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Weird brake leak, what would you do?

jcarne

Well Known Member
Patron
As you can see in the picture I attached I went out to the hangar a few weeks ago and notices a slight brake leak. There was a small amount of fluid on the ground and a hanging drop on the right caliper. I pushed on the pedals and there is still good pressure on both sides and the pressure did not cause any more drops of fluid.

The weird part is that the aircraft has not moved, has always been in a heated hangar at 42 degrees. To top it all off I haven't noticed any more drips in about the last two weeks. The brakes were also never actuated to cause this. Long story short I'm kind of puzzled. I would think if the there was a problem with the o-ring on the piston it wouldn't just stop like this. Thoughts?
 

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I had a situation like that several years ago. I assumed a seal got hot and leaked, then returned to shape when it cooled. Surprisingly, it never leaked again. Almost certainly a dumb move on my part not to pull the piston and replace the seal at that point, but I got away with it.

I have since replaced the o-rings with spiffy viton rings.
 
Jeremy,

I just had exactly the same on my left brake. Pulled wheel pant and the bleed screw AND the bottom caliper bolt have obviously been attracting dirt and were very slightly damp. Cleaned it all up, placed a paper towel underneath it and pumped like crazy and held pressure on it for what seemed to be an eternity. Nothing. I rebuilt both calipers and flushed the system about a year ago. I did use viton o rings. Guess I'm going to pull the left caliper and look at it real close and then rebuild it one more time.
 
Thanks for replies so far guys. I should also add that these brakes are brand spanking new and have never even been run before. They have however been holding fluid in them for more than 6 months before this happened. Therefore, I don't think heat has anything to do with it.

Unfortunately, I will be flying in some seriously cold weather so it sounds like the vitons may not be the best choice for me.
 
If they are new then perhaps a small amount of brake fluid got onto the caliper during assembly and eventually ran down to the point where enough collected and dripped a time or two. I would clean it up and put a paper towel down and keep watch. No more dripping, brake fluid level check and good pressure... I'd say you're good to go.

Edited: IFFFF there continues to be a small drip even only occasionally, I'd break it down and replace the o-ring in the caliper. I always did worry about landing and being heavy on the brakes and with a sudden gush, one goes out. The possibility of loss of control increases. It's actually part of my prelanding checklist. Brake check equal/normal feeling pressures.
 
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Any chance a small amount sprayed/lost fluid during the bleed collected then dripped down? I also had a hairline circumference crack due to overflaring aluminum brake line above the caliper. Noticed a tiny almost dried out trace then investigated further.
 
Make sure it goes back in correctly

If you take the puck out to change to o-ring just make sure it goes back in correctly. It can be put in wrong and you’ll loose your brake when you least expect it. This is the correct way. If it is put back wrong the o-ring will be too close to the edge and will come out when you apply the brakes.

5F1CB1C2-9846-4BB1-91FC-8088DAB905C5.jpg
 
Actions regarding this leak depends on the owners tolerance for being stranded at an outlying airport or worse, stranded in a ditch.

I had a winter leak of left brake a couple of years ago. Fluid on the ramp when i walked up to the bird. Fortunately, I had a spare car there and I was able to drive home in 3 hrs which was a normal thing for crappy weather commutes. I had all the tools i needed in my emergency kit but no fluid to replenish. Fixed it on the ramp the following week.
Being further afield with no wheels would have been a pita.

So... o-rings are cheap so they are getting changed at annual now dispite it being a nasty job. It's better done at the cozy hangar at home airport.
 
nicked the piston

I had a similar problem with a continuous seep from my left brake. I had previously replaced the piston O-ring. It was just a seep and would go for 2 or 3 flights before it appeared again. Checked the connections and the bleed screw, all fine. Eventually pulled the slave cylinder and pulled out the piston with O-ring. There was a very slight scoring in the slot that the O-ring fits into. Some hack mechanic (me) had used a small screwdriver to reach down the side of the slot to pry out the O-ring, and in the process must have slightly scored the groove - it was quite tiny. Ordered new piston (~$100), put in new Viton O-ring (make sure it is brown) and the seep went away. Do make sure the piston is installed in the correct direction, new pistons actually have printing that says "this side out".
I always have trouble getting that ring out of the groove. I have resolved in the future to use a fishhook to stab the O-ring in the middle and pull it out that way.
 
It's not uncommon to get a small crack in the flare on the tube under the nut. Could cause a small occasional leak under the right conditions.

Vince
RV-7
 
Thanks for the replies everyone. The leak is most definitely not coming from the flared connection or the bleeder. Definitely from the piston region. I went out there again yesterday; no leak for the last week... I guess I'll order a few orings and take it apart to be certain. Won't be flying until March as it is. Just wondered if anyone else has found a smoking gun on something like this.
 
That's a head-scratcher.

