Brake fluid leak

[Nev]

Had a cold overnight recently where the temp was below freezing (20°f). During the preflight in the morning I found what appeared to be brake fluid on both tires. It appeared to have leaked from the brake pistons, In the last 225 hours there haven't been any leaks, but both brakes leaked simultaneously on the same night. Has anyone seen this before?
I checked the brakes during run-up and they were operating normally.

EDF5F6E6-F054-4E55-A408-3931E6029865.jpg

[AZBearhawk272]

Nev,

Not uncommon. The O-Ring seals on the piston are compounded from a rubber that is optimized for higher temp operation. At cold temps they shrink and become stiff loosing contact and allowing fluid seepage.
usually seals well once pressurized.

I suspect you are using automotive Transmission fluid as brake fluid? ATF has a lower viscosity when cold than Mil- H- 5606.

Kevin D
# 272
KCHD

[Nev]

Yes spot on Kev, that explains it. Thanks.

[Battson]

ATF has a lower viscosity when cold than Mil- H- 5606.

This makes sense.

We are using Mil-H-5606 brake fluid. I have not seen this problem before despite many, many cold nights spent out in the same climate / elevation range. I have the same wheel and brake assemblies as pictured above. So the fluid seems to be one key difference.

[Nev]

In addition I'm wondering if having the Park Brake OFF with the system unpressurised makes a difference. Other overnights in similar temps (with the Park Brake ON) didn't produce any leaks. Next time I'll put the Park Brake on and see what happens.

[predragvasic]

Since the parking brake is engaging the hydraulics, so the rubber seals are under pressure, there should be no leaking, right?

[Nev]

Just to update this, I got another leak coupled with sub par brake performance and decided to investigate further. It turns out that the brake pads all needed replacing and I probably should have done this earlier as they showed significant wear.

I used the opportunity to replace the seals (although the original seals were still in quite good condition). We'll put the new pads on tomorrow and hopefully the issue should be fixed.

[jaredyates]

How is your disk wear? Just curious for the data point.

[Nev]

How is your disk wear? Just curious for the data point.

Disk wear itself seems ok. I was concerned about uneven wear but fortunately think I caught it just in time.

[jaredyates]

Did you happen to measure their thickness? I wonder how many pad sets a disk lasts for. I'm thinking about replacing mine due to the way they are wearing.

[Nev]

I'll measure them today and let you know.

[Nev]

Brakes are working again as advertised and hopefully no more fluid leaks.

The Grove specifications show new disks as 0.375" thickness, and minimum thickness as 0.35. Mine measured 0.36". This is after 1010 landings. The earlier landings were probably harder on brake wear and about 100 were very high wear where I was deliberately keeping the tail off as long as possible.

Next time I'll be planning to replace the pads at about 800 landings (but also keeping a closer eye on them). I expect I'd get at least another 1000 landings from the disks and then check the disk tolerance again.

[jaredyates]

Very nice thanks. It sounds like if the wear is linear, discs are good for around two sets of pads in our application.

[Nev]

Doesn't sound like a lot of longevity. Perhaps others who have higher airframe hours could give feedback on how many sets of brake pads they went through before the disks reached tolerance limits.

[Bcone1381]

Nev; You've indicated that you'll service the brake pad linings at 800 landings. Have you considered fabricating a small lightweight tool to measure brake pad thickness to determine their service limits?