Randy, Lorn, Scott, et al,
One of the issues raised was the trapping of
pressure in the down side
circuit with the gear in the up position.  In the
current pump
configurations (?? at least back to 1995, probably
much longer),  the
back pressure circuit opens up a low resistance path
back to the
reservoir.  Without it, the low side can rise to the
setting of the
relief valve during retraction, leaks from the high
side, thermal
expansion, etc.  Again, we appear to have some older
configurations that
may behave differently, however, trapping pressure
on the low side up to
its operating operating is exactly what the back
pressure circuit is
supposed to prevent (assumes the spool valve is in
the correct
orientation, of course).
 
The following link is to a video of an intentional
leak from the high
to low sides, with the system pressurized in the up
direction.  I used a
leaking dump valve (removed from my plane several
years ago) as my leak
source.  To accelerate the leak for purposes of
taping, I just barely
cracked the valve.  You'll see the movement in the
high side pressure
gauge as the pressure rises and falls.  Pressure
never builds in the low
side.  All fluid is returned to the reservoir.
http://www.N91CZ.com/Hydraulics/Pump_Cycling_1.WMV
 
 
This second video is sort of a follow on the one
above.  I used a
shutoff valve to isolate the low side of the test
subject - the faulty
dump valve - from the return side of the pump.  In
that way I can charge
the high side, then open the test valve and  let the
pressure start to
build on the low side of the test valve.  I continue
this cycle until I
have about 1000 psi on each side of the faulty dump
valve.  Now I
reconnect the return line to the pump.  The low side
immediately dumps
back to the reservoir.  No pressure is maintained in
the low side
circuit.
 

 
Chris
 
 
 
 
 
 
Chris Zavatson
N91CZ
360std.
www.N91CZ.com