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Inspection of Master Cylinder and
Components
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Discard the primary and secondary cups, the trap
valve and boot. Clean the master cylinder and
piston in brake fluid or methyl alcohol - do not
use abrasive materials or solvent fluids.
Inspect the master cylinder body for wear in the
piston bore. Normally, if the seals have been
operating satisfactorily no wear need be expected,
but if seal failure has occurred, the piston may
have destroyed the bore finish. If there is any
doubt whatsoever, the cylinder must be replaced.
If the cylinder is found to be fit for further
service, ensure the two cylinder ports into the
reservoir chamber are perfectly clear and clean.
Inspect the hose union and switch threads for clean
and satisfactory condition, and the lever pivot
bolt bore for wear or cracks, or fractures. Replace
the master cylinder if any doubt exists.
Check the brake lever for pivot bore wear and
possible wear on the piston thrust face. Replace
the lever if necessary. Next examine the piston for
signs of scuffing on the ground outer diameters,
and any wear on the lever thrust face.
Do not attempt to polish or grind the piston in
any way, using abrasive compounds, as the slightest
residual trace of such materials, even after
intensive cleansing will damage master cylinder and
caliper working parts beyond repair, and can result
in premature brake failure.
Inspect the brake hose for cuts, signs of
leakage or deterioration. Replace if the slightest
doubt exists.
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Master Cylinder Reassembly
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After inspecting and cleaning the master
cylinder, clean the piston, spring, spreader and
cup washer in clean hydraulic fluid, and place in
order of assembly on a perfectly clean working
surface, in a dust free room or workshop.
DO NOT USE A FLUFFY RAG TO WIPE THE COMPONENTS.
ALLOW TO DRAIN.
Soak the new primary and secondary cups in
hydraulic fluid for fifteen minutes, kneading
occasionally to encourage the special rubber cups
to gain their maximum supple state prior to
assembly. Ensure the three relief holes in the
"crown" of the piston are clear and clean.
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1
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Take the "hollow" secondary cup and place it non
lipped side against the ground "crown" diameter of
the piston. Work the cup over the "crown" by hand,
then down the piston body, over the shoulder into
its groove adjacent to the main body ground
diameter.
CAUTION - Do not use any form of tool to
ease assembly of the cup to the piston for fear of
damage to the lip.
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2
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Fit the boot over the piston open end towards
the piston crown and ensure the upper end is fitted
squarely and snugly in the piston groove. The
piston is now ready for assembly.
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3
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Assemble the trap valve to the spring, ensuring
the inner plastic bobbin is accurately seated in
the rubber valve base, and that the small diameter
bleed hole in the bobbin base is clear and free.
Ensure the plastic spreader is securely and firmly
pressed home on the other smaller diameter end of
the spring.
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4
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Offer the spring/trap valve assembly into the
master cylinder, valve end first, keeping the
master cylinder bore upright. It is permissible to
hold the nose of the casting securely in a soft
jawed vice, provided it is in no way overtightened.
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5
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Assemble the primary cup washer into the bore,
open end of the cup facing into the bore, followed
by the primary cup washer. Ensure only one cup
washer is employed at this point. Place this dished
primary cup washer, "hump" upwards, towards the
open end of the cylinder bore. (The effect of the
dished cup washer is to close the primary cup
during the return stroke, allowing oil to by-pass
the cup and replenish the pressure cylinder).
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6
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Offer the piston crown first into the cylinder.
Place the 10 eared boot circlip over the top of the
boot, the slight set on the ears facing away from
the cylinder. Ensure fluid is smeared onto the
secondary cup and into the mouth of the cylinder
bore.
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The next series of operations, to locate the
piston, boot and circlip by means of the brake
lever and pivot bolt can be difficult without the
aid of special tools, and the assistance of a
second operator. However, if care is taken,
assembly can be completed without such help, the
difficulty being to locate the base of the boot
into the cylinder counterbore and finally locate
and snug home the circlip whilst holding the piston
assembly within the bore of the master cylinder,
against the trap valve spring, long enough (and
with sufficient piston thrust pad clearance) to be
enabled to fit the hand lever and to locate it with
the pivot bolt. The method of assembling these
parts is detailed below:
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