In reply to a message from Bryan N sent Sun 23 May 2010:
John,
As a follow up to the referenced previous post, I tried to
measure the relative force required on the brake pedal with
and without brake boost, but I couldn’t adapt my wife’s
fancy electronic kitchen scales to do the job!
However, this may be of some guidance to you in determining
when you do and do not you have brake boost.
I turned the key to position (I) on my car this morning
after it sat for 4 days unused (so the accumulator pressure
would long since have dissipated) and heard the Teves pump
running. When the pump stopped, I switched the key to the
off position and with my hand, operated the brake pedal
easily with boost for 40 applications before the pedal ‘went
hard’.
The horizontal centre line of the top surface of the brake
pedal on my car is approximately 7 inches above the carpet.
With brake boost USING MY THUMB ONLY, I could comfortably
depress the pedal 2� inches (i.e so that the upper surface
of the pedal is now only 4�’’ above the carpet)
After exhausting the accumulator pressure (40 brake
applications later with the key off) so that the pedal ‘goes
hard’, exerting the same amount of force with my thumb, I
could only depress the pedal 1� inches before the resistance
(like hitting a brick wall) was too severe for my thumb
pressure to overcome.
BTW, with the accumulator fully charged (i.e Teves pump off)
but with the key in position (I), the pump will re-start
every time after the second normal application of the
brake and run for a couple of seconds to top up the
accumulator. BUT, if in the same circumstances I hit the
brake pedal hard (i.e stamp on it!), the pump will re-start
after just one such application of the brake.
Neither of those situations is a reliable indication of the
condition of the accumulator sphere - but the number of
applications you get from a fully charged accumulator
after the key has been switched off is. As mentioned, mine
does 40 normal applications before the pedal ‘goes hard’.–
The original message included these comments:
I’ve had no problem detecting when brake boost is lost on my
car. I will try to figure out a way of measuring the force
required to depress the pedal with boost and then when it
‘goes hard’.
Will get back to you.
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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