<continued>
The main difference between the two types of servos--besides their
construction--is that the (semi-) sealed unit uses a metal piston with a
rubber diaphragm instead of the metal piston with leather/rubber seal
around the edge. This is either a) a better way to do it, b) cheaper,
c) neither or d) both (my guess is "b"). I have a feeling this type is
meant to be replaced rather than rebuilt.
The other is a slight difference in the externals and internals of the
body (note there are two cylinders: a slave which is used to operate a
valve which gates either atmospheric air or vacuum (non-braked
condition) to the vacuum chamber, and the M/C, which applies the
pressure to the wheel cylinders). On the metal/leather/rubber
piston-type unit, the tip of the "conrod" from the vacuum piston seals
the M/C piston when brakes are applied and releases allowing fluid to
back-flow when brakes are released. On the diaphragm-type, there is a
small ball bearing which I believe is part of this check-valve setup.
Here comes the favor: when I disassembled my unit, the check-valve fell
out and I didn't see EXACTLY how it is installed so, if you do get your
unit apart watch carefully for the location of this ball bearing and
please, let me know where it goes.
The rebuild kit you bought should provide all the parts you need, and
then some (you won't need the rubber/leather seal for the vacuum piston,
for instance). There are seals for both fluid cylinders, the gasket to
seal the vacuum canister to the servo body and new copper sealing
washers for the bolts that attach the vacuum canister to the servo body
(most kits don't provide new valves for the air/vacuum "switch;" yours
are probably OK). Ironically, if you could get to the bolts attaching
the canister to the body without opening the canister you probably
wouldn't need to open the vacuum canister at all; I think the rubber
diaphragm should last a long time and there's nothing you can do to
service or replace it anyway (this type doesn't need the dry lube on the
inside of the canister as the perimeter of the diaphragm is pinched to
between the two halves of the canister).
If you get the cylinders sleeved, make sure your rebuilder does not
media blast within the air/vacuum valve area; it'll muck up the machined
surface (I don't think it's necessary to blast them regardless, but most
rebuilders do because they want to clean them up and it looks pretty).
Otherwise, just clean everything and reassemble.
I've tried to find the history of these units, but can only find
documentation on the early unit (the most info I got was from the
Victoria British catalog). Like you mentioned, I think the sealed-band
type is a later replacement design.
bs
*******************************************************************
Bob Spidell San Jose, CA bspidell@comcast.net
*******************************************************************
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