"C J Campbell" wrote in message
...
Hydraulic pressure is used to hold the gear up. The pump runs every few
minutes to maintain hydraulic pressure.

Assuming the 172RG gear system is similar to the 182RG and 177RG systems, if
the pump is running on a regular basis, you have a leak somewhere. Not
necessarily one venting fluid, but one allowing fluid from the pressurized
side of the system to the non-pressurized side.

(The pressurized side changes, of course, depending on whether you're
raising or lowering the gear)

The manual system is just another pump, only it is only capable of
lowering
the gear. If you lose hydraulic pressure the manual system will provide
enough pressure to lower the gear and lock it in place if there is any
fluid
in the system at all. You would have to spring a leak at the bottom of the
sump to lose all your hydraulic fluid.

A leak on the pressurized side of the system would allow the hand-pump (or
the electric pump) to pump all the fluid out of the system. You don't need
to spring a leak at the bottom of the sump to lose all your fluid.

[...] The landing gear system is not all that critical anyway. If more
redundancy
is required, the weight penalty is better applied to other systems.

True without a doubt!

Pete