Fixing Soft Brakes
From Bondline
Contents |
Introduction
Mechanical Difficulty: 
Properly maintaining the brake system on Grumman American aircraft is extremely important because the brakes are the only way to steer the plane on the ground at low speeds where the rudder is ineffective.
Although the brake system on Grumman American singles is listed in the parts manual and sales brochures as having "optional" passenger side brakes, few seem to have been built with the "standard" brakes. Accordingly, I'll discuss the brake system as if they ALL have brake pedals on both sides.
Due to the way the pilot's side master cylinders plumb through the co-pilot's side master cylinders, if the fluid is too low in the reservoir(s), the pilot's side brake pedal(s) can be soft and ineffective, and the co-pilot's side can still operate normally -- but perhaps only for a few applications, depending on the size of the leak. This can be used to your advantage if the brakes are checked as part of a pre-landing checklist (step on the toe brakes in flight and see if either is soft) by giving you advance warning of the loss of brake effectiveness and allowing you to either change seats if solo, or to transfer braking responsibility to the co-pilot.
The hydraulic brakes use a Mil Spec fluid MIL-H-5606G, commonly referred to as "5606". This is a red petroleum-based fluid, as opposed to the Glycol based automotive fluids, and the system is not compatible with any other fluid.
The pilot's side master cylinders each have built-in reservoirs that contain the surplus fluid needed as the brake pads wear and the wheel cylinder piston moves out to take up the missing pad thickness. The proper fill level is approximately 1/4" below the top of the reservoir, and can be measured with a pipe cleaner. Filling the system before the fluid level drops far enough to suck in air will prevent the loss of braking and subsequently, directional control on the ground.
Bleeding the Brakes
When air enters the brake system, the brake pedals become mushy, soft, and ineffective. When this occurs, the air must be replaced with brake fluid by "bleeding" the brakes. The brakes are bled by pumping clean 5606 fluid into the bottom bleeder valve on the brake cylinder located on the inboard aft side of each main wheel. A 1/4" box end wrench is needed to loosen the bleeder. Note: Many people get confused about the direction to turn the bleeder screw, since it is upside down.
The best way to force fluid into the system is by using a pistol-grip style oil can with a short section of clear plastic line slipped over the tip. The vinyl line used on aquarium air systems works well, as does certain types of surgical rubber tube. Prepare for servicing the brakes by filling the oil can with several ounces of 5606 fluid and by attaching the flex hose to the end of the can. Finally, pump the flexible hose full of brake fluid to avoid pumping air bubbles INTO the system.
You're now ready to begin the bleeding procedure, which is greatly simplified by having a second person in the cockpit to monitor its progress.
Remove the filler plug from the master cylinder reservoir on the correct side of the brake system, and then put your 1/4" box end wrench on the bleeder fitting and crack it open. You'll know you've opened the fitting when a drop of fluid appears. While leaving the wrench on the bleeder hex-screw, slip the end of the preloaded flex hose over the end of the bleeder screw, loosen the bleeder screw an additional half-turn, and begin pumping enough to purge the air from the lines. My hand pump oil can usually takes 15-20 strokes to accomplish this.
Now, tighten the bleeder snugly--but leave the hose attached. Check the pedal on the pilot's side; if it's firm, then you can stop pumping. If not, open bleeder and continue pumping and checking until the pedal is firm.
After bleeding the brakes, check the fluid level with a pipe cleaner to make certain that it's within 1/4" of the top of the reservoir. If the fluid is lower, bring your pump inside and fill the reservoir. Re-install the filler plug when done.
Other Source of Braking Problems
In addition to loss of brake fluid, there are many reasons why the brake pedal might become soft. A history of fluid loss requires a careful inspection of the brake masters, the flex lines in the cockpit, the hard lines in the cockpit, and especially the aluminum hard lines from the wing walk area to the wheel cylinders. Cracks in the flare of the aluminum tube where it enters the caliper are very common, especially on the older members of the fleet. The entire line can be replaced with a new part from FletchAir or one fabricated in the field from 5052-O aluminum tube (0.035" wall) or many have been repaired with a 6" section of teflon lined steel braided hose and a straight union AN fitting to couple the re-flared line to the female flex hose.
Those planes with rubber covers over the main gear legs will need to split the aft edge of the rubber cover to access the aluminum brake line on the aft side of the main gear leg. The rubber comes in 2 halves and was glued together when installed. The glue can usually be separated with a dull butter knife. It will shear the glue without cutting the rubber cover. The trailing edge can be re-glued with 3M Super Weatherstrip Adhesive, which is available in black or yellow.
Due to a change in the parking brake design on some '78 and all '79 and later 4-place, a rather baffling loss of fluid can occur. The fluid can disappear from one master cylinder reservoir, and over fill the other! The parking brakes are plumbed with a valve that can be closed to hold braking pressure after the pedals are released, and the o-rings in the valve are known to fail such that the pressure is leaked from one side of the system to the other side with no external leaking. The fix is to remove the parking brake valve from the firewall ahead of the co-pilot's rudder pedals and repair it with a new set of o-rings.
