|Time: 2 hr
Tools: Vacuum brake bleeder, Pressure brake bleeder, Friendly assistant
with a good right leg
Applicable Years: All
Tinware: Brake fluid
Tip: Bleed your brakes with different colored fluid so that you know when
all of your older fluid has been flushed through the system
Performance Gain: Stiffer and safer braking
Comp Modification: Replace master cylinder, replace brake lines
Bleeding brakes is not one of my personal favorite jobs. There seems to be a bit of
black magic involved with the bleeding process. Sometimes it will work perfectly,
and then other times it seems like you end up with a lot of air in your system. The
best strategy to follow when bleeding your brakes is to repeat the procedure several times
in order to make sure that you have removed all the trapped air from the system.
The right tools are a necessary part of the job too. A few
days before this book was to be sent off to my editor, I had a chance to evaluate a new
type of pressure brake bleeder kit from Motive Products. Retailing for about $45,
this kit attaches to the top of the master cylinder reservoir and applies pressurized air
to the system. Brake fluid is forced out of the master cylinder reservoir and into
the system. The pressurized kit is probably the best one around because it is the
least likely to create air bubbles in the system. There was a time when no one was
manufacturing pressurized bleeders, but thankfully Motive Products now supplies this
excellent quality kit at a reasonable cost.
The pressurized system works very well because it pushes
the brake fluid out of the reservoir and into the system. In this manner, it is very
unlikely to create air bubbles in the system. When small air bubbles form in the
brake lines, the entire system suffers as the brake pedal becomes soft. This is
because air is much more compressible than the brake fluid. When you push on the
pedal, the air trapped in the lines acts like a spring inside of the system. The air
becomes compressed, absorbing energy from the system, instead of directing the energy
towards pushing the caliper piston against the brake disc.
A second alternative is the vacuum bleeding kit.
This kit works in the opposite manner of the pressure bleeder, applying a vacuum to the
brake system in order to draw brake fluid out of the car. The system works well, but
can sometimes cause air bubbles to form in the lines. Particularly on cars with rear
brake proportioning valves like the Porsche 914, the vacuum system can leave air trapped
in these valves, giving a spongy pedal as a result. When using the vacuum bleeding
system, the best approach is to bleed each corner of the car several times, in order
assure that all the air is out of the system. Simply fill up your brake reservoir,
attach the pump system, pump up some vacuum, and then open the bleed nipple. Brake
fluid should be pulled out of the system when the vacuum is applied. If its
not you may have a problem with your brake lines.
The third and most labor intensive method of bleeding
your brakes involves actually having an assistant press on the pedal while you go around
to each wheel and bleed the system. Without a doubt, this is the most effective
method of bleeding, and should probably be used as a final procedure when performing any
brake system bleeding. This method actually pushes fluid through the system (similar
to the pressure-fed system) at a high rate of velocity. Sometimes, air bubbles that
are in the system can become dislodged and cleared out by the quick rush of brake fluid
when you press on the brake pedal.
The procedure for bleeding the brakes using the brake
pedal is pretty straightforward. Attach a small rubber hose to the brake caliper
nipple and let the other end hang inside an empty container. Ask your assistant to
firmly and quickly press on the pedal 3 times, and hold it down the third time.
Then, open up the bleed nipple by unscrewing it slightly. Brake fluid should come
rushing out and the pedal should sink to the floor. Make sure that your assistant
doesnt remove his or her foot from the pedal, as that will suck air back into the
system. With the pedal still depressed to the floor, tighten up the bleed nipple.
When the nipple is closed, have your assistant remove their foot from the pedal.
I recommend that you use this procedure as a final step,
even if you are vacuum or pressure bleeding. The high force associated with the
pressure from the brake pedal can help free air and debris in the lines. If the
brake fluid doesnt exit the nipple quickly, then you might have a clog in your
lines. Brake fluid that simply oozes out of the lines slowly is a clear indication
that your rubber lines might be clogged and constricted. Dont ignore these
warning signs check out the brake lines while you are working in this area.
Another important thing to remember is that brake fluid
kills paint jobs that is. Brake fluid spilled on paint will permanently mar
the surface, so be very careful not to touch the car if you have it on your hands and
clothing. This of course, is easier said then done. Just be aware of this
fact. Rubber gloves help to protect yourself from getting it on your hands and your
paint. If you do get a spot on your paint, make sure that you blot it with a paper
towel - dont wipe or smear it. Its also important not to try to clean it
off with any chemical or other cleaning solutions.
During the bleeding process, its very easy to
forget to check your master cylinder reservoir. As you are removing fluid from the
calipers, it will be emptying the master cylinder reservoir. If the reservoir goes
empty, then you will most certainly add some air bubbles in to the system, and you will
have to start all over. Keep an eye on the fluid level and dont forget to
refill it. Make sure that you always put the cap back on the reservoir. If the
cap is off, then brake fluid may splash out and damage your paint when the brake pedal is
released. If you are using a pressure bleeder system, make sure that you often check
the level of brake fluid in the bleeder reservoir so that you dont accidentally run
If you are installing a new master cylinder, its
probably a wise idea to perform what is called a dry-bleed on the workbench. This is
simply the process of getting the master cylinder full of brake fluid and
wet. Simply add some brake fluid to both chambers of the master
cylinder, and pump it a few times. This will save you a few moments when bleeding
When you are bleeding the system, start with the wheel
that is farthest away from the master cylinder, and then work your way back towards the
front left wheel. In other words bleed the system in this order: right-rear,
left-rear, right-front, left-front. Bleeding in this order will minimize the amount
of air that gets into the system. Always bleed each caliper more than once, because
bleeding the other calipers can dislodge air into the system. You might be surprised
thatafter 5 times around the car there still might be a little bit of air in the
system. A good rule of thumb is the more you bleed, the better your brakes will be.
There are few little tricks that you can use when
changing your brake fluid. The company ATE makes brake fluid that comes in two
different colors. Its a smart idea to fill your reservoir with a different
colored fluid, and then bleed the brakes. When the new colored fluid exits out of
the caliper, you will know that you have fresh fluid in your system. Make sure that
you use DOT 3 or DOT 4 brake fluid in your car. Some of the later model 911s with
anti-lock braking systems required the use of DOT 4. The use of silicone DOT 5 fluid
is not recommended for street use.
You should also routinely flush and replace
your brake fluid every two years. Deposits and debris can build up in the lines over
time and decrease the efficiency of your brakes. Regular bleeding of your system can
also help you spot brake problems that you wouldnt necessarily notice simply by
driving the car.
Picture 1: One of the most popular
vacuum brake bleeding kits is the MityVac kit. This kit contains a hand operated
vacuum pump and a variety of attachments to allow it to fit just about any car. The
pump draws brake fluid using vacuum pressure created by the hand pump. This
photograph shows the process of emptying the system of brake fluid, prior to removing a