Are there any other leaks in the system? Pump the brakes and see if it develops again. Might be the next easiest step Jereme, rather than tearing right into it.

Just a thought.
 
That's a head-scratcher.

Are there any other leaks in the system? Pump the brakes and see if it develops again. Might be the next easiest step Jereme, rather than tearing right into it.

Just a thought.

Ya I pumped the brakes a few times already. It doesn't cause any more to leak. It happened once, I pumped the brakes, it went away for a week or so and then it dripped again. I pumped em again and it hasn't leaked since (about 3 weeks now).
 
In the last 20 years I have seen these caliper leak conditions.
Miniscule scratch in caliper bore, must have been assembly error, everything looked good. Dressed out with Scotchbrite pad
Not so minuscule scratch caused by nick in piston, definitely assembly error. Toast, replaced caliper, owner very unhappy.
Small nick in o-ring, again assembler did something wrong. Dressed caliper bore just in case, new o-ring.
O-ring shaved in one spot, caliper bore edge very sharp, not deburred & invariably the o-ring would be damaged on assembly. Deburred, new o-ring.
Crack in fitting threaded bore, again obvious production error. Replaced caliper...

Not related but notable, ran into a batch of 1/4" aluminum tubing one time that leaked in spots along it's length, obviously production defects. Just proves anything produced by humans potentially can fail.
 
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Listen to the Hardware

Thoughts?

If it was me, I would listen to the hardware. It is telling you something if a fluid you thought was in a closed system in outside of that closed system. Did some fluid leak when you bled them in the first place, and just now materialized? Did you clean the entire assembly after bleeding the brakes, which makes this explanation unlikely? Maybe, but only you can answer how long that has been, and if the most likely cause is that it was there previously. Is it more likely that you have a piece of junk in an o-ring and it leaks in some cases, but not others? Yes, as others have said, that has happened before. Is it possible you have a split in the tubing, a fitting that didn't seal, a piece of garbage in the seat of the AN fitting? All of these are possible. But you should listen to the hardware. It is telling you something.
 
Jereme---there is a possiblity that the O Ring is contracting slightly during the coldest part of nitetime. The as things warm, it expands slightly, thus sealing the piston. With you having bled the system, and have a firm, solid pedal with no leaks during that process, the o ring contraction is the only thing I can think of right now. If the bleeder or the AN nipple or hose were to be slightly loose, you would have seen fluid before now.

Tom
 
As an addon to the TS flightline idea on temperature. Whenever we took our KC-135 Tanker (Cousin to 707 for you civilians) to somewhere cold like Eilson AK in January from home base, (Hickam AFB) we would always, ALWAYS have hydraulic leaks after sitting overnight on the first night. Then no issues the rest of the time we were there.

Conversely the Eilson based Tankers that would have their hydraulic equipment torqued at much colder temperatures. Had much much fewer leak issues as a whole on their world travels.

So long story short could just be temperature issue.
 
I had a similar problem with a continuous seep from my left brake. I had previously replaced the piston O-ring. It was just a seep and would go for 2 or 3 flights before it appeared again. Checked the connections and the bleed screw, all fine. Eventually pulled the slave cylinder and pulled out the piston with O-ring. There was a very slight scoring in the slot that the O-ring fits into. Some hack mechanic (me) had used a small screwdriver to reach down the side of the slot to pry out the O-ring, and in the process must have slightly scored the groove - it was quite tiny. Ordered new piston (~$100), put in new Viton O-ring (make sure it is brown) and the seep went away. Do make sure the piston is installed in the correct direction, new pistons actually have printing that says "this side out".
I always have trouble getting that ring out of the groove. I have resolved in the future to use a fishhook to stab the O-ring in the middle and pull it out that way.

A better way to get the O-rings out is to not use any tools! Pinch the o-ring like you are trying to pop a pimple.. it’ll rise up on one side, then you roll it out from the groove!
 
Ya It happened once, I pumped the brakes, it went away for a week or so and then it dripped again. I pumped em again and it hasn't leaked since (about 3 weeks now).

There is your answer: It happened twice. There is something going on that is better resolved in a hanger. Whenever I have a seemingly minor issue like this I try to narate the accident report in my head. If I sound like an idiot, then I know I have to correct the issue.
"Several weeks before the incident the pilot had posted on an aviation form that he was having some problems with his brakes. He determined that the issue was not serious and elected to proceed with the flight. During the take-off roll..."
Its not a bad job.
 
There is your answer: It happened twice. There is something going on that is better resolved in a hanger. Whenever I have a seemingly minor issue like this I try to narate the accident report in my head. If I sound like an idiot, then I know I have to correct the issue.
"Several weeks before the incident the pilot had posted on an aviation form that he was having some problems with his brakes. He determined that the issue was not serious and elected to proceed with the flight. During the take-off roll..."
Its not a bad job.

Haha I like that thought process.
 
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