Some facts about brake hoses:
#1. The industry standard average safety life of a brake hose is six years. FYI data SAE: J1401, J1703, J1705, J1873, J1406, J1288, J1403, J1833, J1402.
#2. Brake hoses deteriorate from the inside as well as the outside.
#3. Moisture is absorbed into brake fluid systems through brake hoses.
#4. Contaminants in brake fluid act abrasively on the inner wall of brake hoses.
#5. The brake hose reinforcing fabric deteriorates through expansion and moisture.
#6. High operating temperatures contribute to the deterioration of hoses.
#7. Brake hoses swell with age and restrict flow.
#8. Generally all brake hoses on a vehicle deteriorate at the same rate, so all hoses should be replaced if one is found to be faulty!
Brake hose failure mode:
A. Rupture (burst hose) = age, exercise (too many miles) or impact are the typical cause.
B. Partial internal collapse = the inner ply becomes damaged/detached acting as a partial restriction and/or one-way valve.
C. Full internal Blockage = the inner ply becomes damaged/detached acting as a plug = the brake pedal still feels good but there is no brake application.
There seems to be a bit of black magic involved with bleeding brakes. 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.
Warning: Over-stroking the master cylinder may cause it to fail.
This can happen to even the most experienced mechanic, "Especially" on a high mileage car. The old school manual brake pedal "down" - "up" - "down" - "up" bleed method risks over-stroking the master cylinder. The master cylinder piston seals can get scored if they're pushed into a normally unused - corroded, region of the master cylinder's bore. If this happens, you'll have little or no brake pedal, and will need a new master cylinder. I strongly recommend using a pressure bleeder to avoid the issue.
There are three basic methods of bleeding the brake system:
Pressure Bleeding. This is where you have a reservoir of brake fluid, and place a positive air pressure force on the opposite side of the fluid, forcing it into the brake system.
Vacuum Bleeding. This is where you fill the reservoir, and then apply a vacuum at the bleeder nipple to pull fluid through the system.
Helper Bleeding. This is where you recruit the one family member or friend who owes you a favor and have them stomp on the pedal repeatedly until the entire system is bled. Usually, this results in sore legs and the family member yelling at you "Again...?!?"
The method we will use here combines the first and the third methods described above. Basically, I advocate bleeding the system with the pressure bleeder, and then using a family member to stomp on the pedal to free up the proportioning valve.
The first step in bleeding your brakes is to fill the system with brake fluid. Some people have suggested that alternating colors of brake fluid (ATE SuperBlue) in order to determine when fresh fluid has been flushed through the entire system. I use a pressure bleeder like the Motive Products Bleeder. The system has a hand pump that you can use to pressurize the brake fluid. A small gauge on the front of the brake fluid reservoir indicates the pressure of the brake fluid inside. The very large reservoir can hold about two quarts of brake fluid: more than enough for most brake flushing and bleeding jobs. Retailing for about $45, the bleeder kit is a very useful and cost-effective tool to have in your collection.
The system bleeds by pressurizing a bottle filled with brake fluid from air from an internal hand pump. The procedure is to add fluid, attach the bleeder to the top of the reservoir cap, and pump up the bleeder bottle using the hand pump. This will pressurize the system. Note: brake fluid is highly corrosive and will mar paint very easily. Bleeding your brakes is a messy job; keep yourself away from the paint. The probability of spilling on yourself and then leaning against your car is too great. Check to make sure that there are no leaks around the bleeder, or where it attaches to the top of the master cylinder reservoir.
Now start bleeding the system. Start with the right rear caliper, the one that's located furthest away from the master cylinder. You should remove the rear wheels of the car to easily get to the rear caliper. The whole process is a lot easier if the car is off of the ground, and the wheels have been removed. The front wheels can be turned for access to the calipers. Bleed the right rear caliper by attaching a clear plastic hose to the bleed nipple, placing the other end in a jar, and then opening the valve with a wrench (a 8mm wrench is typically needed). Let the fluid out until there are no more bubbles.
If you don't have a pressure bleeder system, you need to find someone to press on the pedal repeatedly to force fluid through the system. This process usually works best by having the helper pump the pedal 5 or 6 times, then hold the pedal down, while you open the bleeder valve. Watch the hose. As soon as the fluid stops moving, close the valve and have the helper build up pressure in the brake circuit by pumping again. Do not let the hose come off the nipple when the bleeder valve is open.
