Shop Work Basics
(Concluded from precedingpage)
the internal workings of a magneto with
burned grease, bronze bearing material,
and electrode contaminants.
Hold the distributor block up to the
light and look closely at the electrode area
for signs of carbon arcing and material
degradation. Sunlight works best for this
check and will easily show the "rainbow"
look of distributor crossfire or arcing
between electrodes.
You need to see that before going
through any cleaning process. If you have
one of the old, black-colored distributor
blocks, take it outside and throw it as far
away as possible. Do everyone a favor and
replace that old-style block with the new
brown polyester block. It's money well
spent.
OccaSionally, we still see the midgeneration, green distributor block. This
should also be tossed due to some problems with failing distributor gear bushings. It isn't often, however, that you see
these blocks installed in even the oldest of
magnetos.
Look closely at the both sides of the
distributor block for cracking that occurs
around the pressed-in bearing. Some
of this cracking is okay and is common
to the old, black distributor block. The
newer, brown block does not seem to
crack as often, but is still susceptible to the
problem.
Make sure you pay close attention to
the block. In all magnetos, regardless of
make or model, if distributor crossfire
occurs you can plan on making major
engine repairs. This is the only area of a
magneto, aside from impulse coupling
problems, that has a potential to destroy
the engine by firing a cylinder out of
sequence.
This magneto can quit, it can cough,
sputter, backfire and spit, but if the block
cross-fires (arcing between two adjacent
electrodes) and does so at a high power
setting (like climb power), the misfired
piston will be destroyed in short order.
When in doubt, spend your money
on a new block. Also, check for cracking,
electrode erosion, plug tower degradation
or any other reason for replacement.
Next month, we'll finish our inspection and reassemble this beastie. Then
comes the part many find most difficult-timing the thing to the engine.
Don't despair! It's not that hard, and all
will be revealed next month.
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Brake Master (yl Rebuild
The brake master cylinder can last for many troublejree
hours with minimal care. Here's how to do it.
BY MIKE BERRY,
IA
Last issue's article on brake bleeding
generated additional questions on the
master cylinder itself, so we added this
expanded article on the master cylinder,
plus some additional tips on bleeding
the system once the master cylinder is rebuilt. Extra tips are includedfor Pipers.
he call came at 2 AM. The pilot
of a Boeing 737 mashed down
on the brakes a little too hard
and the aircraft was stuck solid
to the cool Montana ramp. The
heat generated by the melted brake discs
wasn't enough to blow the tire but the
ground crew gave the dull-red glow on
each main wheel a wide berth.
Unable to taxi from the gate, the captain requested maintenance assistance in
freeing the brake packs. He didn't consider the enormous mechanical advantage found between the master cylinders
and the main-wheel brake assemblies.
He also failed to comprehend the
magnitude of the problem.
It is said that the secret of patience is in doing something else
in the meanwhile. The maintenance crew decided to have coffee
while the brakes cooled off on
their own. The captain decided to
"break 'em loose" by using whatever power setting was required
to effect the desired result.
In the end, the "result" was
predictable; a spare airplane was
flown in from San Francisco, resulting in missed connections for
about 48 passengers, and a written reprimand to the captain for
leaving 150 feet of Goodyear rubber on the tarmac. It's amazing
how much ruin can be generated
by that little cylinder attached to
the end of your toe.
T
fluid under pressure in one direction
while relieving pressure and filling the
cylinder in the other. Its purpose, of
course, is to push the brake caliper puck
against a brake pad, which, in turn,
pushes against the brake disc. Each main
wheel brake has its own master cylinder
for individual brake selection and the
degree of braking can be varied depending on runway conditions.
Master cylinders come in all shapes
and sizes but there are generally only
two distinctly different types used in
modem light aircraft: those with an
integral hydraulic fluid reservoir and
those plumbed to an external reservoir
can. Most Cessna aircraft use a master
Master cylinders come in all shapes and sizes
but the two main types you'll find are cylinders
with an integral fluid reservoir (like the Cessna
model on the right) and those using a remote
reservoir (ommon to the two on the left.
DESCRIPTION AND TYPES
The brake system master cylinder
is a simple device that pumps
MAY2011 . .
real advantage in
this design is in the
servicing of the system. It seems every
Cessna made has a
fluid-soaked section
of carpet around the
left rudder pedals
while most all other
airplanes stay reasonably clean in that
area.
