In the 980421 MMD Steve Greatrex brought forth again the question of
how to rebuild leaky cross valves. I will not recommend to him the
round valve seat replacement unless he fully understands that there
is a huge difference in the open area of the cross valve seats compared
to its round valve replacement. The original valve has an opening
of .1468 square inches and the replacement valves are 33 percent larger
at .1962 square inches.
Since valve performance is really just a balancing act between the
forces holding the valve on its seat (at rest) and the forces of the
pouch pushing the valve off the its seat (at play), one can easily see
that if all other things are equal, the replacement valve seat requires
a stronger pouch force to activate the valve. What this means is the
same valve with a new round valve seat will not activate a vacuums
below about 4 inches and will be sluggish to repeat at vacuums below
about 7 inches. The results are very unsatisfactory.
The new round valve seats can be made to work, but other changes are
required. Those changes include, reducing the mass of the valve body,
or reducing the diameter of the valve body to 9/16 inches, increasing
the size of the pouch to 1-1/8 inches or just putting a lifter disk of
5/8 inches on the existing pouch. Any combination of the above should
Steve's suggestion that the original valve plates are bent is highly
unlikely. What could possibly bend them while they have been screwed
in place for sixty years?
Steve, if you don't want to replace the cross valves with round valves
and you think that your cross valves are bent, I will sell you a set of
original cross valves cheap. I have thousands of them. I have been
down this road many times.
The cross valves leak perhaps more than any other valve made and can
completely ruin the performance of a Duo-Art. The round valve seat
replacement can fix the leaking problem, but introduce new problems of
poor valve performance.
I like both systems -- when they work properly.