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- Replacing the pistion
in
a 4 pot caliper
(by Kyle)
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If you need to replace
one or more of the pistons in a four pot caliper then this piece is
for you. As you can see from the photograph below the bottom left hand
piston had badly corroded at its extremities causing it to stick when
trying to retract back into its housing.
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Caliper with sticking piston |
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The
pry bar shown in the picture below was not able to move it far enough back
into the bore to allow the new pads to be fitted. There was no real alternative
but to replace the piston. The piston on the right of the right hand pic
is the replacement, the difference between the two is only too clear. |
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After
raising the wheel in question and putting the car on axle stands remove
the road wheel. Now turn the steering wheel full lock to the opposite side
of the car so the caliper can be seen in all its glory. Remove the cap of
the master cylinder reservoir, then clamp the rubber hose connected to the
“T” piece with a pair of vice grips taking care not to damage
rubber. Using an 11mm brake pipe spanner unscrew the brake pipes and lift
the pipes up and push them out of the way. Force out the pins keeping the
pads in situ using pliers and/or a punch. Now using a pry bar or long sturdy
screwdriver prise the pads apart, forcing the good pistons into their housing
and remove the pads and anti rattle clips. |
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Using a punch to push out
pad retaining pin |
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Now
tackle the two retaining bolts (19mm). The bottom one should come off fairly
easily using a 19mm socket. The top bolt is trickier as there is very little
space around the head of the bolt to enable a socket to slip on, I used
a 19mm socket that I had ground down around the edge for a prior job. Take
care to support the caliper as you slide out this second bolt as it will
now drop. Put the
caliper into a vice and using an 11mm socket (two of the nipples cannot
be tackled with a pipe spanner due to space) remove the three bleed nipples
and clean them using carb cleaner. Now remove the four bolts which attach
the two sides of the caliper together and separate the two halves. To
do the piston swop job I had to buy a new cylinder and seal kit. It would
be false economy not to replace the old seals when inserting the new piston.
I also had a missing end ring and seal from one of the other pistons which
necessitated the kit anyway. Please refer to the exploded picture of my
caliper below for information.
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Use
a pair of circlip pliers or a small pick or screwdriver to remove the metal
top seal retainer and the seal itself. At this stage the half of the caliper
with the dodgy piston should be placed in a vice (make sure the vice doesn’t
prevent the piston in question being removed) and extracted. The method
I employed for doing this was to attach a pair of vice grips to the edge
of the piston and hammer the vice grips in the direction of travel you require.
This worked very well for me. Once the piston came out I could see that
all was well in the bore but the piston itself was in a dreadful state.
I removed the inner O ring seal taking care not to damage the bore. The
next task is to clean out all the fluid inlets and the bore with carb cleaner
(carb cleaner because it simply evaporates and therefore doesn’t react
with the brake fluid). It is extremely important to keep the bore spotlessly
clean and free of grit. Now slip the new seal into the channel near the
end of the bore and slowly depress the new cylinder into the housing. You
may use a clamp to help this procedure. The outer seal and retainer can
now be squeezed into the aperture around what is now the top of the fully
depressed piston. The best way to do this is to introduce the seal into
the retaining ring and try to fit them both as a unit. Now re-bolt everything
back together again, mount the caliper back in the car with it’s new
pads, pins and anti rattle clips, put the pipes back on and the brakes are
ready for bleeding. Don’t forget to remove the vice grips and put
the master cylinder cap back on though. Good luck! |
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