when the government mandated the use of dual master cylinders. With two reservoirs the brake system was split into two separate hydraulic circuits, so if one failed due to a leak the other was still operational. At one time it wasn’t unusual to update a car’s brakes and keep the single reservoir master cylinder (the single Mustang “fruit jar” master cylinder was a popular substitute), but today there is no reason to use anything other than the dual type.
When it comes to selecting a master cylinder there area a variety to choose from, with bores generally from 7/8 to 1-1/8 inch (just remember, a larger master cylinder bore produces more volume and a smaller master cylinder bore produces more pressure). The issue then becomes selecting a master cylinder that provides adequate pressure with sufficient volume. With the brakes properly adjusted the master cylinder must be able to supply fluid to all the components with less than two-thirds travel of the available stroke. Basically too hard a pedal and poor braking means too large a master cylinder, while excessive brake pedal travel indicates insufficient volume.
Along with bore size, volume type of brakes being used will dictate the master cylinder required: Drum brake master cylinders will have small, dual fluid chambers that are the same size and will have residual valves built into the outlets (some aftermarket master cylinders do not have these valves so they must be installed in the lines externally).
As the front disc brakes require more volume than drum brakes, a disc/drum master cylinder will have a larger fluid reservoir for the discs. OEM disc/drum masters had a built-in residual pressure valve to the rear drum brakes only.
Four-wheel disc master cylinders have two large reservoirs and provide the increased volume and pressure rear disc brakes require. Using the wrong master cylinder on a four-wheel disc system will result in ineffective brakes; a soft pedal with excessive travel.
Increasing the Mechanical Advantage
Another way of increasing braking efficiency is with more mechanical advantage. An easy way to do that is with a longer brake pedal. Another method to increase pedal pressure is with a brake booster, the most common being the vacuum type. With these types of boosters the larger the diameter of the internal diaphragm the more assist is provided. However in some cases smaller-diameter boosters with multiple diaphragms can be used if space is an issue.
Another type of brake booster taps into the power steering line and uses hydraulic pressure to supplement the pedal pressure applied to the master cylinder. There are also electric boosters that deliver hydraulic pressure created by an electric pump to a special master cylinder.
Brake System Valves
Metering valves, also called a hold-off valve, are used in the brake system to better balance the front to rear brakes. The valve does not allow the pressure to rise at the front disc brakes until the pressure at the rear drums has risen sufficiently to overcome the brake shoe springs. At this point the valve opens to allow full pressure to build at the front brakes.
Proportioning valves modulate the pressure to the rear brakes. They minimize rear wheel lockup found in heavy braking and compensate for differences in braking conditions in front disc/rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced, preventing rear brake lockup. Adjustable proportioning valves are used to “tune” front to rear brake balance.
Residual valves maintain a small amount of pressure in drum brake systems to keep the wheel cylinder cups expanded. This prevents air from being drawn into the system and allows the brakes to react quicker. A 10-pound valve is common in drum brake systems. Normally disc brake systems don’t have residual pressure valves, however when the master cylinder is mounted below the floor, and is lower than the calipers, a 2-pound valve is used to prevent the calipers from draining fluid back to the master cylinder.
A combination valve incorporates metering and proportioning functions into one valve. These are available for disc/drum or drum/drum systems and often have warning light provisions to indicate if one half of a dual brake system has lost pressure.
In operation a brake system may produce well over 1,000 psi, which requires lines, hoses, and fittings that can withstand pressure reliably. As for brake lines there are only three options that are appropriate-steel (usually with a tin coating to prevent rust), stainless steel (that is often polished), or NiCopp (seamless copper-nickel alloy tubing that is DOT approved for hydraulic brake systems).
There are a couple of common misconceptions about brake lines. One is there is a relationship between brake line size and hydraulic pressure-there isn’t. The master cylinder establishes the pressure in a brake system; all the lines do is deliver the pressurized fluid. Brake lines are most often 3/16- or 1/4-inch diameter and, while there will be no pressure difference between the two, there will be a difference in the amount of fluid delivered. The bigger tubing will carry more volume, so 1/4-inch line may be preferable in some instances, disc brake calipers with large-piston displacements).
The second misconception is that stainless brake lines cannot be double flared. While it is true that some stainless steel tubing will crack when double flares are attempted, double flares can be formed if the appropriately annealed tubing is used.
