Brake System

[Bob Steward writes] 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.

Althought 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.

The hydraulic brakes use a Mil Spec fluid 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 directional control on the ground.

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 that many people are 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 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 with the flex hose installed and pre-loaded with fluid. 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. A drop of fluid ought to come out to let you know you have opened the valve enough for the fluid to flow. Leaving the wrench on the bleeder screw hex, slip the end of your flex hose over the end of the bleeder screw, [EQ: loosen the bleeder screw an additional half-turn], and begin pumping. This procedure inevitably a small unwanted air bubble into the system, so you will need to bleed it [EQ: what's the antecedent for "it"?] enough to purge it from the lines to the master cylinder on the passenger side and then through the lines to the pilot's side master. My hand pump oil 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.

Now 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 filler plug when done.

Beside 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 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.

[EQ: do you think it's worth mentioning that on some models ('78 and '79 4-place), it's possible for one set of pedals to become dangerously soft while the other set remains functional. This is an excellent argument for learning to land the plane from the co-pilot's seat...]