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Brake pressure 'hanging' in cold weather

 
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Mountain Doctor
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PostPosted: Mon Mar 11, 2019 5:58 am    Post subject: Brake pressure 'hanging' in cold weather Reply with quote

My past couple of flights (OAT in the teens) my brakes are 'delayed'.

It takes twice the pressure to get them to work, then they stay 'on' for about 15 seconds as the pressure bleeds off and then the plane rolls normally.

On landing the wheels are freee and normal, but then when I use them they are delayed in onset and offset again.

Both brakes behave the same.

They have been serviced annually by my AI.

Any ideas?

Is this normal? (I don't recall this in years gone by)

It seeems like the fluid is too viscous, or the O rings are too tight...

Thanks for your help.
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Andy Young
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PostPosted: Mon Mar 11, 2019 10:48 am    Post subject: Reply with quote

I’d check two things, to start:

1. Are the caliper pins free-floating in the torque plates? Seems like they could be binding in there, due to corrosion or dirt/mud/slush/ice.

2. Are the brake pedals free-floating, and not binding on their pivots? Seems unlikely, but worth a quick check.

I doubt it’s hydraulic.
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riverbuggy
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PostPosted: Mon Mar 11, 2019 2:18 pm    Post subject: Reply with quote

Check the items that Andy mentioned. If no joy replace the brake fluid by bleeding the brakes enough to replace all the fluid. Could be water in the brake fluid. I had that situation on a customer's airplane. Don't know how it got there, but there was water in the system.
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Ray
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TimB
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PostPosted: Mon Mar 11, 2019 3:08 pm    Post subject: Reply with quote

Check you caliper for corrosion as it is a magnesium alloy casting, the piston is aluminum. This should be done at least every other year and O-rings replaced at the same time. If you use your bird off pavement service calipers every year. And check your parking brake locking lever P# 10046B and shafts for damage from the lever and shafts jamming in the master cylinder top bushing with brakes on during landing. Have had two aircraft damaged due to this.
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Mountain Doctor
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PostPosted: Tue Mar 12, 2019 5:43 am    Post subject: Reply with quote

Thanks guys. I'll check these things and report back.

It seems odd that both brakes are affected exactly the same.
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riverbuggy
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PostPosted: Tue Mar 12, 2019 3:45 pm    Post subject: Reply with quote

It does seem strange to me too. The cold operating temperatures is the main clue here. Was the correct fluid used the last time they were serviced?
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gbarrier
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PostPosted: Wed Mar 13, 2019 4:59 am    Post subject: Reply with quote

Change your fluid. Probably has moisture on it. I bet it looks milky when you remove it.
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Mountain Doctor
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PostPosted: Thu Mar 14, 2019 5:23 am    Post subject: Reply with quote

Update:

I was going to bring the plane to my AI to have it checked.

The OAT was about 40 and the brakes are operating normally.

The fluid was just changed last annual and the plane is hangared so I doubt water could be a factor. As far as incorrect fluid maybe, but I use a very established AI for my annuals.

I think I'll let it be and obeserve.

My next annual will be before next winter so I'll have the fluid changed and the brakes inspected then.

I'll keep you guys informed. It may help the next guy. Very Happy
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andy
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PostPosted: Sat Mar 16, 2019 3:11 am    Post subject: Reply with quote

Sounds like maybe some water got into the calipers and froze. You mentioned it's been about 40F so in flight it can easily get below freezing. If there's any water trapped around the pucks, it could freeze in flight and then stick when you land and use the brakes. Try pumping the brakes when you take off to get rid of water.
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Andy
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