REFLECTOR: Hydraulic system pressure

[John Dibble]

I checked and found that the main cylinder is leaking at the forward end 
which is under pressure when the gear is down.  Hoping to delay the 
repair for cooler weather, I have been able to stop the leak by pulling 
the breaker once the gear is down and then cracking the dump valve to 
relieve pressure once on the ground.  Now I'm wondering why the pressure 
needs to be so high?  The gear will come down by itself with the dump 
valve open.  The pressure has no benefit in keeping the main gear down.  
When the nose gear is over center, pressure from the nose strut actually 
pulls on the cylinder rather than pushes, so the hydraulic pressure has 
no benefit there either.  If the linkage somehow got under center, then 
the nose gear would push on the cylinder.  However, with the dump valve 
closed and little or no pressure from the pump, the hydraulic fluid, 
being non-compressible, would resist compression from the gear.  So I'm 
thinking that significantly reducing the hydraulic pressure in the down 
position could greatly extend the seal life.  Comments appreciated.

John

On 7/17/2013 2:38 PM, Scott Baker wrote:
> Hello John,
> Unless your aircraft has something other than standard factory 
> equipment, the speed brake is operated by an electric linear actuator 
> (no hydraulics).
> The speed brake actuator opening in the floor of the aircraft is 
> serving as an exit place for a leak in the hydraulic system.
> Remove the keel cover(s) and keel inspection plates - grab a 
> flashlight, mirror, and clean rag - and begin searching for puddles 
> and drips.
> If you are lucky, it will be a loose hose fitting.  If the cause is 
> bad cylinder seal, it is reasonably easy to fix.  This will likely 
> involve removing the cylinder from the aircraft for service.
> Good luck!
> Scott B.
>
> On 7/17/2013 2:52 PM, John Dibble wrote:
>> A small amount of hydraulic fluid is leaking at the back side of my 
>> speed brake.  Looking on the inside it seems to originate where the 
>> "cylinder" connects to the brake.  Does the speed brake use hydraulic 
>> fluid?  If so, does it come from the retract system, or is it separate?
>>
>> Thanks,
>>
>> John
>> _______________________________________________