August 2014- For as long as I can remember, bleeding the brakes on the Cherokee family of aircraft (all PA-28, PA-32 and PA-34 models) has been a problem. For years I’ve listened as many mechanics and owners suggested the different approaches that they have taken to bleed the brakes—all with a limited amount of success.
The procedure that I’ve used for over 45 years (which may not be original) has worked for me every time without a great deal of trouble or expense. Owners who are not A&Ps must work under the supervision of their mechanic per the FAR/AIM.
Before we get involved with the actual process, I would highly recommend the brake hoses—from the parking brake to the master cylinders to the actual brake calipers—be inspected very closely. I have seen many old hoses actually swell when brake pressure is being applied.
If these hoses are over eight years old or have over 1,000 hours on them, replace them. Brakes are the best insurance you can have. If you were to taxi into another plane on or off the runway due to a ruptured or leaking hose, the insurance company might have a good case for not paying up since the factory recommends replacement every eight years or 1,000 hours.
What you need
Once you’ve confirmed the hoses are in good working order, you can proceed with this tried-and-true procedure.
In the way of tools and equipment, you will need a ¼ inch box end wrench (it would be best to have two ¼ inch box wrenches; one for each brake caliper) to make the brake bleeding process easier. I’ve found it very helpful to reduce the thickness of the box end of the wrench by grinding it, which allows the tubing to have more area to fit over the bleed port.
You will also need 25 feet of clear 3/16-inch (inside diameter) plastic tubing—50 feet of tubing if you plan to do both sides simultaneously. This flexible tube material (Tygon is one brand) can normally be found in the plumbing department of most hardware stores, and it’s not very expensive.
Other supplies you’ll need to accomplish this job include masking tape and .032 safety wire. You will also need at least one quart of MIL-PRF-5606 hydraulic fluid. (This red, mineral oil based hydraulic fluid has replaced MIL-H-5606. —Ed.)
MIL-PRF-5606 is normally available at most FBOs or aviation repair shops. It may also be purchased in gallon quantities from Aircraft Spruce and other aircraft supply houses. If you do not have any on hand, I would suggest purchasing at least two quarts, especially if you have replaced hoses or overhauled components of your hydraulic system. If you have just installed new O-rings in the calipers or are just bleeding the brakes, one quart should be enough. However, it’s always a good idea to have some spare MIL-PRF-5606 around since it is also used in the landing gear as well.
Beginning the process
Once you have all of your materials, open the left engine cowl or remove the top cowl for access to the brake fluid reservoir which is mounted on the left side of the firewall. If you bought 50 feet of plastic tubing, cut it into two 25-foot sections.
Place one of the ¼ inch box wrenches on the bleed port fitting which is located on the lower side of the brake caliper (right or left brake).
Slide one end of the plastic tubing over the end of the brake bleeder port fitting. Wrap a small coil of safety wire (like a clamp) around the end of the tubing to secure it to the brake caliper bleed port.
Using a tie-wrap or masking tape, secure the tubing to the existing brake hose (or the landing gear strut) to make sure the plastic tubing will not loosen from the caliper bleed port.
Run the tubing up and over the leading edge of the wing. Continue with the tubing, taking it up and over the top of the cabin. Secure the tubing to the top of the windshield with masking tape and continue down across the windshield to the brake fluid reservoir.
Remove the brake reservoir cap and insert the end of the tubing into brake fluid reservoir.
Repeat the preceding steps on the other side of the plane.
Once both tubes are immersed in the brake fluid reservoir, you are ready to move on.
Bleeding the brakes
Make sure your brake reservoir is topped off with MIL-PRF-5606 hydraulic fluid before you get started and continue to check the reservoir fluid level throughout the bleeding process. Even though the reservoir may appear full now, keep an eye on it, as you don’t want to the reservoir to run low on fluid.
It’s helpful to have an assistant in the plane during the brake bleeding process, but it isn’t essential.
Loosen the bleed port fitting and start pumping the brakes. Continue the pumping action and observe the hydraulic fluid flowing in the clear tubing. Check the reservoir to make sure it’s topped off with fluid. (Once the tubing is full of 5606, you won’t have to worry about the reservoir fluid level.)
Continue to pump the brake pedals as well as the hand pump (parking brake) while observing the flow of hydraulic fluid. Keep going until you no longer see any air bubbles flowing inside the tubing.
Continue pumping the brakes (alternating from the hand brake to the pedals) for about five minutes to make sure you have all the air removed from the system. Close the bleed port fitting on the brake caliper and repeat the same operation for the other side.
Normally, when the bleed tube runs solidly with fluid (no visible air bubbles), you will be done and should have a hard brake pedal. In most cases I have found that this procedure works the first time around.
Getting rid of trapped air
If you carefully bled the brakes (watching closely that all of the air was removed) yet find the brake pedals or parking brake still spongy, you probably have some trapped air in one of the master cylinders. This is normally caused by an air bubble trapped near the top fitting of one or more of the master cylinders.
To remove the air bubble, place an old towel under the master cylinder you plan to bleed. Pump the hand brake and lock the handle.
With a 9/16-inch wrench, momentarily loosen the top hose fitting and then immediately retighten the hose fitting.
Release the hand brake and check the brakes again. If needed, you may need to perform the same procedure for each brake master cylinder, although I rarely have had to do this.
When you are finished with the brake bleeding, remove the safety wire and tubing from the bleed port. Immerse the end of the tubing in the container of MIL-PRF-5606. Carefully remove the tube from the reservoir and allow the fluid to drain back into the container. Repeat this for the other side as well.
After you are finished, check to make sure your reservoir is full of fluid. Reinstall the cap on the reservoir and re-cowl your engine (or latch the cowl door)—you are now ready to go flying with good solid brakes.
Dick Russ is a multi-thousand-hour commercial, multi-engine and instrument-rated pilot. He’s also a flight test engineer and an A&P/IA who has restored many Pipers. In addition to his career as a freelance writer and aviation business owner, he was senior engineer on the Shuttle Enterprise Approach and Landing Test Program at Edwards AFB. Russ holds three patents on aviation components. Send questions or comments to editor@www.piperflyer.com.
RESOURCES
PFA Supporters that stock aircraft hydraulic fluid
Aircraft Spruce & Specialty
aircraftspruce.com
(Royco)
Chief Aircraft
chiefaircraft.com
(Aeroshell Fluid 4, Phillips 66 X/C)
Wag-Aero
wagaero.com
(Phillips 66 X/C)
