Brake fluid leak

[Nev]

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?

Making a tool is a very good idea. Just to clarify though, I'm expecting to have to replace the brake pads at about 800 landings, but I'll be checking them regularly before that.

[Steve W]

So I had previously bled my brake systems on the amphibs and confirmed there weren’t any leaks. Taxied around a bit. Parked the plane in the hangar and checked for leaks the following two week. I then went to work for three weeks and came back a few days ago for my time off. Went up today to ready the plane for the upcoming Transport Canada inspection. Both lines were completely drained out on the ground under the main wheels.
I had cleaned the callipers and replaced the puck O rings with new when I assembled the gear earlier this year. I did use transmission fluid which I have utilized for every aircraft I have owned and it was an open bottle that has been used in other aircraft but it’s about 10+ years old.
I’m at a loss as to why the fluid found it’s way out. Either the fluid is old or not compatible with newer O-ring composition. I brought the callipers home, cleaned them and assembled with another set of new O rings. I will use Shel 41 and bleed the system out tomorrow. I didn’t see any sign the O rings were being compromised in any way. No cracking, softening or stiffening.

[Nev]

Steve, on mine I think the brake pads were worn down enough that the O rings were being pushed right out of the housing, and thereby allowing the fluid an easy path out. Not sure if this helps.

[AZBearhawk272]

Steve,
You might confirm that the O-Rings are actually the correct ones, sizes and cross section. Mil Spec vs Parker Industrial vs Metric. There are differences.
Supply chain is a bit messed up and the word of the day is, close enough....

Kevin D
#272

[Steve W]

Thanks guys.
New discs and pads and Parker Mil spec O-rings of the correct part number were used. As I said, I’m at a loss other than old transmission fluid. I’m heading back up to the hangar tomorrow and will redo with Shell 41.

[Steve W]

So Kevin, your saying the package could contain the improper size within?
The rings didn’t leak during two weeks I was keeping an eye on them. Then both developed a serious leak and dumped all the fluid over three weeks I was away. I suspect a reaction of some sort between the tranny fluid and new rings.

[AZBearhawk272]

Based on the Packaging and identification
thoes look like pre COVID, MIL Spec parts.
The real deal.

[Battson]

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.

It's about 3 or 4 sets of pads to a disc in my experience, I imagine there is some variability.

The discs don't wear evenly, they wear faster at the rim and slower at the base (more wear at greater diameter where angular velocity is higher), and more slowly where the rivets are (smaller area in contact with the disc) so their profile is rather uneven when worn. Suggest measuring the average thickness throughout the pad area to gauge wear on a fair basis.

My new discs are 6.5mm, the worn discs have a maximum dimension 5.4mm and minimum of 4.6mm at the rim. I had absolutely no issues with these discs and they seem plenty strong, I just changed them to be on the safe side.

Worn disc - you can clearly see the uneven wear if the photo is viewed full size - high areas in the middle where the brake pad rivets sit,.
Worn discs.jpg

Calipers after cleaning and refurbishing
Calipers.jpg


New discs installed complete
New discs.jpg

[Nev]

Good to know - thanks.

[Nev]

To add another data point here, I've now been through two sets of brake pads. The first set lasted 800 landings, the second set, 460 landings. The reason for the difference is wear of the disk, and the limiting factor is that the piston O rings are getting pushed out far enough that a small fluid leak appears.

A friend and engineer took a look and mentioned that my brake units would be larger than those on many light twins. He said the disk should have another 1000 landings on it before even contemplating replacement. His suggestion is to install brake pads with more thickness, or alternatively, shims, to match the disk wear, in order to utilise full disk life.

The grove brake pads are 66-106. Does anyone have a part number for a thicker set of these pads ? It would be about.025 - .035 additional pad thickness.

[Nev]

Update

After experiencing another small fluid leak on one side a few weeks ago, I checked the current brake pads for wear. They are currently about 1/3 wear. The first time this occurred the pads were well worn and needed replacing. But the next time occurred with less wear.

My brake rotors are a more heavy duty than what is usually fitted. As both the brake pads and the rotors wear, it appears to be allowing the pistons to extend too far (and further each time), resulting in a fluid leak. I've now added shims (0.050") on both sides and the leak appears to have immediately stopped. I'm making a second set of shims as well. When the rotors are worm and need to be replaced I expect to remove the shims and start the process over.

[Jim Herd]

When pulling my BHP backwards into the hangar, I am hearing some noise from the left brake pads. My brake lines are solid aluminum to the calipers. Surely this is not good practice. Why? Because it impedes the "float" of the calipers. Consequently, the pads can be a little biased and touching the rotor and thereby premature pad wear, and/or misalignment causing noise during ground movement. Is this rational? And perhaps good reason to change to flex lines to the calipers?

[jaredyates]

Update

After experiencing another small fluid leak on one side a few weeks ago, I checked the current brake pads for wear. They are currently about 1/3 wear. The first time this occurred the pads were well worn and needed replacing. But the next time occurred with less wear.

My brake rotors are a more heavy duty than what is usually fitted. As both the brake pads and the rotors wear, it appears to be allowing the pistons to extend too far (and further each time), resulting in a fluid leak. I've now added shims (0.050") on both sides and the leak appears to have immediately stopped. I'm making a second set of shims as well. When the rotors are worm and need to be replaced I expect to remove the shims and start the process over.

Nev, what type of method would you use to measure the thickness of the disks? I ran into this with 303AP. There were lots of highs and lows on the disk, such that I figured if I was to get a good measurement with something like a caliper or micrometer, I would need to first true the disk on a lathe. In the end I just replaced them, though I did save the old ones for science, at a later date. In my case I had a caliper lock up, and I attributed it to the piston being too far out and getting crooked. Once I realized what it was doing, I shut down, hopped out of the plane, cracked the bleeder at the bottom to relieve the pressure, then manually pressed the pistons back in a little. That was enough to get me home but I didn't want to fly it again until something was changed. The new pads and discs have been holding up well with no problems so far.

Jim, I do get some noise from my brakes, even after switching to the flex lines. It may be that they aren't quite long enough to allow for a full freedom of motion.

[Nev]

I haven't figured a better way of measuring them other than a caliper which as Battson mentions above tends to be an estimate due to the variation in thickness.

[Jim Herd]

Replacing the solid brake lines to the calipers with flex lines didn't actually fix the dragging problem, though it still needed to get done. It made the right caliper "rattle" when moved back & forth by hand. But the left side still had no free play. The solution was a 0.020" shim plate at the outboard caliper of the left brake. The caliper just wasn't adequately aligned with the disc to provide free play. Now no dragging, scraping noises, or difficult pushing/pulling.

[jaredyates]

Replacing the solid brake lines to the calipers with flex lines didn't actually fix the dragging problem, though it still needed to get done. It made the right caliper "rattle" when moved back & forth by hand. But the left side still had no free play. The solution was a 0.020" shim plate at the outboard caliper of the left brake. The caliper just wasn't adequately aligned with the disc to provide free play. Now no dragging, scraping noises, or difficult pushing/pulling.

Jim, in which axis were you misaligned?