AIR TRACTOR AT-502A, Секция обслуживания

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Maintenance Page 4 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
If a spot of brake fluid appears on the cockpit floor on the L/H side it does not necessarily mean that
the master cylinder is leaking. The usual cause is over-filling the brake reservoir, so that when brakes are
used, and fluid returns to the reservoir, it sometimes leaks through the vent hole in the reservoir cap.
Brake Bleeding
If air enters the brake system because of worn O-rings or the replacement of brake system
components, the brake pedal will become “soft” and the brakes lose some of their effectiveness. It will then
be necessary to bleed the brakes to remove the air.
The conventional practice of placing a pressure pot line at the bleed screw location on the wheel
cylinder and forcing fluid up through the system generally does not work. The factory practice is to place one
person in the cockpit and another at the wheel cylinder with a bleed screw wrench and a Coke bottle to catch
the fluid. Before the bleeding process begins, it is necessary to obtain a short piece (about 6" long) of
windshield wiper hose which will fit snugly over the bleed screw head so that all the fluid can be caught in the
Coke bottle and not wasted.
The procedure is as follows:
1. The cockpit man pulls on the parking brake and leaves it on. Then he pumps the pedal on the side
being bled several times until the pedal begins to get firm. The parking brake is left on since it is
actually a one-way check valve.
2. Then the cockpit man
pushes on the pedal and shouts “Open” to the man at the wheel cylinder who
proceeds to open the bleed screw with a 1/4" open-end wrench. Brake fluid and air then rush
through the bleed screw, through the hose, and into the Coke bottle.
3. The cockpit man,
who has held the pedal all the way down, then shouts “Close” as soon as the pedal
reaches the down position, and the other person closes the bleed screw immediately.
4. The cockpit man uses the procedure outlined in (1) to obtain a firm pedal and again repeats step (2)
and (3).
5. The above procedure is repeated as many as a dozen or so times until a firm brake is obtained with
the parking brake off. Be sure to add brake fluid to the reservoir before the reservoir is emptied or
else you will have to start
all over again. Don’t use the fluid in the Coke bottle as it will have bubbles
of air. When the bubbles are gone it is OK to use, however.
6. Repeat the process for the other wheel.
Parking Brake
The parking brake valve used on later Air Tractors is a Scott p/n 4500A-2. Two of these valves are
mounted together with a common lever to pull the valve to the “ON” position. O-Rings within the valves may
be replaced when needed.
TAIL WHEEL
The tail wheel is a Cleveland p/n 40-168. Keeping the bearings properly lubricated is the key to long
bearing life. A grease fitting is installed in the side of the wheel.
To remove the tail wheel, jack the wheel clear of the ground per previous instructions and remove the
long AN6C64A bolt that attaches the axle to the fork. Note the method of safety wire on the large stainless
jam nut and cut the wire loose from the fork. The tail wheel is now free and ready for dis-assembly. Back off