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One word--- DIRT.

Foreign Object Contamination or FOC (as in oh FOC that damn pump is cycling) is a
leading cause of problems in hydraulic systems. FOC is especially prevalent in
composite homebuilts due the the vast quantities of abrasive dust, fibers, grit and
chips that are generated during building. To compound the problem, new hydraulic
pumps frequently shed metal particles as they "wear in".

All it takes is a sufficiently large FOC to be under the dump valve seat when it
closes and you will have a permanent leak. One builder replaced the valve three
times. Either he received three consecutive bad valves or FOC in the system failed
all three valves.

The solution is to change the hydraulic fluid frequently (after the first 10 hours
and then every 100 hours) and rig a magnet in the resaviour to trap any ferris
particles.

On a IV I would further recommend that a 3 micron filter be placed in the return
line. I used an Airflow Performance (864 576 4512) fuel filter for this purpose.

BTW a device's burst pressure will be at least three times the rated pressure,
therefore a hose rated to 600 psi will withstand 1800 psi without rupture.

Brent Regan

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