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MMD > Archives > November 2006 > 2006.11.28 > 05Prev  Next

Testing Exhausters & Reservoirs
By John A. Tuttle

Hi All,  Daniel Cox writes,

> G'day all,  I've just finished redoing my lower section and when
> I block off all the outlets and pump, I can get between 20-22 seconds
> of vacuum until the equalizers are fully open again.  Should I do
> more work on it to get more suction, or should I be happy with what
> I have so far?

There are a number of schools of thought on this matter.  One company
said that the minimum factory specification was 12 seconds.  I would
hope that most good rebuilders shoot for 45-60 seconds.

However, there are certain things that need to be considered when
testing the lower section.  Primary among them is the way the controls
inside the Tempo Control Box are activated.  If they are the sliding
valve type, you must make sure that both sliding valves are closed.

Naturally, this is contrary to the way the control box actually
functions in real time, since either the fast reroll port or the tempo
port are always open as you switch from Play to Reroll.  Some lower
sections even have a bleed opening so that when the tempo is set to
Zero, the air motor governor won't lock up.  But I'm straying...

The point is, if the fast reroll port and the tempo port aren't
completely closed, air will leak in through the holes where the control
rods pass into the control box.  So, you have a 'built-in' leak.

Also, as I've written about before, wood gets porous as it ages, and
unless all of the internal surfaces are sealed while the lower section
is dismantled, air will leak in through the microscopic pores in the
wood.  However, this problem can be solved by sealing all of the
outside surfaces with a good sealer.

Naturally, it is presumed that all the gaskets were replaced and that
they are air-tight.  But, if there's a question about the integrity of
the seals, applying a bead of sealer to the gaskets while the unit has
vacuum will reveal the truth.

Another possible source of leakage is the flap valves (both internal
and external).  The integrity of the external flaps can be checked by
sealing the holes with duct tape.  The integrity of the internal flaps
can be checked by removing the external flaps and collapsing the
reservoirs manually.

Another possible source of leakage is where the cloth is glued to the
wood.  All too often, the seal may be physically strong but not air
tight.  Here again, applying a bead of sealer where the outside edge of
the cloth meets the wood while the unit has vacuum, will reveal the

All that said, the best way to check the integrity of the bellows is to
test them before assembling the unit.  Also, the trunk can be checked
before assembly.  Point is, if you know that all of the parts are great
before they're assembled, you have narrowed down the possible problems
to the points where the pieces come together.

For information about recovering a bellows, see

John A Tuttle

(Message sent Tue 28 Nov 2006, 23:47:44 GMT, from time zone GMT-0500.)

Key Words in Subject:  Exhausters, Reservoirs, Testing

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