Another solution is to get a check valve and place it on the nipple while you stomp on the pedal. This will work for getting fluid into the system but you will still need a second person for the final step - to make sure you have bled the system completely. If your rear caliper has two bleed nipples (some have one, others have two), bleed the lower one first.
When no more air bubbles come out, then move to the next caliper. Bleed them in this order:
- Right Rear Caliper
- Left Rear Caliper
- Right Front Caliper
- Left Front Caliper
Bleeding in this order will minimize the amount of air that gets into the system.
Repeat until you can no longer see any air bubbles coming out of any of the calipers. Make sure that you don't run out of brake fluid in your reservoir, or you will have to start over again. It is wise to start out with about a 1/2 gallon of brake fluid in the pressure bleeder, and another 1/2 gallon on the shelf in reserve. Depending upon your car, and the mistakes you may make, it's wise to have an ample supply.
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.
Installing a new master cylinder requires a bench bleed to ensure all the air is removed from the unit before you mount it on the vehicle. If done correctly, you can install the assembly, do a simple quick line bleed at the master cylinder and have 90% full brake pedal = this saves time, energy, irritation when bleeding the individual wheel brakes.
Now, make sure that all the bleeder valves are closed tightly. Disconnect the pressure system from the reservoir. Now, get your helper to press down repeatedly on the brake pedal at least five times, and hold it down. Then open the bleeder valve on the right rear caliper. The system should lose pressure, and the pedal should sink to the floor. When the fluid stops coming out of the bleeder valve, close the valve, and then tell your family member to let their foot off of the pedal. Do not let them take their foot off until you have completely closed the valve. Repeat this motion for each bleeder valve on each caliper at least three times. Repeat this entire procedure for all the valves in the same order as described previously.
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. Then, let the car sit for about 10 minutes. Repeat the bleeding process at each corner. The pedal should now feel pretty stiff.
If the pedal still feels spongy, make sure that you have the proper adjustment on your rear calipers or drum shoes. Also, you may need a new master cylinder, have a leaky caliper, or have old spongy flexible brake lines.
The company ATE makes a brake fluid called SuperGold. It's a very good product that I recommend. Make sure that you use DOT 3 or DOT 4 brake fluid in your car. Attention: The use of silicone DOT 5 fluid is not recommended for street use, is not compatible with any other fluid, and requires extreme cleaning plus replacement of all rubber components to convert back to DOT 3-4.
Mercedes-Benz service procedures mandate flushing and replacing the brake fluid every two years. Contamination, deposits, and debris can build up in the system over time decreasing 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. Also, never reuse brake fluid - always use new fresh fluid. In addition, don't use brake fluid that has come from a partially empty re-sealed bottle that has been sitting on the shelf "unknown time". Brake fluid is "hygroscopic" a scientific word that means it absorbs moisture, even from the atmosphere after the shipping seal is broken. This moisture 'boils' out of the brake fluid when you start using the brakes, at best it results in a spongy pedal, at worst a small steam explosion that blows a piston seal or hose. A good rule is never buy more than you can use in a month, and don't save partial bottles.
Shown here is a Motive Power Bleeder attached to the brake fluid reservoir of our project Mercedes. Once the adapter cap is screwed on to the reservoir, open the bleeder tank and fill it with about a quart of new brake fluid. Screw the pump handle on to the tank and pressurize it to about 15psi. Keep in mind that anything over 20psi may cause damage to the various seals in the braking system on your car. Read the directions thoroughly that come with the power bleeder before starting. One other thing to remember is that when disconnecting the power bleeder, always relieve the pressure from the tank by unscrewing the pump handle from the bleeder tank and not from the brake fluid reservoir on the car.
With the car firmly supported on jackstands, remove the wheels and locate the bleeder screw on the brake caliper. Pry off the protective rubber cap on the bleeder screw (green arrow).
Underneath the cap, you'll see the bleeder screw (green arrow). Attach a clear plastic hose to the bleeder screw and use an 8mm wrench to open the screw until brake fluid starts to flow out. Make sure that you have a suitable container to collect the old brake fluid that pours out.
In this picture, you can see the stream of air bubbles flowing out of the braking system. You'll want to keep the bleeder screw open until there are no more bubbles in the line.
In this picture, you can see the new fluid flowing through the line. Once the fluid and air have been purged, close the bleeder valve, remove the plastic hose and refit the protective cap over the bleeder screw.