Posted for "Christopher Zavatson" <Christopher.Zavatson@udlp.com>:

With a properly assembled hydraulic pump, the most pressure a leak from
the high to low side can generate in the low side circuit is about 150
psi.  The key point is that the pump must be properly assembled.  For a
number of years they were not.  The LNC2 pumps have a back pressure
circuit that allows the extra fluid volume produced by the retracing
cylinder rods to get back to the reservoir without causing hydraulic
lock.  If the back pressure circuit is not there or disabled due to an
incorrectly installed spool valve, pressure on the low side builds until
the low pressure relief valve opens.  The relief valve setting is higher
than the pressure switch and thus the switch is tripped/opened. Cracking the dump valve relieves this pressure momentarily so that the
pump will start running again.  If you install pressure gauges on the hydraulic lines and observe the
readings during gear cycling with the spool valve installed incorrectly
and correctly you can see the influence of the back pressure circuit and
the spool valve orientation with regard to relieving pressure in the low
pressure circuit.
When everything is assembled properly and the gear is retracted, you
can pump as much fluid into the low side circuit as you wish.  It will
not raise the pressure above the back pressure relief valve setting. It seems a miscommunication between Oildyne and Lancair led to the
delivery of incorrectly assembled pumps for a couple of years.  The pump schematic is available on-line at Oildyne's web site.  Look up
108 Series Hydraulic Power Units.  We use the "LB" circuit.  Note that
up/down and high/low are reversed as we use high pressure to retract the
cylinder.  The schematic assumes high pressure extends the cylinder.
Chris Zavatson
N91CZ
360 std