Washington, D.C. Centre
Tech-Tips
Home
About
Join
Board
Calendar
the Spark
Gallery
Tech-Tips
Regalia
Classifieds
Links
Contact

Gimme a Brake III

by Jim Lunson

 

One last thing on brakes - changing the fluid. I said earlier that DOT 3 and 4 fluids need to be changed every few years because they absorb moisture that can rust and corrode the insides of brake cylinders, calipers, and lines. This periodic bleeding is important, especially with a timeless car like our MGs where years of ownership slip by very rapidly as there is little attention paid to the age or model year and the driven mileage is usually low. And rust and corrosion creep up very slowly and are rarely noticeable until a disaster strikes. The way to do this fluid change is to bleed out the system, slowing replacing all the existing fluid with new.

This process is a fairly easy step to do, but takes a little time, patience and willingness to reach up under the car. The process begins by bleeding out the system from the brake farthest from the master cylinder - the right rear brake (unless your car has right-hand drive in which case its the left rear). Start by filling the master cylinder with new, clean fluid, and begin by bleeding the system at this first bleed screw until the fluid comes out clear. This is best done by using a 3/16" clear plastic hose so you can easily see that the fluid coming out has no bubbles and can tell when it changes from a rust colored brown to clear. This first brake usually takes significant fluid to change as it is flushing all the old fluid from the master cylinder, most of the pipes and the brake at hand. These rear brake bleed screws are also the ones hard to reach as they are inside the rear tires and getting to them involves obstacles like the rear axle, emergency brake cable, and exhaust pipes. It takes a little stretching to get back there.

Once, the fluid running out clear from this brake you are well on your way to flushing out the entire system. Tighten up this bleed screw and move to the next farthest brake and repeat the process. This time it is amazing how little fluid needs to be removed before clear material flows out. Then on the next farthest brake and again, only after few bleeds, you will be done. And finally, the fourth and closest brake is last. Once this one is done, the job is complete and ready for another 3-5 years of safe stopping.

To assist the process, there are several brake bleeding systems available for purchase. They are designed to facilitate the process by being able to accomplish the job with only person, but still involve the same operation. They work by using air pressure to force new fluid through the system and out the open bleed screw. I have used both a system that works off the air pressure in the tires attached to a special cap on the master cylinder, and a system that has a hand pump at the bleed spot to induce a vacuum in the system, sucking out old fluid. There are several variations on these mechanisms available. They work, but I have found them be usually be more trouble than they are worth to set up. Plus they also require maintenance when they are only used every 3-5 years or greater. What they do, when they work properly, is to allow one person to reach under the car at the brake to be bled, open the screw and let the fluid flow out until it is clear and contains no bubbles and shut the system when complete. You still have to hit all four brakes to get the job done.

I personally prefer the old tried and true brake bleeding system of using two people, in my case, my trusty mate, Ann. A child, mother, neighbor or any assistant also works well as the task required by this person is not real complicated, consisting of sitting in the drivers seat, stepping on the brake pedal when requested and then releasing it, also when requested. I do the hard part, including the reaching up under the wheels.

This system works like this: attach the clear plastic hose to the brake to be bled. Insert the end of the hose into a jar with about one inch of old fluid in it, making sure the hose end is submerged in the fluid. Then open the brake bleed screw about 1/2 turn. Then ask the trusty assistant to slowly step on the brake pedal until it goes all the way to the floor. Observe the fluid that flows out the tube into the jar. When the pedal reaches the floor, the fluid stops flowing, then tighten up the bleed screw as it was before starting. Then have the assistant release the brake pedal. Repeat this operation until the fluid in the tube runs clear. The only change to this cycle is that after about 5 repetitions, you need to get out from under the car and make sure the master cylinder remains full of fluid or refill it as necessary. If the fluid level here gets too low, air bubbles will get sucked into the system, and additional bleeding will be required until they are flushed out. Usually 3 to 4 pumps are sufficient to flush even the farthest brake, and the closer ones only usually take about two pumps to finish. Not too complicated and lasts for several years.

The only two possible complications occur in that the brake pedal probably has not gone all the way to the floor in a long time. Note I say to push it in SLOWLY, especially the first time. Often the hinges, and pivots in the pedal mechanism may be stiff from non use to their maximum range and could break if forced too suddenly. Also, the master cylinder piston has probably not gone its full travel length for some time either and if forced too suddenly, can break the seals on it. So proceed slowly with that first brake pump. The other complication occurs if the bleed screws have not been opened for a long time and may have frozen. Sometimes, a penetrant will free one to turn; otherwise, it is likely to break off when a wrench is applied. This will open the system immediately and all the fluid will drain out. So if you have trouble turning a screw, make sure you have another large jar ready to catch a lot of fluid coming out. Plus then you have to remove the broken bleed screw and replace it with a new one; a much more complicated task. A test of whether the screws can be loosened is advisable before the process is begun. Then you will be ready without this one surprise. One last note, brake fluid is hard on car paint finishes, so be careful when filling the master cylinder not to spill any, or cover the fender to protect it from damage.

I have discussed flushing the brake system as a maintenance step, but the brakes also need bleeding if there is a problem and air has entered the system. This can occur either due to a leak somewhere, or if the fluid level in the master cylinder gets too low. This bleeding step is the same as outlined above, and should be done if the brake pedal feels spongy, or gets soft after being applied during a prolonged stop. So check your brakes again, find out when the last time you changed out the fluid, and spend some time keeping the car running (and stopping) in tip top shape. Again, this is a step that is easy to ignore, but is crucial to maintaining our MGs. Plus it is a step that most shops charge up to $150 to accomplish (takes two people, remember.) So this winter, find an assistant and get busy on those brakes.

 

Back to Technical Tips

Home