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  #1  
Old 02-05-2020, 08:59 AM
bkervaski's Avatar
bkervaski bkervaski is offline
 
Join Date: Mar 2017
Location: Birmingham, Alabama
Posts: 1,866
Default The Mystery of the Missing Brake Fluid

Gang,

Last October during a taxi I noticed my brakes getting mushy.

Upon inspection, the brake reservoir was empty.

This is the Beringer brake system.

I check the brake fluid at least once per month and the last check, a few weeks prior during the annual, it was the normal 3/4 full.

Here's the mystery: Beyond one tiny drop on the reservoir connection there was no leaked break fluid anywhere. I checked everything. Every connector in the airplane, the entire path of the lines. Nothing. Just a small drop on the back end of the reservoir but nothing pooled up. No stains in the plane or on the hangar floor. Nothing.

I didn't post then because I wanted to see if it would happen again and it hasn't, the reservoir has maintained 3/4.

The only conclusion I can reach is that perhaps there was a large volume of air somewhere in the system that finally cleared itself, however, prior to the magic disappearing brake fluid the brakes were working great, no issues and very consistent.

Thoughts?

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Last edited by bkervaski : 02-05-2020 at 09:07 AM.
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  #2  
Old 02-05-2020, 09:58 AM
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flyboy1963 flyboy1963 is offline
 
Join Date: Oct 2008
Location: Lake Country, B.C. Canada
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Default evaporation????

strange! it's an 'open' system as the reservoir is vented, but seriously.....

I 'topped' up my reservoir every year, at least, at annual, for 8 years. It was down about 1/4 to 1/2". no fluid anywhere, seeping, dripping, weeping, etc.
and the 1/16" of pad wear certainly wouldn't add up to that volume....
( ok, someone do the math?!?)
;-)
the mystery continues......
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  #3  
Old 02-05-2020, 10:02 AM
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bkervaski bkervaski is offline
 
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Location: Birmingham, Alabama
Posts: 1,866
Default

I don't think it's coming out when the earth gets upside down on me, I have one of those "Reservoir Dog" caps that seals when inverted.
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2019 Bronze Lindy

Last edited by bkervaski : 02-05-2020 at 10:05 AM.
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  #4  
Old 02-05-2020, 11:59 AM
bscycleman bscycleman is offline
 
Join Date: Jan 2011
Location: San Antonio, TX
Posts: 58
Default Caliper piston

I've heard another pilot relay the same story, but it may not apply here. In his case, the brake pads were worn down close to the wear indicators. This allowed the piston in the caliper to unseat and leak fluid. He also checked all of the connections and never found a puddle under the wheels.

Brian
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  #5  
Old 02-05-2020, 10:28 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,686
Default

Quote:
Originally Posted by flyboy1963 View Post
strange! it's an 'open' system as the reservoir is vented, but seriously.....

I 'topped' up my reservoir every year, at least, at annual, for 8 years. It was down about 1/4 to 1/2". no fluid anywhere, seeping, dripping, weeping, etc.
and the 1/16" of pad wear certainly wouldn't add up to that volume....
( ok, someone do the math?!?)
;-)
the mystery continues......
Don't be so certain. calipar piston/bore is about double the size, in diameter than that of the reservoir or a bit less. Therefore, 1/8" of piston travel (2 pistons) should be in the neighborhood of 1/2" in the reservoir (a 2X increase in diameter equates to roughly a 4X increase in the area and ultimately volume). If the piston were only 1.5 times larger, I would expect around a 1/4-3/8" drop; Don't feel like doing the math, as I would need a calculator on that one.
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Last edited by lr172 : 02-05-2020 at 10:45 PM.
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  #6  
Old 02-05-2020, 10:48 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Default

Quote:
Originally Posted by bscycleman View Post
I've heard another pilot relay the same story, but it may not apply here. In his case, the brake pads were worn down close to the wear indicators. This allowed the piston in the caliper to unseat and leak fluid. He also checked all of the connections and never found a puddle under the wheels.

Brian
This would be my first guess. Though one would expect to see signs of that once the wheel pant is off. Not to mention the poor braking performance once the pads are soaked in it. The stuff doesn't evaporate, so it had to go somewhere.

Larry
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  #7  
Old 02-05-2020, 10:52 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Default

Quote:
Originally Posted by bkervaski View Post
The only conclusion I can reach is that perhaps there was a large volume of air somewhere in the system that finally cleared itself, however, prior to the magic disappearing brake fluid the brakes were working great, no issues and very consistent.

Thoughts?
That is pretty unlikely. That would be a LOT of trapped air and would have resulted in very mushy pedals.
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  #8  
Old 02-06-2020, 02:25 PM
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bkervaski bkervaski is offline
 
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Location: Birmingham, Alabama
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Default

The mystery continues ...

Quote:
Originally Posted by lr172 View Post
This would be my first guess. Though one would expect to see signs of that once the wheel pant is off. Not to mention the poor braking performance once the pads are soaked in it. The stuff doesn't evaporate, so it had to go somewhere.
Brakes are like new. No residue.

Quote:
Originally Posted by lr172 View Post
That is pretty unlikely. That would be a LOT of trapped air and would have resulted in very mushy pedals.
Agreed, but the pedals have the same travel now as before, no issues braking.

See why I posted? Makes no sense ...
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  #9  
Old 02-06-2020, 07:18 PM
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PaulvS PaulvS is offline
 
Join Date: Aug 2019
Location: Western Australia
Posts: 770
Default Bleed valve

Perhaps it went out through a marginally loose bleed valve on the caliper, under unusual pressure from heavy braking? Most of the evidence would be out on the tarmac, though there'd likely also be some residue in the bottom of the spat.
At least it's OK now.
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Last edited by PaulvS : 02-06-2020 at 07:30 PM.
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  #10  
Old 02-07-2020, 04:29 AM
F1R F1R is offline
 
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Location: ____
Posts: 911
Default Caliper Piston O Rings

How many years old are the calipers since new?

More accurately, how many years since manufactured?

In My first airplane I could take off with full reservoirs, fly 140 miles to pick up my wife and land with no brakes (and empty reservoirs) Really honey it's a great airplane...

Re fil and sit and hold heavy brake pressure for 10 minutes. Not a drop and no leaks to be found. One week later, the floor under the mains was spotless and dry. Inside, no leaks and the reservoirs still topped up.

Fly for 1 hour and land with no brakes & empty reservoirs

The culprit was old o rings in the caliper pistons that were no longer live and round, but rather a D form from sitting many years in non use, before being put into service. The slip stream at speed in the air was creating just enough negative pressure to pull the brake fluid out in flight. O rings can be just good enough to seal under pressure , but not live enough to hold slight vacuum or negative pressure.

A new set of o rings cured the issue.

Last edited by F1R : 02-07-2020 at 04:34 AM.
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