In reply to a message from mouton sent Tue 30 Aug 2005:
‘‘Seizing calipers…never (to my knowledge) result[s] in brakes
locking on, since the symptom is the opposite: pistons don’t
move, so no brakes at all.’’
‘‘Ho, ho, ho!’’ said everyone living near roads that get wet often,
and salted every winter. You California/Texas/Arizona/Florida dudes
don’t know you’re born!
FWIW Jerry, I have never personally experienced a deteriorated hose
locking brakes on, although it happens. But I’ve had at least 15-20
cases - 2-wheeled or 4 - of calipers seizing on due to corrosion.
Admittedly that’s only my anecdotal experience, but it’s pretty
much the norm at the bottom end of the motor vehicle food chain
where I exist. Subsistence motoring it’s called, or ‘Bangernomics’!
In fact is is my view that most hydraulic brakes on vehicles used
in all weathers in the UK would eventually seize on due to
corrosion if no maintenance were performed. I’ve had seizure ON
(i.e. dragging brakes) due to corrosion on the caliper sliding pins
in single psiton designs like my old Honda CB 750 and many Yamahas.
I’ve had seizures due to piston corrosion on Moto Guzzi (Brembo),
Triumph (Nissin), Classic Triumph (Lockheed), Fords (Girling),
Renault & Peugeot (Bendix), Citroen (own brand system)…need I say
more?
Many of the above were not due to hygroscopic brake fluid either,
but to external moisture getting past dried/torn dust covers into
the piston/cylinder gap outboard of the seal, as John described on
his 40K XJ6. The fluid areas behind the seal are usually fine
except on really old clunkers like my E-type OEM calipers.
On my current Triumph (which has brakes acknowledged to be
absolutely stunning in power and feel) the pistons are stainless
and caliper body alloy but the brakes still drag after a year or so
without maintenance or if left standing for a long period after a
wet ride - especially the back brake which is exposed to more crud.
From memory the pistons have no separate dust cover, or a tiny one.
They are mostly just the exposed pistons (4 per caliper) sticking
out past the caliper body as the pads wear. When they get deposits
of pad dust and general muck they have trouble retracting properly
and a good clean is called for. After 6 years I guess I need to do
a pre-emptive seal ‘refresh’ too, since the twisting of that square-
section rubber part in its groove is all that pulls the piston
back. Maybe they are slightly ‘cooked’ now.
Put it this way, it will take far less corrosion to bind/seize a
caliper ON because the piston is too stiff to be pulled back by the
weak seal return action, than it would to seize the brake OFF
because it is corroded so solid that even major hydraulic pressure
does not push it against the disc…
Single-piston designs can play different tricks whereby only the
piston side pad wears if the caliper is seized on its sliding pins,
so in effect one half of the braking effort is seized OFF, which
comes close to your supposition, but we were talking about piston
seizure.–
Peter Crespin 66 2+2 ‘E’
Buxton, United Kingdom
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