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Brake Fluid Loss Mystery

Radioflyer

Radioflyer

Well Known Member
During my condition inspections, I find my brake fluid reservoir almost empty. No tell tale leak indications are found at the calipers or master cylinders and the brakes work well. Since the reservoir is on the engine side of the firewall, might it be possible that the fluid slowly evaporates over a year due to heat? Do others also experience diminishing fluid levels after a year from their engine side reservoirs?
 
My brake resivior is on the engine side of firewall as well. It's only been low twice and I had a leak both time. Otherwise it's always pretty much where it was at the last annual CI.
 
Radioflyer said:
During my condition inspections, I find my brake fluid reservoir almost empty. No tell tale leak indications are found at the calipers or master cylinders and the brakes work well. Since the reservoir is on the engine side of the firewall, might it be possible that the fluid slowly evaporates over a year due to heat? Do others also experience diminishing fluid levels after a year from their engine side reservoirs?
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How much wear do your calipers show? As brakes wear, the fluid goes from the reservoir to the caliper. It may be time you inspected your pad wear.

When changing pads, as you push the pistons back into the caliper, fluid will rise in the reservoir.
 
You wouldn’t have any firewall insulation do you? My tech advisor just related a story of a customer who had the same issue. No obvious leaks were found till they noticed the insulation on the firewall was moist.
It had been leaking at the reservoir port and soaking into the insulation.
 
Mel said:
How much wear do your calipers show? As brakes wear, the fluid goes from the reservoir to the caliper. It may be time you inspected your pad wear.

When changing pads, as you push the pistons back into the caliper, fluid will rise in the reservoir.
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Fair point. That may be part of the issue, but the discs are within spec and the pads still show the wear notch. I don't think this is the whole story.
 
Clayvt said:
You wouldn’t have any firewall insulation do you? My tech advisor just related a story of a customer who had the same issue. No obvious leaks were found till they noticed the insulation on the firewall was moist.
It had been leaking at the reservoir port and soaking into the insulation.
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Nope, no insulation, but I will check that fitting again. Thanks.
 
Walt said:
Hyd fluid doesn't evaporate.
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+1

Also, the volume of fluid required to take up the space from pad wear is a fraction of the reservoir volume. If you find the reservoir empty that was once full, you have a leak somewhere. The fact that you haven’t found it yet, doesn’t change the physics.
 
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Hydraulic fluid does evaporate (otherwise it wouldn’t turn to goo at the site of a leak) but the question becomes at what rate? You should be able to find that information on the Material Safety Data Sheet for 5606 and compare it to other common liquids. The big take away - which you probably realize but hasn’t been said so I will be the one - is that you should be checking and topping off the level more than once a year. Don’t want to start sucking air into the system.
 
rmarshall234 said:
Hydraulic fluid does evaporate (otherwise it wouldn’t turn to goo at the site of a leak) but the question becomes at what rate? You should be able to find that information on the Material Safety Data Sheet for 5606 and compare it to other common liquids. The big take away - which you probably realize but hasn’t been said so I will be the one - is that you should be checking and topping off the level more than once a year. Don’t want to start sucking air into the system.
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Perhaps you could enlighten us?

From the MSDS:
Evaporation Rate: No data available

Maybe I'm not really burning engine oil, could it be evaporating?
 
Walt said:
Perhaps you could enlighten us?

From the MSDS:
Evaporation Rate: No data available

Maybe I'm not really burning engine oil, could it be evaporating?
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www.machinerylubrication.com

Determining Oil Evaporation Loss

All oils are subject to evaporation loss, some more so than others. In certain circumstances, this can have disastrous effects on your equipment.
www.machinerylubrication.com www.machinerylubrication.com

The msds does have the vapor pressure though…
 
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Radioflyer said:
During my condition inspections, I find my brake fluid reservoir almost empty. No tell tale leak indications are found at the calipers or master cylinders and the brakes work well. Since the reservoir is on the engine side of the firewall, might it be possible that the fluid slowly evaporates over a year due to heat? Do others also experience diminishing fluid levels after a year from their engine side reservoirs?
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What aircraft do you have. I don’t know about other models, but the 14 has brake fittings close to the top of the gear legs in the fuselage that could leak and not be noticed.
 
