| Unfortunately, I don't know the XJ6 SII braking system too well, but
here goes.
Assuming that the vacuum connection and non-return valve are OK, and
the servo unit is fine since it was replaced, there are a couple of
points you can check.
1. Is the pedal to servo operating rod clearance correct? If not then
the servo may not be resetting properly between pedal applications.
2. Is AIR getting in to the opposite side of the servo diaphragm. This
is mostly applicable to older servos (such as my 3.8S) which has an
air filter on the air intake side of the servo. If this is blocked
then the servo will not operate very reliably.
If vacuum, air and mechanical adjustement is fine, then is the problem
in the master cylinder? It may be that the master cylinder is not
operating correctly. Given the original problem you had, it may well
be that the master cylinder is the cause of the problem.
A quick crude check can be made by disconnecting the servo vacuum hose
and trying the brakes with the pipe disconnected.
**** CAUTION **** I don't advise doing this any anything more than 2
mph, as, even if you are a 20 stone gorilla, it will be hard to stop
1.5 tons of XJ6 with a faulty master cylinder and no servo!. But at
least you should be able to isolate, or eliminate, the servo as the
problem, if the brakes are worse or non-existant with the servo
disconnected.
Assuming that the SI XJ6 had the same set-up as the 420, (it certainly
used the same 3-pot calipers) it would have had single circuit
hydraulics. Many dual circuit master cylinders require the pistons
resetting manually, in the case of a single circuit hydaulic failure,
before the system will bleed properly and operate correctly. This is
normally obvious if there is a hydraulic circuit warning light, but may
be less obvious if you have not wired one in.
I hope this helps.
Brian
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The saga continues. Now that the weather has improved and the
cat is stirring in the garage, I decided to get the auxilliary starting
carburretor fixed. I stripped it cleaned it and renewed the rubber
hoses to the inlet manifold. It works now. But guess what, so does
the brake servo and I can't get it to go wrong now. The rubber hoses
to the inlet manifold were in bad condition. Does anyone think that
I was losing enough vaccuum to affect the servo but not performance?
Cheers Bill
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I've heard of similar problems to this - it was because the rubber
hose was two ply (inner hose, cord reinforcing, outer sleeve) and
the inner hose had partly come away from the re-inforcing, so that
the vacuum inside the hose closed the pipe. From the outside the
hose looked old, but ok, but the servo was nigh-on useless.
Also occurs on water pipes but the opposite effect. Water gets through
inner hose to the re-inforcing and then goes between the plys. This
is visible though as a blister on the pipe.
Tony
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