Hydraulic leak - can't find the source

AV8ing · #1 09-01-23, 02:45 AM

I've got a 337D that's been leaking hydraulic fluid from the belly for some time, usually dropping 5" to 10" puddle but only after 7 to 10 days AFTER last flight.

I've pulled up the floor and there's hydraulic fluid just about everywhere from firewall to firewall !

Many hours and a few liters of white spirits and gas later I've gotten the mess cleaned up.

So I've jacked the plane and swung the gear multiple times and can't see any leaks.

Two weeks later, I've got more fluid and a puddle under the plane.

Wash - rinse - repeat and still unable to find the source despite using mirrors and borescope ....

That said, it appears to be under or near the main actuator , but I haven't actually been able to positively confirm this.

I don't want to pull out the main actuator unless I'm 100% sure it's the culprit.

Running out of ideas - any help appreciated !

Rick Erwin · #2 09-01-23, 12:19 PM

Try cutting about 3”x3” squares of white pig mat and place under each hydraulic joint to see what shows up.

I’m doing the same thing right now.

I’m thinking that should help to narrow down the location.

Rick

Skymaster337B · #3 09-01-23, 05:13 PM

My experience with that kind of leakage is the main gear actuator. It’s the only one big enough to put out that much fluid in the belly.

It happened to me. I overhauled the main actuator and discovered the back-up ring was installed the wrong way. It came out of the factory that way. But still worked for 50 years.

AV8ing · #4 09-02-23, 02:20 AM

Skymaster337B - That's my current thinking and I'm pretty sure it's been going on for many years.

Did you overhaul your actuator or send it out to Cox or another Repair Station ?

wslade2 · #5 09-02-23, 06:28 PM

I had a belly leak that was dripping from seam in belly skin. Similarly left a hydraulic stain about size dinner plate after a week. Floorboards up and hydraulic fluid all over from front to back by mains actuator.

After clean up and quite a bit of time (hard because the fluid weeps everywhere) finally found I had a drip from the emergency hand pump. At first didn’t suspect it because your brain goes to the worst case scenario, kept examining the main’s actuator.

I finally found via a good clean up and using a flexible scope (like from harbor freight, very affordable). I would see wetness in the well around the shaft. Suck it out with corner of paper towel. Wait a while, scope it and start seeing the fluid build up in the shaft well. The scope magnifies things making the tiny leak findable.

The pump was an easy rebuild. Just some o rings.

AV8ing · #6 09-03-23, 04:23 AM

Wslade2 - Actually, I started with the hand pump since I used to own a Cessna 210 and the only hydraulic fluid leak I ever had was from the hand pump.

I can say unequivocally that the hand pump is not leaking.

So will probably bite the bullet and pull the actuator next week.

Any tips on rebuilding the main actuator ?

wslade2 · #7 09-04-23, 08:38 AM

Then it probably is mains actuator. Give us tips and tricks for pulling that. I have had it in my mind to prophylactically pull and overhaul mine.

But two other things (I'm sure you looked at): Brake system also uses hydraulic fluid and has flexible lines at pilot left foot and near rear passengers hip that won't last forever, could crack and leak. These areas could flow into belly. You are D model, so "power pack" (more properly control pack since engine driven pump powers things) has a hydraulic fluid catch tray under it and drain tube runs loosely into belly. Be sure "power pack" not leaking.

Frank Benvin · #8 09-05-23, 03:39 PM

We rebuild the acuators and replaced all hoses . After flying it for a while I topped up reservoir and went flying Parked the plane and found a small bit of fluid dripping from the belly Looked for the leak everywhere and could not find anything We even put the plane up on jacks and cycled the gear to look for the leak. We figure that the reservoir was overfilled and leaked out the overflow while in the air getting sucked into the belly It has stopped leaking since.

Skymaster337B · #9 09-05-23, 06:53 PM

Quote:

Originally Posted by AV8ing

Skymaster337B - That's my current thinking and I'm pretty sure it's been going on for many years.

Did you overhaul your actuator or send it out to Cox or another Repair Station ?

I overhaul it by removing the aft end. It unscrews like a jar top. Then Jack it to pull the piston out.

AV8ing · #10 09-12-23, 01:15 PM

Finally found the leak(s) ! ! !

Turns out it was the up-lock actuator - the last place I looked, all the way in the back under the rear seat rail. I can't believe that it made the mess that it did, that said there's no doubt it's the source.

Has anyone compiled a list of O-rings and packing required to overall ALL of the gear actuators ?

So The plan is to overhaul all of the main gear actuator except the big main actuator that's dry as a bone.

Thanks!

Frank Benvin · #11 09-12-23, 01:22 PM

We just went to the parts manual and looked at them up and ordered them Do you have the parts manual book if not sent me a private message with your email address and i will sent it to you or if you are on any of the Facebook skymaster sites i have downloaded them there under files

Frank

AV8ing · #12 09-12-23, 02:25 PM

I have the "Cessna 337 Series Parts Catalog - 1965 - 1969" .

In Fig 46 is shows the main gear door strut actuator but no break-out for the internal seals.

The up-lock and maingear doors are detailed in Figs 122 and 122A

That said , I looked up the MS2774 & 5 o-rings - not found.

Further the "S1121-389 CESSNA AIRCRAFT PARTS & ACCESSORIES RING, BACK UP TEFLON" listing @ $129 !!!

Surely someone has complied a shopping list using generic PN s?

Kim Geyer · #13 09-12-23, 07:48 PM

You should look up MS28774 and MS28775 orings and I think the backup Teflon rings should have the same dash number. The -329 is huge
I think I still have a book and will check in the morning

Skymaster337B · #14 09-12-23, 07:55 PM

It’s good that you caught that. If the fluid drained out the gear would not unlock. The spring locks the gear up. And only hydraulic pressure will release it.

AV8ing · #15 09-13-23, 04:31 AM

Quote:

Originally Posted by Skymaster337B

It’s good that you caught that. If the fluid drained out the gear would not unlock. The spring locks the gear up. And only hydraulic pressure will release it.

Yikes, that's not a good "fail" mode ....