LOCK-O-SEALS
Screwing the inner housing out of the reservoir
body will allow for O-ring and packing replacement. Be sure the reservoir is clean and free of
all dirt.
cylinder with an attached reservoir bowl.
A plastic plug is screwed into the top of
the cylinder, and filling the reservoir is
accomplished by using a squirt can of
5606 (H being the latest version), a long
hose, and the Braille method of aircraft
servicing.
Piper and Beechcraft, among others, generally use a master cylinder with
an external hydraulic reservoir. The
The inner workings of the Cessna master
cylinder can be accessed by removing the set
screw and turning the inner housing out of the
reservoir body.
. . MAY 2011
Master cylinder
operation is Simple.
Pushing the piston
into the cylinder closes a spring-loaded
fill valve and pressurizes the fluid in the
cylinder. The resulting pressure and flow
is transmitted to the brake wheel caliper,
which moves the brake puck against the
disc.
When the piston is allowed to retract,
the spring-loaded fill valve opens, allowing reservoir fluid to enter the cylinder.
The cylinder will only fill with enough
hydraulic fluid to make up for any loss in
moving the pucks in the caliper-very
little circulation of fluid ever takes place
in normal service.
Regardless of the style, the master
cylinder is comprised of a single piston
and a few O-rings. The body is equipped
with fittings and a series oflinkages for
dual pedal operation and the installation
allows for pivoting on both ends. The
most complex part of the system is the
fill valve and its operation. It operation
is also the most problematic.
The spring-loaded fill valve is
nothing more than an O-ring seal
attached to a floating washer or spacer
that moves against spring pressure,
sealing the reservoir supply from the
pressure side of the cylinder. There are
several names for the assembly but in
a generic way this fill valve is called a
Lock-O-Seal.
The name is derived from Cessna's
version of the seal that is a flat steel
washer into which has been fitted an
O-ring. This one-piece pack is then
installed on the shaft and is located
against the piston. The same kind of
arrangement is made for other styles of
master cylinders as well.
As the piston is moved up and
down, the Lock-O-Seal alternately
seals and then opens with each movement of the piston. If the seal is cut,
ripped, chaffed, or just plain worn out,
the brake will feel mushy, and braking
action will be greatly diminished.
PROBLEM IDENTIFICATION
The degree to which the Lock-O-Seal
leaks will determine the failure mode
for the cylinder. A slow leak will prOvide
a spongy pedal feel. If the seal is cut or
missing, there will be no pedal action at
all. In all cases, master cylinder failure
can be separated from other brake
system problems by recognizing whether
or not fluid movement can be felt in the
pedals.
Brake systems that have trapped air
in the calipers or lines can feel soft but
it's possible to still feel fluid movement
through the master cylinder. In cases like
this, pumping the brakes will eventually
provide a stiff (relatively) pedal once all
the air in the lines has been compressed
as much as pOSSible.
Failures in the master cylinder,
though, can be identified by the flat feel
of the pedal. There will be little if any
resistance to piston movement and the
only thing felt in the pedals will be the
return-spring action. Generally speaking, when the fill-valve fails (assuming
the system is full of fluid), the pedal will
go all the way to the floor without providing even a hint of any braking action.
At this point, master cylinder rebuild
will be required. Fortunately, the job
is easy and requires no special tools or
equipment. It does require A&P supervision for certified aircraft, however.
REMOVAl AND RESEAL
The most difficult part of rebuilding the
master cylinder is gaining access to the
component. Stuck behind the rudder
pedals, the master cylinders are usually
mounted to structures underneath the
floorboards or intertwined in an array of
tubes and linkages.
Piper cylinders are the easiest to remove but you pay for the convenience in
another way. Bleeding the brake system
on many Piper aircraft can be an exercise
in pure frustration. More about that later.
For most installations, you'll need a
Phillips screwdriver, a set of open-ended
wrenches, a cotter pin puller, a good set
of Allen wrenches, and some #4 plugs
to cap the hydraulic lines after they're
LIGHT PLANE MAINTENANCE
removed from the cylinder. It would also
be helpful to have a good flashlight and
several shop rags at hand.
.A Removing the master cylinder reW' quires no special instructions; however, it
will require that your body yield to some
contortions usually seen at the circus.
To make matters worse, you'll need to
do this while holding on to various tools
and supplies.
If you're patient, the job will go
smoothly, but even then it can draw out
some behaviors you may have thought
were only the afflictions of youth. Like
what happened when your neighbor's
dog left a gift for you in your yard.
