What's this thing above my battery in my M20J

[Thedude]

I was replacing my battery for the first time yesterday and there's a reservoir above the battery with some kind of oil/grease in it, but I have no idea where it goes. Does anyone know what it is?

It's the canister mounted to the left on the back of the baggage compartment.

[Aerodon]

that would be the brake fluid reservoir with a long line going to the brake pedals.

[EricJ]

+1 that that's the hydraulic fluid reservoir for the brakes.

Congrats on the new battery!

 

[LANCECASPER]

3 hours ago, Thedude said:

I was replacing my battery for the first time yesterday and there's a reservoir above the battery with some kind of oil/grease in it, but I have no idea where it goes. Does anyone know what it is?

It's the canister mounted to the left on the back of the baggage compartment.

Since you're learning about your Mooney, I thought I'd throw this in there:

Resist the urge to remove your hydraulic reservoir to clean it up or paint it to improve the appearance. One of the most frequent posts on Mooneyspace is when newer owners do any kind of brake work and get air in the lines and fight trying to remedy the soft brakes. I t doesn't mean that your brakes shouldn't be bled by someone that really knows Mooney brakes to get rid of the dark gelled nasty brake fluid that's likely in the system But let someone that has years of experience deal with that.

[1980Mooney]

My J was in a partnership back in the 90’s.  One of the partners, attempting to be helpful, filled the brake fluid reservoir. Unfortunately, he filled it with DOT automotive brake fluid. It turned the fluid in the system to gel and one of the brakes failed when landing.  Luckily they didn’t wreck the plane but overhauling the entire brake system was expensive and the partners were upset. 
 

[Thedude]

Thanks for all the insight here, definitely good to know. Fortunately the breaks are working well, so I'm not messing with this thing ATM.

[N201MKTurbo]

You should check the level and top it off if low. You need to use 5606 hydraulic fluid. Buy a pump oil can and some clear tubing to stick on it. Stick the tubing in it and pump till it is full. 
 

Monitoring its level will give you a good indicator of your brake health.

[Thedude]

How do I tell the level? The reservoir doesn't seem to have any external indicator.

[EricJ]

3 minutes ago, Thedude said:

How do I tell the level? The reservoir doesn't seem to have any external indicator.

You can make a little dipstick out of a piece of paper or cardboard.

[A64Pilot]

35 minutes ago, EricJ said:

You can make a little dipstick out of a piece of paper or cardboard.

If you do, make sure whatever you use is long enough or has a bend or something that it can’t be accidentally dropped into the reservoir.

My advice is as you don’t know how old the fluid is to bleed your brakes and flush all of the old fluid out, when it gets old 5606 turns into a gel, and then you have a real problem that can be avoided. There is a better fluid that doesn’t gel but merely flushing the system every few years prevents the gelling too

Edited Saturday at 03:33 PM by A64Pilot

[PT20J]

48 minutes ago, Thedude said:

How do I tell the level? The reservoir doesn't seem to have any external indicator.

I use a coffee stir stick.

[65MooneyPilot]

19 hours ago, A64Pilot said:

If you do, make sure whatever you use is long enough or has a bend or something that it can’t be accidentally dropped into the reservoir.

My advice is as you don’t know how old the fluid is to bleed your brakes and flush all of the old fluid out, when it gets old 5606 turns into a gel, and then you have a real problem that can be avoided. There is a better fluid that doesn’t gel but merely flushing the system every few years prevents the gelling too

Yes hydraulic fluids that are compatible with 5606 are 83282 or 87257. They are synthetic and fire resistant.  There is a thread in here talking about the new fluids and whether they are approved in our Mooneys by Mooney. 

Hydraulic Fluids 

Most aircraft manufactures require specific fluids for their hydraulic systems. Always refer to your aircraft manufacturer's maintenance instructions for the proper fluid.

All Grove brake systems incorporate Buna-N (Nitrile) O-Ring seals that are compatible with MIL-H-5606 hydraulic fluid which has been the aircraft industry standard for many years. 

MIL-H-5606 hydraulic is a mineral-oil based fluid with excellent operating properties over a temperature range of -65°F to 274°F. A major deficiency of MIL-H-5606 is its high degree of flammability. Because of this, the commercial aircraft industry has developed and uses hydraulic systems based on phosphate-ester based hydraulic fluids. However, these fluids ARE NOT compatible with MIL-H-5606 or its systems. 

The military led the way in developing a more flame resistant hydraulic fluid that is compatible and miscible with MIL-H-5606. As a result, a synthetic-hydrocarbon based fluid MIL-H-83282 (now MIL-PRF-83282) was developed which is superior to MIL-H-5606 (now MIL-PRF-5606) in fire resistant properties, but lacks good qualities in low temperature viscosity.

More recently MIL-PRF 87257 was introduced in order to address the concerns over the low temperature viscosity weakness of MIL-PRF-83282.

Bottom Line: MIL-H-5606 has been replaced with MIL-PRF-83282 and MIL-PRF-87257. Each of these fluids, and their associated systems, are compatible and miscible with each other. You can mix any combination and/or amount of these fluids together without any ill effects.

If, and/or when, you change over from MIL-H-5606 you can either: 
1. drain your hydraulic system and refill with MIL-PRF-83282 or MIL-PRF-87257, or 
2. merely top off the reservoir with MIL-PRF-83282 or MIL-PRF-87257 as needed.
Both methods have been used with great success with no reported problems

[ArtVandelay]

I use a cable tie, the fat end I think will prevent it from falling in. Also you’ll need a small funnel if you don’t want to make a mess.

