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Old 04-04-2021, 02:18 PM
ctennis ctennis is offline
Join Date: Aug 2014
Location: Columbus, IN
Posts: 120
Default Bleeding brake woes

RV-10, 99.9% done. My brakes are dragging, and trying to debug I'm running into an interesting issue I'm not sure of how to troubleshoot:

* If I do the traditional bleeding method (push brake, hold pressure, have another person loosen fitting at brake, push out fluid, tighten, repeat) - this works fine from the RIGHT seat. Fluid comes out the valve at the brake. I can do this and draw down the reservoir as you would expect.

If I do this from the LEFT seat, it seems like it sucks in air. It pushes out fluid at the brake, but nothing draws from the reservoir, and after retightening, the brake gets mushy. The only way to unmushy is to push fluid from the RIGHT seat again.

* I can't seem to force fluid "up" from the bottom. If I do, it seems to displace any air that might be in the line, but as soon as that's quickly displaced, no more fluid goes up. The reservoir level never changes. And yes, I am loosening up the fitting the fluid is connected to in order to let it go up into the break.

I've quadrupled checked the plumbing. I'm not finding any leaks at any of the fittings, including in the tunnel or under the seats. It's acting almost like there's a check valve between the left and right sides, but even then, I'm not sure why when pumping from the left side it wouldn't draw via the reservoir.

This happens for both the left and right brakes independently.

Any obvious thoughts I should try? My next step will be to loosen each fitting and verify I'm getting fluid to it when pumping from both directions.
N29213 - RV8 completed 2017
N955CT - RV10 completed 2021
Columbus, IN - KBAK
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Old 04-04-2021, 02:53 PM
BobTurner BobTurner is offline
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,356

Originally Posted by ctennis View Post
RV-10, 99.9% done. My brakes are dragging, and trying to debug.....
It sounds like one or more of your brake cylinders are ‘hung up’, and not returning to the full open position (max extension). This is not uncommon if the bottom brake pedal pivot bolt is too tight or binds. In this case when bleeding via the ‘bottom up’ method fluid is blocked at the cylinder from getting to the reservoir. Some have installed helper springs to insure the brake cylinder returns to full up (open) when you remove your foot, although I suggest you fix whatever is causing excess friction.
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Old 04-04-2021, 04:11 PM
ctennis ctennis is offline
Join Date: Aug 2014
Location: Columbus, IN
Posts: 120

Totally makes sense, and not something I had thought about. Will investigate into the cylinder extension. Thank you!
N29213 - RV8 completed 2017
N955CT - RV10 completed 2021
Columbus, IN - KBAK
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Old 04-04-2021, 04:16 PM
Taltruda Taltruda is offline
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 665

I was thinking the same thing. . The traditional, "normal" way to bleed aircraft brakes is from the bottom up. Try unbolting the masters from the pedals and see if they weren't "all the way up". They should allow fluid to easily transfer to the reservoir. Be careful that you don't overflow it and spill everywhere!
Las Vegas
RV-8 empenage finished 10-2020

Wings Started.. 11-2020
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Old 04-04-2021, 04:35 PM
rocketman1988 rocketman1988 is offline
Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,440
Default If

If they are the matco master cylinders, they last 1/16” of the extension is very important...
Aerospace Engineer '88


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Old 04-04-2021, 04:54 PM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 5,895

Originally Posted by rocketman1988 View Post
If they are the matco master cylinders, they last 1/16” of the extension is very important...
Yes! The fill port is closed off by the plunger in the first small amount of travel.

If the master plunger does not return you will have serious issues, be sure it returns after each application of the brake. Your peddle pivot pins may have friction, there should be none.

Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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Old 04-05-2021, 08:03 AM
WingsOnWheels WingsOnWheels is offline
Join Date: Mar 2009
Location: Plano, TX
Posts: 2,137

Also, to make bleeding "up" easier, lift of the pedals so they are close to horizontal (put a box or something under them). This helps bleed out the air trapped in the loop between the left and right sides.
Colin P.
RV-6A #20603
Complete 5/10/19
I donate every year on my B-Day (in Dec), but donated early in Sep'19.
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Old 04-05-2021, 10:33 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,998

My read is that you have a leak in the left master cylinder(s). If you can traditional bleed from the right but not the left, it implies that the stat-o-seal or other o-ing in the left M/C is leaking. Somewhat common on the Matcos after 10 years of service life.

if you apply the brake and it draws air into the line instead of drawing fluid from the supply side, it implies that an o-ring in the M/C has failed. There are three of them plus the stat-o-seal, which is similar to an o-ring. ACS sell a rebuild kit for $11.

On my 10, the M/C's were new, but 12 years old. They sucked air into the line from the initial installation (rubber had aged and hardened). No amount of bleeding effort would make them firm and you could see the air bubble being introduced on brake application. Both rebuilt and worked normally.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 04-05-2021 at 10:41 AM.
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Old 04-05-2021, 11:41 AM
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Walt Walt is offline
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,020

Standard practice for aircraft is to pressure bleed from the bottom up.
Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
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RV7A built 2004, 1900+ hrs, New Titan IO-370, Bendix Mags
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Old 04-05-2021, 12:07 PM
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Mike S Mike S is online now
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Mike Starkey
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Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

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