Mike,
Check the archives ... this has been discussed a lot in the last year.
To sum:
A servo rebuild isn't a whole lot trickier than a master cyl rebuild.
Read your shop manual--you do have one, I hope--until you understand
the principle of operation. Basically, the servo has two cylinders;
the "primary" is what the master cyl applies pressure to and is used
to switch between vacuum and ambient air in the vacuum canister (there
are bypasses in case the servo isn't functioning--at least you'll have
SOME braking). The "secondary" cylinder is driven by the vacuum/air
piston and applies the pressure to your wheel cylinders. The vacuum
cylinder holds engine vacuum until you apply the brakes, which causes
ambient air to be applied to one side of the vacuum piston--with
vacuum remaining on the other side--which pushes the secondary piston.
The force is amplified, proportionally, by the large surface area of
the vacuum piston against the smaller area of the secondary piston.
Two points have come up:
1) there is/was a dry lubricant material on the inside of the vacuum/air
canister
that keeps the leather seal on the piston from sticking. If this is gone the
piston can stick in the actuated position, locking your brakes. There is a
spray-on dry lubricant from the Sandstrom company that works, but unless the
servo has way more than 100,000 miles you probably don't need it
2) both the "primary" and "secondary" cylinders--don't think the manual calls
them that, but it's appropriate--are hardened aluminum lining only; i.e. no
liners. This has a finite life and when it goes you'll be pulling the
cylinder again and putting sleeves in. I rebuilt mine at approx. 130,000
miles and the cylinder walls looked OK so I didn't sleeve; have about 10,000
on the rebuild and so far so good (if you send the cylinder body out for
re-sleeving make sure the re-sleevers DO NOT sand/media blast the inside of
the vacuum/air valve chamber; it has a machined sealing surface and the
blasting will rough it up)
It appears at one time the rebuild kits provided a foam rubber packing
for the leather seal that didn't fit well ... the ones in the kits
I've purchased fit fine. The kits don't provide the two hard rubber
seals that are on the T-shaped air/vacuum valve; you probably don't
need to replace them but if you want to British Car Specialists has them.
BTW, the most common failure mode is the servo starts inhaling brake
fluid, but still works (for awhile, at least). This is due to the
seal--called the gland seal--that seals the shaft around the vacuum
piston shaft breaking and/or hardening. Sounds like you have a
sticking primary piston (or air/vacuum valve, but I don't think this
is likely).
I found this an interesting task and learned a lot about my Healey's
braking system. I think any rebuild is a crapshoot; some Listers have
had good luck with rebuilders, some have not. You can do this
yourself if you're careful; the hardest task is getting the servo out
and back in the car. Many opt fot an aftermarket Lockheed
replacement; it requires a special bracket but is otherwise a drop-in
replacement. The rebuild kits cost about $80, a typical "pro" rebuild
is about $300 and the Lockheed part with bracket is around $350 (I
think). Figure $50-60 apiece if you need to re-sleeve the primary or
secondary cylinder(s).
Ah, one last thing: when you pull your servo, DO NOT break the lines
at the servo-- break them at the two three(four?)-way fittings. It's
pretty much impossible to re-install the lines at the servo.
bs
********************************************
Bob Spidell San Jose, CA bspidell@pacbell.net
'67 Austin-Healey 3000 '56 Austin-Healey 100M
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