In reply to a message from PeterCrespin sent Sat 5 Oct 2013:
The thing which causes such confusion is that the hydraulic
system and the vacuum system do not operate independently of
each other . The vac system is in fact modulated by the
hydraulic system and as a result if there is any sticking of
the hydraulic system ( particularly the pedal master
cylinder ) this will result in vacuum being retained in
the forward chamber of the servo and thus allowing the
atmospheric pressure in the rear chamber to keep the servo
piston pressed forward slightly, thus retaining hydraulic
pressure in the brake circuit after the servo.
The factory manual at page L.X.s.3 and 4 spells out
precisely how the system works , and an understanding of
this explains why pulling the reaction valve manually -
resulting in the whoosh of air into the forward chamber of
the servo - helps to release the brakes
My experience of this problem - and it is quite extensive
- is that it results from the poor lubricity of modern brake
fluids which result in the HYDRAULIC seals binding on the
bores of the cylinders. This is particularly so in the case
of the servo shuttle which is exposed to more heat. I have
cured it in the past by rebuilding the servo cylinder using
copious quantities of red rubber grease - I bought a 1 lb.
jar - but I have a suspicion that an easier way might be to
dissolve perhaps an ounce of red rubber grease in some dot 3
fluid and then flush the system through with this so that
the system is now completely refilled with adequately
slippery fluid–
christopher storey
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