REPAIRS PAGE 107
SEIZED REAR ENDS
No, it’s not an advert for laxatives and all that revolting stuff, but the more
uncomfortable effect of the dreaded seized rear calipers.
These are notorious for seizing up to the extent of causing MOT failure, and are not
exactly the easiest things to work on.
I have done a few in my time, and it seems to me that the problems that do occur
are partly related to design and partly related to usage—
a) Design. In common with most rear disc brake cars the handbrake mechanism
operates directly onto the caliper piston. To do this there is-
i) A lever crank which pivots on the caliper body, which actuates a push rod that
enters the rear of the cylinder via an O-ring seal.
ii) A self-adjusting screw thread mechanism onto which the piston is screwed
(don’t ask me how it self-adjusts as I’ve not yet taken one fully apart.)
All this makes the rear calipers more complicated than the fronts (which by
comparison give little trouble.)
b) Usage. The handbrake inevitably tends to get left on for longish periods, and in
our damp climate linkages and surfaces tend to ‘stick.’
Indeed
the usual advice for long-term storage is to leave the handbrake off and use
chocks to block wheel rotation. This helps, but is no guarantee against seizure.
So,
that’s design and usage, what’s the usual failure mode?
I
would have thought that might be the exposed crank on the caliper body, but it
doesn’t seem to be. (This does not include the handbrake cables- they’re a
different kettle of fish for a separate article.)
Most
of the problems I’ve encountered relate to the piston ‘stiffening-up’
inside the cylinder bore, to the extent that it resists any attempts made by the
self-adjusting mechanism.
So
why does this happen? There are various possible causes-
a) External corrosion on the piston nose and adjacent cylinder wall. This
usually occurs when the rubber ‘bellows’ perishes/splits and allows water
ingress. Given that these seals can be up to 19 years old it’s hardly
surprising.
b) Piston seal compression – where the seal gets clamped onto the piston
so tightly that the handbrake mechanism can’t overcome it. I don’t think it
is due to the seal ring swelling up, but more due to minor corrosion in the
cylinder wall groove in which the seal sits. Even this marginal corrosion seems
to cause the seal to grip the cylinder very tightly.
c) Micro-corrosion’ in the piston bore and cylinder in-board of the
seal ring. This is like a super-fine
Jewellers Rouge deposit as opposed to ‘pitting’, and is fairly easily
removed with a little care.
Of these 3, piston seal compression is the most common factor that I have seen.
It is fairly easily cured, but naturally the seal has to come out - with a lot of care to avoid damaging it. I have evolved special tools
to achieve this, and once removed the cylinder groove is fairly easily cleaned
out, I use an ‘L’ shaped scraper made from brass shim.
When the cleaned up rubber seal is replaced in its groove - the right way round! -
the piston is usually a lot freer in the bore.
That, in conjunction with cleaning everything else up – spotlessly – can often
revive a flagging caliper.
This is not of course meant to be an ‘A to Z’ procedure – but a guide to key
points to watch for the ‘DIY-ers’ out there.
ALAN JONES