Power Boosters: Power boosters were needed when disc brake systems were being used more and more on factory cars.  The amount of boost created from the booster is directly related to the square inches of the booster and the inches of vacuum imputed from the engine.  Since the disc brake calipers required a greater volume of fluid due to the size of the pistons and the clamping force (some times up to 6 tons), the master cylinder requires a bigger diameter bores to push the required volume of brake fluid.  When you increase the bore size you reduce the output pressure of the master cylinder.  In order to boost the pressure output of this larger bore master cylinder the factories installed a power booster.  Power booster range in size from 7" to 11".  Most street rods have floor mounted pedals so the master cylinders are generally located under the floor boards.  This creates a room problem so the 7" booster was incorporated to use with the 1" and 1-1/8" master cylinders.   The biggest problem with using a power booster is it requires vacuum to operate and most hot rods have 3/4 race cams so there is little or no vacuum.  If you are currently using a power booster and having problems stopping, take a vacuum gauge and check the inches of vacuum.  To work properly it takes 16-18 inches of vacuum anything much less than this forget it.

Recently the dual diaphragm boosters have become available to the hot rod industry.  The dual diaphragm 7” has a total area of over 76 square inches about the same as a 10” single.  This doubles the vacuum assist of the single diaphragm.   I would recommend anyone using the older single diaphragm to update to this booster.

In the past I have seen everything from remote vacuum canister to electric vacuum pumps to increase or store the vacuum.  So what happens when the engine dies or you loose your 12 volt electricity or take more than two pump on the brake pedal?  No vacuum and no brakes!

The newest thing is to install a hydrostatic system driven by the power steering pump.  This is a after market version of the systems often found in one (1) ton pick-ups.  I had a 1 ton crew cab that was used to pull a 48 foot 5^th wheel trailer with a gross weight of 21,000 pound.  I once lost a serpentine belt descending off Mt Ashland (Oregon) on I-5.  No brakes!   I was luck enough to have 3 miles of straight highway.  It is a good thing I had good trailer brakes or I might not be writing this article.

It basically boils down to the fact that there are all sorts of supplemental equipment to assist your lack of vacuum, but if you keep everything SIMPLE you will not need them.  Someone once said, “keep it simple stupid”.

That is why race cars depend on manual master cylinders.

This is the formula to figure your output booster pressure.

Force in pounds = (Diaphragm area in square inches) x (manifold vacuum in inches Hg) x ½

Example: 7” single diaphragm booster with 17 inches of vacuum.  3.5” x 3.5” x 3.14 = 38.465 square inches x 17 inches of vacuum x 50% = 326.95 psi

Here is quick table for your reference;

Maximum diaphragm force in pounds

                                Vacuum                Diaphragm diameter

Of course this it based on the booster being 100% efficient, a good rule of thumb would be 80 to 90 percent efficient.

Booster Master Cylinder Combinations:  When every possible you should always replace the existing booster with another one of similar size and design.  Engineers designed the booster/master cylinder based on the weight, tire size, calipers, rotors, etc.  However when you change anything on your vehicle it effects other parts that was engineered to be compatible with the part you changed. 

When choosing a booster master cylinder take all the factors into consideration. 
     1. Do I need a booster?  Who will be driving the vehicle?  How big is that person?  The bigger the person the more applied force there is available to the brake pedal.  It is much easier for a 250 pound person to apply 150-200 pounds of applied force to the brake pedal than a person weighing 110 pounds.
     2. Location of the booster?  Valve covers, shock towers, floor boards, clutch linkage are some restriction that will restrict the size of the booster.  The last thing you want to do is remove your booster to take your chrome tall valve covers off.
     3. Amount of vacuum?  If possible measure the available vacuum.   18-20 inches no problem, 16 inches marginal, 12 inches or less forget it.  And please do not even consider a electric vacuum pump.
     4. Of course we would want to put the small bore master cylinder with the booster we have chosen, but if you choose too small of a bore, you might run out of brake fluid or you might have too much out put pressure causing your pedal to feel spongy or to sensitive.

     Brake Lines: Think of your brakes lines as the blood system in your body.  Just like your body there are important things that need to be implemented when running your brake lines.  Never run your brake lines near any source of heat, such as headers or exhaust pipes.  Use steel brake lines as much as possible and keep the length of the flexible line as short as possible.  In selecting brake lines always use thick wall tubing and steel braided teflon lined flex hose.  The rigidity of the brake system is a must, you do not want any part of this to flex.  Use 3/16" brake lines on most applications, the smaller 3/16" line will fit the need of 99 percent of the applications.  Always double flare the steel lines, even if you are using AN type fittings.  We first double flare the lines with a 45 degree, then flare it with a 37 degree flaring tool, when using AN type fittings.   Use .025 or .035 stainless lines and single flare at 37 degrees with AN fittings.

I can not say enough about having good tools for the job.  Beg, borrow or buy a good set of Ridgid flaring and bending tools.  After you have used a cheaper flaring tool you will know what I mean. 

It seems that the latest "fad" is to route your brake lines inside your boxed frame.  I for one think the brake should be where you can get to them for service and inspection.  How do you know if the lines are leaking, unless you buy tubing in 20 foot lengths the line inside your frame has a connection.  Was the brake line mounted to the lower rail?  Outside rails? or Inside rails?  If you have to drill into your frame where is the brake lines?  How was it mounted and where?  Clamps?  Was the clamps held down with machine screws?  Will the machine screws work loose?  Unless the brake lines were stainless, steel lines do rust, so how do you replace the brake lines?  Simple is always the best, route your brake lines where you can service them.  

Copyright reserved by Dean Oshiro.  Reproduction without written approval is a violation of Copyright Laws.  1994-2006