[N201MKTurbo]

It is hard to imagine how the flammability of 5606 is a concern for us. We don’t have a constantly pressurized hydraulic system with 10s of gallons of fluid in them that runs near and through our engine compartments. 
 

We have maybe 2 cups of unpressurized fluid that is nowhere near any ignition sorce. 
 

Have you ever tried to light 5606? The only way to get it to sustain a flame is to use a wick. A rag soaked in 5606 will burn, but not very agressively.

 

Here is some trivia, the low flammable hydraulic fluids were not developed for aircraft use. The army used 5606 in its tanks to rotate the turrets and raise the guns. If a hydraulic line got hit by a bullet or shrapnel it would make a high pressure mist of 5606, not a good day for the crew. They tested the fluids by shooting containers of hydraulic fluid with incendiary rounds..  

[65MooneyPilot]

Yes excellent point. I was thinking about the 5606 turning into pancake syrup while in the system too long. It happened to me and I did not realize it would do that. It got to the point that the breaks barely worked. I removed the break pistons and cleaned them out. I don’t know if the new synthetic does that but I doubt it. 

[jetdriven]

This is the brake fluid from a Cherokee 180 that we own. I noticed that the brakes appeared to drag and release very slowly, and I thought it was a combination of old brake fluid and brake caliper slide pins that were not lubricated, but this is absolutely astounding. The stuff has the consistency of mayonnaise. Now, if you change this 5606 out for Aeroshell 31 it won’t do that anymore because the synthetic fluid lasts longer and is better at colder temperatures. 

[A64Pilot]

Actually I think the new Synthetic doesn’t work well at Stupid low temps, AF didn’t put it in the B-52 back in the day because of that, but we are talking stupid low temps. Not an issue for most of us surely

https://www.radcoind.com/blog-post/a-brief-history-of-u-s-military-aviation-hydraulic-fluids/

Other than that the new Syn is superior in every respect to 5606, and it’s completely miscible, no need to do a complete flush, but it’s better if you do.

However if you choose to stick with 5606 if you flush the system whenever you put new pads on, you will never have a problem.

If you have a bleeder ball flushing is as simple as hooking it up and let it flow until it’s flushed out enough to make you happy.

Brake fires on GA airplanes are unlikely but can happen, Cirrus had issues with brake fires, remember they steer with brakes, so a long taxi with a stiff cross wind and you could get a brake fire, of course even if it didn’t catch fire the brakes get real hot, they are not serviceable, and brakes aren’t cheap.

https://www.avweb.com/briefs/nprm-addresses-cirrus-brake-fires/

 

Edited Sunday at 04:39 PM by A64Pilot

[1980Mooney]

3 hours ago, A64Pilot said:

Brake fires on GA airplanes are unlikely but can happen, Cirrus had issues with brake fires, remember they steer with brakes, so a long taxi with a stiff cross wind and you could get a brake fire, of course even if it didn’t catch fire the brakes get real hot, they are not serviceable, and brakes aren’t cheap.

FYI - if you have not been paying attention to the latest by Private Equity.  In 2022, Parker Hannifin sold the entire Cleveland aircraft wheel and brake business to Kaman Aviation Corp.  In April of 2024, Kaman Corp was acquired by Arcline Investment Mgnt (which completed the acquisition of Hartzell at the end of 2023.)

• Using the Cleveland 600 X 6 Main Wheel Model No. 40-97A Assembly (Includes brake disc, bearings, wheel rims, etc) as an example (found on most Cessna 172, 177 and 182 and therefore probably the most common in General Aviation)
  • In the June 1, 2023 Aircraft Spruce Cat. - $1,159
  • In the Current Aircraft Spruce Online Cat. - $1,723

THAT IS A 49% PRICE INCREASE FOR NO APPARENT JUSTIFIABLE REASON

[jetdriven]

It will double beyond that. Look at the price of a 10–24D master cylinder. I think it’s about three times what it was a year ago. And it wasn’t cheap a year ago.

[A64Pilot]

On 1/26/2025 at 3:34 PM, 1980Mooney said:

FYI - if you have not been paying attention to the latest by Private Equity.  In 2022, Parker Hannifin sold the entire Cleveland aircraft wheel and brake business to Kaman Aviation Corp.  In April of 2024, Kaman Corp was acquired by Arcline Investment Mgnt (which completed the acquisition of Hartzell at the end of 2023.)

• Using the Cleveland 600 X 6 Main Wheel Model No. 40-97A Assembly (Includes brake disc, bearings, wheel rims, etc) as an example (found on most Cessna 172, 177 and 182 and therefore probably the most common in General Aviation)
  • In the June 1, 2023 Aircraft Spruce Cat. - $1,159
  • In the Current Aircraft Spruce Online Cat. - $1,723

THAT IS A 49% PRICE INCREASE FOR NO APPARENT JUSTIFIABLE REASON

I had no idea, what happened to no trusts?

The reason of course is to jack up the short term profits, I say short term because jack them high enough and somebody does STC’s to get in on the profits.

Beringers look good, But I have no idea of the cost especially as you have to buy wheels too, but I think you get sealed bearings? Which ought to last essentially forever with no maintenance.