It would be advisable to look closely
at how the master cylinder is attached to
the brake linkages. Take a photo, make a
drawing or have a diagram handy.
Note the position of any washers,
spacers, and overlapping link arms and
rod ends. Also, be sure to loosen the
brake lines before removing the master
cylinder mounting hardware. Then cap
all lines and fittings.
Once removed, the cylinder can be
disassembled and cleaned. On Cessna
_
master cylinders, remove the setscrew
, . , located along the side of the cylinder
toward the top. 'Then unscrew the cover.
When loose, the cover will pull out of the
body, taking the rod, piston, and Locka-Seal assembly as a complete unit. On
Piper and Beech aircraft, remove the
snap ring located in the clevis end of the
cylinder and pull the rod and piston assemblyout.
Check the fill-valve seal (or Lock-OSeal) for condition and make sure nothing is lying in the bottom of the master
cylinder body. Remove all a-ring seals
and packings and lay them off to the side.
The Lock-a-Seal, regardless of make, is
held to the rod with a nut, washer, and
spring.
Note the position of these parts before
disassembly and draw a picture of the
fit, if necessary. Clean all parts in solvent
and air dry. Check for scoring on the
cylinder walls and make certain any
nicks or gouges are removed from the
piston rod.
Also, check any gland nuts or inserts
. . for wear. The piston rod will move in and
. , out of the cylinder end while it rides on
an internal O-ring seal. Often, the rod is
subjected to a side load that wears away at
the aluminum gland nut. Check for any
www.lightplane-maintenance.com
excessive wear in
this area. It will be
helpful to leave the
clevis pin attached
to the piston rod
so rudder pedal
rigging will remain unchanged.
00 NOT DAMAGE
LOCK-O-SEAL
ASSEMBLY AND
INSTALlATION
All seals, gaskets,
and packings
should be replaced with new. Parts are
readily available and can be ordered from
your local shop. The most expensive
item will be the Lock-a-Seal but when
totaled, you'll likely spend under $20 for
all the parts you need.
Replacing a-rings and seals is no
problem, but the Lock-a-Seal needs to
be properly adjusted in order to provide
the check-valve action required to fill the
cylinder. After confirming that all the
parts are installed in their proper locations' adjust the nut and spring-loaded
piston so that there is no clearance between the piston and the Lock-a-Seal.
Then back off on the nut to allow for a
0.040-inch measured clearance between
the piston and the seal. Other brands
of master cylinders may be adjusted in
a similar fashion but remember that if
the clearance is too large or too small,
you'll need to pump the brakes to get the
desired result.
a-rings should be bathed in clean
5606H hydraulic fluid prior to assembly.
Be sure to remove any dirt or debris from
the master cylinder reservoir or pressure
chamber. 'The old 5606G and new 5606H
fluids are compatible (except natural rubber a-rings may not be OK with H).
Reinstalling the master cylinder is
accomplished by reversing the removal
process. Make sure to cotter pin any
castle nuts and clevis pins and ensure
that there is no binding or catching of
the linkages during full application of the
rudder pedals. The cylinder needs to float
freely through all pedal ranges without
interference.
Connect the hydraulic lines but do
not tighten the "B" nuts. 'They'll need to
be loose during the bleeding procedure.
REMOVING THE AIR
There are as many opinions about brake
bleeding as there are about engine
Most master cylinders have some form of a
lock-O-Seal fill valve. Typically, a spring-loaded
piston will move against the seal, closing off
the reservoir from the pressure side of the
cylinder until the brake is released. The piston
then moves away from the seal and fluid is
allowed to fill the chamber.
break-in procedures. Some methods are
simple and straightforward while others
are wrought with complex maneuvers
and multiple functions. All of them will
accomplish the same thing with the
only variable being the time it takes to
complete the job.
Of the various makes and models of
brake and master cylinder configurations, the Cessna system is the easiest to
bleed. Piper aircraft, on the other hand,
have four vertically mounted master cylinders with a fifth parking brake cylinder
fixed to the panel. 'This system is difficult
to bleed in the conventional manner and
requires some special considerations
during the procedure. For all aircraft
other than the Piper brand, a bottom-totop bleeding procedure will work well.
Once the master cylinders are reinstalled, attach a clear hose to the end of
a hand-held squirt can full of Mil Spec
5606 hydraulic fluid, loosen the brake
bleeder screw on the wheel brake caliper,
and slide the clear hose over the screw.