Walt said:
Perhaps you could enlighten us?

From the MSDS:
Evaporation Rate: No data available

Maybe I'm not really burning engine oil, could it be evaporating?
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Haha I believe it DOES evaporate also.. just because there’s no data for it, doesn’t mean it doesn’t evaporate. My money is on either a leak at the firewall or possibly at the masters, and leaks onto the carpet.
 
Taltruda said:
Haha I believe it DOES evaporate also.. just because there’s no data for it, doesn’t mean it doesn’t evaporate. My money is on either a leak at the firewall or possibly at the masters, and leaks onto the carpet.
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Maybe if you make a big puddle on the floor and wait long enough some may evaporate, however in closed system with a 1/4” cap how long would it take?
 
This happened to me, it's one of those things you're convinced nobody will believe and you're embarassed to ask. I had my brake fluid on my 14 just simply disapear similarly as to you. No leak indicators, no fluid, no puddles or stains, locked hanger, no little green men. I couldn't even begin to tell you how it happened and seems impossible. I just tightened everything up and re-bled, never happened again.
 
Walt said:
Maybe if you make a big puddle on the floor and wait long enough some may evaporate, however in closed system with a 1/4” cap how long would it take?
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It would take forever to evaporate out of the vent. I can confirm that it will eventually dry and leave a sticky film on the firewall if you have a miniscule leak. Long odds, but I wonder if the OP does any sustained inverted flight or anything using negative G's?

That said, there are several potential leak points in the system at various fittings.
 
Yes oils can evaporate, but at room temperature, it takes years and the amount very small. So, as long as were talking about fluids disappearing in months or a year, you can generally say it is not evaporation. The thick goo that you find is oil that has oxidized, not evaporated. Very different reaction and usually needs heat.
 
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Seriously, as several pointed out, the vapor pressure values are found in the SDS. Both Aeroshell 4 and 41 have vapor pressures 5x that of motor oil. I'm just sayin'...
 
Humble opinion, wheel pants make pad inspection a once a year event. As previously posted the fluid fills the void in the caliper. Also the portion of the brake lines on the outside of the Airframe are exposed to 150+ knots which can atomize the 5606 (somewhat). I really don't think evaporation in our semi-closed systems are the answer to the mystery
 
sansoneservices said:
Humble opinion, wheel pants make pad inspection a once a year event. As previously posted the fluid fills the void in the caliper. Also the portion of the brake lines on the outside of the Airframe are exposed to 150+ knots which can atomize the 5606 (somewhat). I really don't think evaporation in our semi-closed systems are the answer to the mystery
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Assuming an RV, remove front half of wheel pant, air up tires every 3 months and inspect linings at the same time.
 
I really like the Gremlins hypothesis. Curiously, my engine oil evaporates about 1 quart after about 7 hours of flight (joke). Seriously, evaporation (though technically possible) is probably not the cause. I will be looking high and low for a leak, especially at the inside firewall area.
 
Desert Rat said:
This whole conversation reminds me of when my daughter told me that water isn't wet...it just makes other stuff wet.
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Actually from a purely scientific perspective, she is correct. However, from an everyday perspective she is incorrect. I certainly wouldn't argue it.
 
Mel said:
This is true, BUT, this will show up with evidence directly under the wheel.
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Remarkably, in my case, it didn't. Not until I was stuck due to finding the pedal flat when I hopped in to start. Nothing on my hangar floor, no indication of poor braking on the previous flight. My best guess is that it was pumping a little out each time I braked hard on landing, but when taxiing I wasn't pushing as hard and anything that leaked out while landing had time to distribute.

There was evidence of dripping on the wheel when I took the wheel pant off to fix the problem.
 
