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Home > Technical Articles > Bleeding Brakes

Guest Technical Article

Bleeding Brakes

Difficulty Level 2

Difficulty scale:
Adding air to your tires is level one
Rebuilding a 911 Motor is level ten

Wayne R. Dempsey

This article is one of the sample chapters from Wayne's latest book, 101 Projects for your Porsche 911.  The book has 240 pages of full color projects deailing everything from headliner replacement to timing the camshafts.   With more than 350+ full-color glossy photos accompanying the more than 100 projects, this book should be a staple in any 911 owner's collection.  See for more details.
Time: 2 hr
Tools: Vacuum brake bleeder, Pressure brake bleeder, Friendly assistant with a good right leg
Applicable Years: All
Talent: 2
Tab: $20
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 it’s 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 doesn’t 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 doesn’t 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.  Don’t 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 - don’t wipe or smear it.  It’s also important not to try to clean it off with any chemical or other cleaning solutions.

During the bleeding process, it’s 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 don’t 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 don’t accidentally run dry.

If you are installing a new master cylinder, it’s 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 the brakes.

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.  It’s 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 wouldn’t necessarily notice simply by driving the car.

PowerBleederPro.jpg (9130 bytes)

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 brake caliper.


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