Be sure to pump out any air trapped
in the clear hose before attaching the
line to the brake screw. 'Then, loosen the
fittings on the brake lines attached to the
master cylinder and remove the filler
plug from the top of the cylinder. While
your assistant pumps the fluid up to the
master cylinder, you will need to monitor
what air and oil seeps from the loose
fittings .
Keep plenty of rags available to soak
up the draining fluid. When it looks like
only clear fluid is leaking from the lines,
MAY2011 . .
,)1
,A
I
I
12
I
I
11
1Sl-_--10
NOTE
Filler plug (17) must be vented so
pressure cannot build up in the
reservoir during brake operation.
Remove plug and dr1ll 1/16" hole,
30 0 from vertical, If plug Is not
vented.
o. 040 ±O. 005 INCH
1.
2.
3.
4.
5.
6.
Clevis
Jamb Nut
Piston Rod
Cover
Setscrew
Cover Boss
7.
8.
9.
10.
11.
12.
ASSEMBLY OF PISTON Body
Reservoir
O-Ring
Cylinder
Piston Return Spring _
Nut
Above is an image of a master cylinder repair
from a 19705 vintage Cessna 182 maintenance
manual. Always have the current maintenance
manual on hand before tackling the job-it's
an FAA requirement to have the data. Also,
check that you will have any mandatory
replacement parts on hand before you start or
the plane (ould be down for maintenance for a
long time waiting for parts.
tighten the lower fitting and continue to
pump until the upper fitting runs clear.
Then tighten the upper fitting. During
this process, you will also fill the Cessna
master cylinder reservoir with fluid. In
all probability, hydraulic fluid will be
spewing out the top of the cylinder, too.
With the oil can removed and the
bleeder screw tightened down, stroke the
brake pedal several times to completely
fill the pressure side of the cylinder and
. . MAY 2011
13.
14.
15.
16.
17.
18.
Piston Spring
Piston
Lock-O-Seal
Compensating Sleeve
Filler Plug ;
Screw
check the operation of the Lock-a-Seal.
Even if the pedal feels tight, pump the
cylinder a few times and hold pressure
on the pedal while your assistant loosens
the bleeder screw again.
The fluid pushed down from the master cylinder will force any remaining air
trapped at the bleed screw out through
the caliper. Top off the reservoir and
install the plastic plug.
It should be noted that the filler plug
must have a 1I16-inch hole drilled in the
side to provide a vent for the reservoir
and to accommodate any fluid expansion. Other aircraft models using a separate reservoir can mounted externally
should be checked for proper fluid levels
when finished.
The procedures used for Piper aircraft
are a little different because the master
cylinder piston pushes fluid up through
the cylinder rather than down into the
brake line. This upside-down configuration has tested the patience of the best
of mechanics and has been the topic of
many hangar discussions.
While any of several methods will
work, the best we've found uses a top-tobottom bleed procedure and requires the
capable assistance of at least three people.
One will monitor and top off the external
reservoir during the bleeding procedure,
the next will pump the brakes, and the
last helper will need to coordinate the
opening and closing of the bleeder screw.
To start the bleeding procedure,
tighten all the brake lines to the bottom
of the master cylinders, leaving the top
lines loose on all four cylinders. Also,
loosen the exit line from the parkingbrake cylinder.
While slowly pumping the parking
brake handle, tighten the first loose line
and all subsequent lines in succession.
Tighten the fittings only when the parking brake handle is in the full out (or
pressurized) position.
When all air is bled from the lines and
all the lines have been tightened down,
apply and pump both pilot and co-pilot
right brake pedals and the parking brake
with one smooth motion. When the pedals feel tight, yell to your assistant to open
the bleeder screw on the caliper.
Do not allow the pedals to move
upward until the bleeder screw is once
again tight and the system secure. Otherwise, air will be pulled in from the loose
bleeder screw and you will need to start
the procedure all over again.
Repeat the sequence several times
until all the air is removed from the
system and the pedal feels tight. Repeat
the procedure for the left brake pedals. It
should be noted that if your assistant is
entering his twilight years (anything over
40), and is suffering from the natural loss
in hearing found with the so afflicted,
this procedure could go on indefinitely.
Use two-way communications, if necessary. Two tin cans and a string will do.
Aside from reinstalling panels and
carpeting, you will need to evaluate the
integrity of the brakirlg system. A pedal
that slowly bleeds down under load or
has dead motion in the stroke indicates
a problem remains with the master cylinder. Again, make sure there's adequate
fluid in the reservoir and check the Locka-Seal for condition and adjustment.
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