One of the most common types of hydraulic connections found on automobiles is the SAE 45-degree double-inverted flare. It is unique in that the tubing is doubled or folded back on itself for increased strength and resistance to cracking. The tube’s flare is clamped between the nut and the seat in the fitting when screwed together so when the assembly is tightened, a leak-proof metal-to-metal joint is created and no sealer or Teflon tape is necessary.
While they aren’t used on production automobiles, AN fittings are often found on modified cars. (The reference “AN” stands for Army/Navy and it’s a system devised by the government to ensure interchangeability and compatibility of parts made by various manufacturers.) AN fittings use tubing with 37-degree single flares with reinforcement sleeves (37-degree flares will also be found on JIC hydraulic fittings).
The AN system uses numbers to identify the various sizes of metal tubing and the corresponding fittings, and the same numbers are also assigned to the hose and their ends. Called dash numbers, they equate to a 1/16 inch. As examples -3 line is 3/16 inch, -8 is 1/2 inch.
When measuring for flex lines it is critical that suspension travel and wheel movement are taken into consideration. The front wheels must be able to turn lock-to-lock and both axles must be able to go through full suspension compression and rebound without putting stress on the hoses. More than one driver has checked compression travel and then jacked their car up only to find out the hoses weren’t long enough when the wheels dropped.
Classic Trucks Editor Ryan Manson has been putting the finishing touches on an F-1 chassis equipped with four-wheel discs and we followed along as he plumbed the system to get a few proper pointers.
Ryan Manson’s F-1 chassis, fresh from powdercoating by Eddie Motorsports, has been equipped with four-wheel disc brakes and components from Inline Tube.
For this application a 1-inch bore disc/disc master cylinder was used. A Total Cost Involved pedal assembly activates it.
Prior to installation, master cylinders should be “bench bled.” With short pieces of tubing directing the fluid back into the reservoirs the pistons can be pushed with a screwdriver.
Brake pedal leverage is an important consideration when installing a brake system. Manual brakes usually require additional leverage.
Due to space constraints, Manson opted for a small-diameter, dual-diaphragm vacuum booster.
To plumb his F-1’s chassis, Manson chose 3/16-inch stainless steel tubing from Inline Tube.
A quality tubing cutter is a necessity to produce “square” cut when fabricating brake lines.
After cutting lines to length the ends should be deburred prior to flaring.
There are a variety of handheld tubing benders available. This example is from Eastwood.
In some cases you may have to improvise. In this case an oxygen cylinder is perfect for forming a brake line for a rearend housing.
As well as benders there are tubing straighteners. This one from Eastwood uses a series of rollers.
Flaring tools come in a variety of configurations. This example from Eastwood will make 45-degree double flares. A 37-degree version is also available.
Flares are made by clamping the tubing in a precision two-piece block with the desired angle, 37 or 45 degrees.
With the tubing clamped in place the die that corresponds to the angle required is forced into the tube to form the flare.
This is a finished 37-degree flare-note the uniformity of the flared sealing surface.
AN flares require a nut and a reinforcement sleeve. Always remember to put them in place before flaring the tubing.
Making 45-degree flares requires a two-step operation to fold the tubing back onto itself.
To help prevent fittings from leaking Koul Tools offers lapping tools for 37- and 45-degree fittings.
There are a variety of fittings used in brake systems. This is a 37-degree to pipe thread adapters that might be found in some calipers, residual valves, or other components.
Here is a typical 37-degree AN line with a female fitting attached to a male adapter.
Standard automotive 45-degree fitting use an inverted flare seat to seal against the tubing.
This is a typical male 45-degree fitting with a pipe thread adapter. Neither 37- nor 45-degree fittings should require Teflon tape or any other type of sealer.
Often used with drum brake wheel cylinders with a 45-degree inverted flare seat, this adapter allows the use of an AN flex line.
Another common adapter-this example connects 45-degree rigid lines to an AN hose. The groove is for a retainer clip to hold the fitting in a frame bracket.
These are the components used on Manson’s FI. Fittings are AN, note the through the frame fittings (arrows) to connect the stainless hard lines to the front flex hoses.
Because the master cylinder is mounted lower than the calipers, 2-pound residual pressure valves were installed in the front and rear lines. Note the adjustable proportioning valve in the rear line.
Stainless -3 flex lines from Inline Tube were used to connect the front calipers. Always check to ensure the lines don’t get tangled in the suspension when turning.
As the rear calipers float on the mounting pins in operation they’re connected to the rigid line on the rearend housing with flex lines.
Another flex line is used between the frame and the rearend housing. Note the bulkhead fitting to secure the T to a mounting tab-all of which is available from Inline Tube.