Editing Brakes (section)
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==Brake system== A high performance brake system will often use specific brake fluid for the application, stainless steel hard brake lines, teflon lined braided flexible brake lines, air ducting and venting, along with all the other necessary items to aid in controlling brake component temperatures. The picture below shows a typical hotrod brake system which utilizes an adjustable proportioning valve, residual pressure valves, a master cylinder, and a vacuum brake booster. This is an under-floor brake system setup. [[Image:Hot_rod_brake_system.jpg|border|450px]] ===Proportioning valves=== An adjustable fluid valve that allows you to set the amount of pressure being applied to a wheel cylinder by increasing or decreasing the fluid by way of a adjusting valve. ===Residual pressure valves=== A one-way valve that allows fluid to flow through it at any pressure, but limits the amount of return pressure to a certain amount by way of a spring loaded check-valve. Usually comes in 10 psi (red color) for drum brakes and 2-3 psi (blue color) for disc type brakes. These valves are mainly used in under-floor systems where the calipers are higher than the master cylinder reservoir and to compensate for the return spring tension in drum brake systems. RP valves eliminate excessive brake pedal travel in both systems. On a normally functioning brake system having the MC above the calipers, there is no need for a RP valve. If there seems to be a need for a RP valve in a disc brake system having the MC above the calipers, look for the root cause, rather than putting a 2 psi band aid on it. The reason a RP valve shouldn't needed is because disc brake calipers have no retraction mechanism like a drum brake. A drum brake has springs that pull the shoes away from the drum, calipers don't. It would take 'vacuum' applied to the brake fluid to cause the caliper piston to retract- and that's what happens when the MC is BELOW the calipers and the fluid tries to run "downhill" to the MC. If you have a soft pedal w/a 4-wheel disc non assisted brake system it could be due to air in the lines- bleeder screws on the bottom (calipers swapped side to side) will cause air to be trapped and a soft or spongy pedal. A too-high pedal ratio, a defective MC, or too small MC bore will also cause it. ===Master cylinders=== A dual reservoir master cylinder (MC) is used in any modern vehicle and should be used on older vehicles that had single reservoir master cylinders. Usually a drum brake master cylinder will not work for disc brakes because the bore size is too small, and the drum brake MC may have a built in residual valve. ====Power master cylinder diameter and pedal ratio==== Most times, a master cylinder for a power brake system should have a pedal ratio of about 4:1 when using a master cylinder bore diameter of 1-1/8". ====Manual master cylinder diameter and pedal ratio==== Most times, a master cylinder for a manual brake system should have a pedal ratio of about 6:1 when using a master cylinder bore diameter of 1". As a rule of thumb, a pushrod attachment point on the brake pedal about 1" above where the power brake attachment point was (closer to the pivot point) will be about right for a manual brake system. If using a bore larger than 1" on a manual system, the brake pedal effort can become very high. [[File:Pedal ratio1.jpg|thumb|700px|left|Pedal ratio guidelines from mpbrakes.com]] <br style="clear:both"/> The problem sometimes encountered when using a disc/drum manual MC on a 4-wheel disc system is inadequate volume to the rear disc brakes. Calipers take more volume than drums. If there's not enough volume, the pedal will bottom before the brakes are fully applied and the pedal will not be firm. To use a disc/drum MC on a 4-wheel disc system, the MC needs to be capable of about 1200 psi and has to have adequate volume to operate the rear disc calipers. The MC bore needs to be about 7/8" to 1", and the pedal ratio needs to be around 5:1. Be sure to remove the residual pressure valve to the rear drums. An adjustable proportioning valve can be used to adjust the front-to-rear brake bias. =====Pedal ratio/bore size vs. pressure output===== <table border="1" cellpadding="2" cellspacing="0" width="595"><tr bgcolor="#CCCCFF"><td class="tdName" width="20%">'''Pedal Ratio'''</td><td class="tdName" width="20%">'''Bore Size'''</td><td class="tdName" width="25%">'''Pounds Input'''</td><td class="tdName" width="25%">'''Approx. PSI Out'''</td></tr><tr class="trLight"><td>6:1</td><td>1-1/8</td><td>75</td><td>450</td></tr><tr bgcolor="#eeeeee" class="trDark"><td>6:1</td><td>1 </td><td>75</td><td>575</td></tr><tr class="trLight"><td>6:1</td><td>7/8</td><td>75</td><td>750</td></tr><tr><td bgcolor="#oooooo" colspan="4"></td></tr><tr bgcolor="#eeeeee" class="trLight"><td>5:1</td><td>1-1/8</td><td>75</td><td>375</td></tr><tr class="trDark"><td>5:1</td><td>1 </td><td>75</td><td>475</td></tr><tr bgcolor="#eeeeee" class="trLight"><td>5:1</td><td>7/8</td><td>75</td><td>625</td></tr><tr><td bgcolor="#oooooo" colspan="4"></td></tr><tr ><td>4:1</td><td>1-1/8</td><td>75</td><td>300</td></tr><tr bgcolor="#eeeeee" class="trDark"><td>4:1</td><td>1</td><td>75</td><td>380</td></tr><tr class="trLight"><td>4:1</td><td>7/8</td><td>75</td><td>500</td></tr></table><p> ===Brake boosters=== In most cases the vacuum required to operate a power brake vacuum booster should be at least 18" for best results. In most cases anything less than 14" of vacuum will not be enough. Using a vacuum reservoir is not a very good substitute for inadequate vacuum. In cases where there's not enough vacuum, a vacuum pump may be used, or the system changed to manual brakes. If space is a problem, a dual diaphragm booster might be enough smaller in diameter to help, as long as the booster has sufficient pressure to do the job. ====Some guidelines from MP Brakes:==== A midsize GM car with disc brakes in the front and drum rear brakes will require at least 900-1,000 psi to the wheels to lock them up. The pressure output of the booster is directly proportional to its diameter, the larger the booster the greater the power assist. The following assumes 18 in/Hg of vacuum at idle, and 120 psi of pedal force: *Four wheel disc = 9" dual diaphragm (1200 psi) *Front disc/rear drum = From the 9" dual (1200 psi) down to a minimum of a 7" dual (900 psi) *The 8” dual diaphragm booster makes 1,000 psi *9” single diaphragm 900 *7” dual diaphragm 900 *7” single diaphragm 800 ===Brake lines and fittings=== [[File:SM_brake_fitting01.jpg|border|300px]]
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