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Old 14-04-2020, 08:53   #1
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Air in hydrolic line

My autopilot drive is a self contained hydraulic system but has a long (about 15 feet) low-pressure line feeding fuild from a reservoir in the engine room. Doesn't seem to have a clamp on the hose and the hose is one of those stuff plastic non pressurized hoses. I can see that the line has air in it.

Do these pumps have a way to bleed out the air or should I attempt to do so. The line seems very weak if there is not supposed to be any air. It is more like an overflow hose.

Thoughts? I am new to hydraulic autopilot.

Oh yeah, the drive seemed to have a slight leak in the shaft seal. We need to cross the gulf before I can get any real help with it. We crossed the Caribbean as is. Just worried fluid level may be getting low and it isn't drawing fluid like it is supposed to. Also, any hints on how to check the fluid level?
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Old 14-04-2020, 08:58   #2
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Re: Air in hydrolic line

Pics of drive
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Old 14-04-2020, 09:00   #3
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Re: Air in hydrolic line

Better pic
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Old 14-04-2020, 09:02   #4
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Re: Air in hydrolic line

I now see a small clamp but you can also see the air pocket of about 2 feet of hose
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Old 14-04-2020, 09:39   #5
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Re: Air in hydrolic line

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I now see a small clamp but you can also see the air pocket of about 2 feet of hose
Looks like a header tank

The header maintains positive oil pressure at the pump

A air bubble is insignificant

If you unclamped the hose at the top... the tank .. the bubble should float out

Messy job
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Old 14-04-2020, 09:39   #6
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Re: Air in hydrolic line

I’d be willing to bet that the other end of that line is attached to a header tank that should be filled with oil. This line wound be essentially 0 pressure but supply oil to the cylinder/pump unit.
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Old 14-04-2020, 10:36   #7
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Re: Air in hydrolic line

That is definitely a low pressure line going to the pump/cylinder unit.

What happens to the oil level in the tube when you cycle the rudder from full port to full stbd and back?
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Old 14-04-2020, 11:02   #8
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Re: Air in hydrolic line

There should be a place to let any air bubbles out, but I cant tell you where on your system.
Seal: If it just drips occasionally that's fairly normal. a Maintenance guy once told me "if it ain't leaking , it ain't hydraulic"
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Old 14-04-2020, 11:27   #9
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Re: Air in hydrolic line

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"if it ain't leaking , it ain't hydraulic"
The mantra of a lazy, or inept, technician
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Old 14-04-2020, 12:01   #10
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Re: Air in hydrolic line

Does the reservoir have a plugged vent? Just opening the vent and wiggling the line might be enough to allow hydraulic oil to flow down and the bubble to float up. It's not unusual to have a bubble in the low pressure line oscillate back and forth following the the ram's movement.
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Old 14-04-2020, 13:17   #11
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Re: Air in hydrolic line

Thanks all. I guess you are right...that bubble has been there after 11 days at sea and I think it dripped a few drips when it was working hard with 11 foot following seas.

It should be fine for 5 days of slow sailing in 4 foot seas.
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Old 14-04-2020, 22:57   #12
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Re: Air in hydrolic line

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Originally Posted by garyfdl View Post
The mantra of a lazy, or inept, technician
Hydraulic hoses and valves shouldn’t leak, so you are correct there, but cylinders and hydraulic motors are a lot more expensive to keep perfectly leak proof. Most of my experience is on 1000-3000psi non-marine hydraulic systems.
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Old 14-04-2020, 23:30   #13
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Re: Air in hydrolic line

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The mantra of a lazy, or inept, technician

Sadly we can't all live in the perfect world in which you appear to reside.
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Old 15-04-2020, 03:25   #14
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Re: Air in hydrolic line

I am assuming this is just the autopilot so if the worst happens then you can steer manually.


Hydraulics should not leak. If there is a leak it will only get bigger. A complete seal failure is unlikely, but could get serious so take plenty of oil and keep an eye on the level in the header tank. If all else fails any oil, cooking oil, engine oil will work, you are in for a rebuild at the other end anyway.
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Old 15-04-2020, 09:43   #15
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Re: Air in hydrolic line

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Hydraulic hoses and valves shouldn’t leak, so you are correct there, but cylinders and hydraulic motors are a lot more expensive to keep perfectly leak proof. Most of my experience is on 1000-3000psi non-marine hydraulic systems.

Hydraulic cylinders actually need to ooze a little to keep the seals lubricated for longer life. Question is when does an ooze become a drip and a leak!

I once had this massive electro hydraulic device (computer controlled robotic device) that I was responsible for. It used a poison, synthetic, non-flammable hydraulic fluid that was florescent green and easy to see leaks. I was constantly on the backs of the techs that maintained the system to keep the catch basin clean and neat and I received lot's of respectful advice from the techs about leaking massive hydraulic systems and the difference between an ooze and a leak. Such is the life of a middle manager.
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