Snowflake said:
Remarkably, in my case, it didn't. Not until I was stuck due to finding the pedal flat when I hopped in to start. Nothing on my hangar floor, no indication of poor braking on the previous flight. My best guess is that it was pumping a little out each time I braked hard on landing, but when taxiing I wasn't pushing as hard and anything that leaked out while landing had time to distribute.
There was evidence of dripping on the wheel when I took the wheel pant off to fix the problem.
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I'm surprised. Usually that fail mode renders a puddle rather quickly.
 
Snowflake said:
Brake pad wear will empty the reservoir 100% if the wear is enough to let the o-ring unport from the piston... Um... Apparently...
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And it will very slowly empty it if the o-rings are aging out. My reservoir was going down very, very slowly over several *years*, and I attributed it to just "normal" loss, with zero evidence of a leak anywhere. Finally, replacing pads again this year, I saw a bit of fluid around the puck on one side...sigh. O-rings needed replacement. Should have done them a few years ago, I guess, and not *assumed* the very slow drop was normal.

Have I mentioned what a PITA bleeding the system is? I'm going to make that cool bleed tank from the other thread :).

In the OP's case, if he's finding it empty after a year, he's got a leak somewhere, for sure. O-rings might be a good place to look.
 
Mel said:
This is true, BUT, this will show up with evidence directly under the wheel.
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Not necessarily! Not an airplane though. I drove Champ Car for a few years. One race, flying wide open down the back stretch to an off camber, blind, downhill left turn, I did my normal sequence at the marks. Clutch in, heel toe shift to 2nd. Pedal hit the floor. Thankfully the car still had an E-brake. After the cloud of dirt cleared, I limped it back to the pits. The pads were gone and part of the caliper was gone. Just enough to blow the seal and all the fluid. My buddy thought he would try a cheaper set of pads.

But, yes. Normally there would be evidence. 🤣
 
Radioflyer said:
During my condition inspections, I find my brake fluid reservoir almost empty. No tell tale leak indications are found at the calipers or master cylinders and the brakes work well. Since the reservoir is on the engine side of the firewall, might it be possible that the fluid slowly evaporates over a year due to heat? Do others also experience diminishing fluid levels after a year from their engine side reservoirs?
Click to expand...
I’ve had to chase down a few leaks with a similar symptom. It always turned out to be one or more leaks.
I’m assuming this is on a RV6A with a reservoir on the firewall.
Most folks don’t check every fitting in the system. Starting at the reservoir there are fittings there and then down to the masters. This is the low pressure side but surprisingly I find leaks there. If there are dual brakes, you have those hoses and fittings to check.
Then the masters to the calipers and any fittings in between check the top of the masters for fluid too.
No two RVs are alike so you have to trace them all down. There may be fittings you are overlooking.
Check inside the fuselage on the floors, under the seats, etc for telltale fluid. Also the outside bottom of the fuse can show a drop of fluid.
I bet there is fluid somewhere and you just aren’t seeing it.
You can also refill the reservoir and have a helper push the brakes while you observe.
Also check movement of the pedals causing leaks in the unpressurised side.
Keep at it and you’ll figure it out.
Good luck.
 
Or write it off to evaporation and top-off the reservoir more frequently (where’s that cool ducking the flying tomato emoji we used to have on BT?)
 
FWIW I just found (and am now dealing with) a leak in a solid line that developed over many, many years due to chafing. It's the line that crosses over to the right caliper on the firewall. Aluminum line was rubbing on a clamp and developed a hole.
 
Found a fitting on the cockpit side firewall today with red sticky goo for the left brake. There are no major leaks but i see brake bleeding events in the past maint. history. I don't mean to hijack the thread but do I just clean and tighten the fitting or does this need a major surgery?
 
Needs whatever it takes.

If the fitting is a pressure line to caliper and you open the line, you'll need to bleed that side after reassembly.
 
If there is a leak, it is ultra small, slow, and intermittent. Maybe it leaks only in the coldest of winter flights.
I did find that the plastic "T" fitting (cabin firewall side) going into the reservoir seemed not as tight as I would expect. May be the culprit and firming it up will hopefully resolve the riddle. Strangely, no wetness, staining, stickiness, or grime trail in that area or down any of the lines.
 
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