|The Answers to Your Brake Maintenance Questions|
|Written by Jacqueline Shipe|
|Thursday, 24 February 2011 09:46|
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What parts are involved in the braking system?
The average brake system on Cessna airplanes consists of a reservoir/master cylinder at each of the pilot’s rudder pedals, linkage to connect the copilot’s rudder pedals to the brake system, brake lines and hoses, wheel cylinders, and the brake linings and discs.
What maintenance does the master cylinder need?
The master cylinder/reservoir rarely requires maintenance other than replenishing the fluid if it is low.
What can I do to prevent and/or fix leaks?
The wheel cylinders are generally the area where leaks develop. These leaks can be from the o-ring that seals the piston, a loose line connection, or a crack in the housing of the wheel cylinder itself. The most common place for a crack to form is out from where the brake bleeder assembly threads into the cylinder. Cracks form here because the wheel cylinder housing is made from aluminum while the bleeder assembly is steel with a tapered thread; if the bleeder assembly is over-tightened, it cracks the wheel cylinder, causing a leak around the bottom of the assembly.
Although this is the most common type of leak, leaks can occur for other reasons. Most of these problems require the o-ring to be replaced. One problem is that worn brake linings or discs allow the piston to move out further than normal, causing the o-rings to leak.
Other problems occur because the piston gets rough spots around the edges over time and has to be sanded with a lightly abrasive paper. The wheel cylinder housing also gets pits and high places and needs to be smoothed out. Sometimes the housing gets so pitted or worn that it leaks even with a new o-ring and has to be replaced. These are pretty expensive items when purchased new. Once and awhile salvage yards have them but they are often almost as worn as the ones that came off.
Occasionally, the aluminum line connecting the fuselage fitting with the wheel cylinder will chafe on the gear leg or the aluminum fairing installed around the leg and causes a leak. If this happens, it’s best to replace the line rather than trying to repair it.
How do I bleed the brakes?
Each master cylinder is connected directly to its appropriate wheel cylinder. There are no other lines interconnected with tee fittings to transfer fluid anywhere else. This makes the Cessna brake system very easy to bleed compared to other makes.
To bleed the brakes, connect a pump-style filler can with a small clear hose over the bleeder on the wheel cylinder. The bleeder should be wiped off and cleaned before connecting the hose to be sure no dirt is pumped into the system. The fluid should be pumped out to the end of the hose before connecting it to prevent pumping air into the system. A rag can be wrapped around the body of the master cylinder behind the rudder pedal to catch excess fluid. The top of the master cylinder should be wiped off to prevent dust from entering the reservoir. Remove the plastic filler plug and pump fluid up from the wheel cylinder bleeder until it runs out the top of the master cylinder reservoir. The rag should catch the fluid unless there is a lot of it. Excess fluid should be wiped off carpet or upholstery, but it won’t hurt a thing if a little spills down into the floor panels because it actually is a great corrosion preventative.
What can I do to release a stuck parking brake?
The parking brake system on the 150 and 152 models seems to have been a bad design. The brakes won’t release properly even though the parking brake has been pushed back in. Usually one or both wheel cylinders lock up after the plane has taxied forward just a short distance. If this happens, the only way to get them to release is to crack open one of the lines or open the bleeder for just a second to release the pressure. Several flight schools and rental aircraft owners placard them as inoperative or remove the knob altogether so that it doesn’t cause a problem.
Can I use a different fluid than 5606?
The brake fluid used in general aviation airplanes is 5606 red hydraulic oil. Automotive brake fluid causes 5606 to congeal if it comes in contact with it and it also causes the o-rings in the system to swell up. This can result in an extremely hard pedal but absolutely no braking action because the wheel cylinder remains stuck in place. No one should ever attempt to service the reservoir with any other fluid but 5606. Struts and shimmy dampeners also use 5606 fluid.
What precautions should I take when servicing the brake linings?
Brake linings are held on the backing plate with two or more steel rivets. A special brake tool is used to push out the old rivets and flare the end of the new ones. Care has to be taken not to over tighten the new rivets. This can cause either the lining itself or the flared part of the rivet to crack. Under tightening can allow the lining to scoot and chatter on the backing plate.
Brake disc replacement requires removal of the wheel assembly. The disc manufacturer has minimum thicknesses and wear limits established for linings and discs. The discs occasionally develop cracks in them and have to be replaced prior to these limits. The cracks typically form from the outer edge and progress in toward the center. Sudden cooling of a hot disc causes some cracks. For that reason, I try to avoid taxiing through puddles, especially after landing when the brakes have been used.
Aircraft brakes have a very important job. A little maintenance on the brake system will keep everything in top shape so that the airplane stops when needed — a tradeoff any aircraft owner should be happy to make.
From the April 2010 issue of Cessna Owner
|Last Updated ( Thursday, 24 February 2011 09:48 )|