2003 Mooney M20R Brake Bleeding Problems

[-M-]

Hi Everyone.

I purchased a 2003 Mooney M20R last April and during the pre-buy, we noticed mushy left brake pedal on the pilot side.  The issue was fixed by bleeding the brakes bottom-up and then at every fitting under the belly where small amounts of air bubbled out.  Since then the problem slowly got worse and this annual we decided to figure out the root-cause.  Here's a list of what we have performed:

1. Identified that Swagelok B-400-7-2 fitting that connects the plastic tubing coming from the reservoir to the T that feeds to the low side of the Pilot master cylinders.  Replaced the fitting and the leak stopped.  Flushed caliper up and problem persisted.  This probably was a problem but not the root-cause.

2. Removed all 4 master cylinders (pilot and co-pilot) and re-sealed them.  Although some seals were worn out the inspection before and after revealed no external or internal leaks.  We re-sealed anyway.  Flushed the system caliper up both sides but the problem persisted.

3. Checked the calipers, lines, fittings and flushed the system reservoir down, caliper up, applied vacuum at the reservoir many-many times.  Problem remained.

4. We noticed that the maintenance manual has different bleeding instructions for a/c with pedals on both sides.  We blocked the reservoir, pushed fluid up the calipers with a high-flow fitting and cracked open the fittings at the parking brake manifold to see minimal amount of bubbles.

I heard from a hangar buddy that when he replaced his a/c's tire, the two parts of the rim didn't fit perfectly because the valve stem could only fit in 1 of 3 positions.  This caused the caliper to be pushed back enough during taxi that the first pump pushed the pads close to the disk and the second press held perfectly, unless he was taxiing faster.

There are no visible leaks at all and we're kind of lost here.  Any help is greatly appreciated.

Thanks

 

Edited May 7 by -M-

[Fritz1]

The masters are mounted sideways, to get all the air out you have to take them loose and turn the fittings up, turning the calipers upside down and replacing the bleed screws with high flow fittings also helps. I put a pressure pot on the reservoir and both calipers and pumped the fluid back and forth. When a hose blew off one of the pressure pots and I had 10ft red fountain in the hangar I knew I was getting close.

[A64Pilot]

I do NOT have passenger brakes and I do think that complicates things.

But I have found that some aircraft, Mooney’s being one of them do much better if a bleeder ball is used to bleed the brakes, a bleeder ball can push a significant amount of fluid through the system thereby forcing air out. It can push a continuous high volume of fluid through the system, where for example an oil can pump cannot. Whenever I bleed, I flush the system, replacing the old fluid with new and cleaning everything out, a way is to replace the tank plug with a nipple temporarily, put a hose on it and flush into an empty water bottle or similar, I actually just stick a slightly larger piece of clear vinyl hose into the plug hole. 

Finally I’ll throw in my push for 83283 hydraulic fluid as opposed to 5606. Military, Army anyway changed from 5606 I’d guess 40 years or so ago, 83283 is much more fire resistant, but more importantly in my opinion is it never turns into jello or goo, which 5606 does when it gets old, if your flushing fluid anyway, why not switch to the good stuff?

Note 83283 gets thicker in real cold temps, but we are talking like -65F cold, not normal cold, so it’s not normally a problem, but if it is there is a syn fluid now that works even in stupid cold, but I have no personal experience with it.

https://www.groveaircraft.com/fluid.html

 

[-M-]

On 5/7/2024 at 3:10 PM, Fritz1 said:

The masters are mounted sideways, to get all the air out you have to take them loose and turn the fittings up, turning the calipers upside down and replacing the bleed screws with high flow fittings also helps. I put a pressure pot on the reservoir and both calipers and pumped the fluid back and forth. When a hose blew off one of the pressure pots and I had 10ft red fountain in the hangar I knew I was getting close.

Thank you so much for the quick response.  We were able to get a long stream of air when we lifted the front and pulled the landing gear while pushing fluid from the caliper.  The pilot left pedal is now only going down half way  first time we push every 5 minutes.  There has to be more air somewhere else.  Such a pain.    

[I have this friend..]

Hi,

1979 M20J spongy left brake.

I have dual brake sets. 

The last time this happened I bled about 1/2 gallon top down and solved the problem for about 2 yrs.

My method was to have someone pump the pedal down until pressure, then open and close the bleed screw on caliper (acting like a one way valve) then repeat.

It was painful process, but resulted in very solid brakes.

I am considering bleeding bottom up this time (e.g. pressure pot)

For a collection container connection, does anyone know the thread and diameter size of the plug in the brake fluid resevoir over the battery?

The Mooney part number is 850009-000 plug, filler

I would just measure it, except it is 4 hrs of driving to get to the plug.

Trying to make something with a barbed fitting so i can connect to a tube obviously.

Any help appreciated.

[I have this friend..]

I am hoping the plug for the brake fluid resevoir (located above the battery) is a 1/16th in NPT thread. Will find out after lots of driving

[-M-]

On 5/14/2024 at 2:03 PM, I have this friend.. said:

Hi,

1979 M20J spongy left brake.

I have dual brake sets. 

The last time this happened I bled about 1/2 gallon top down and solved the problem for about 2 yrs.

My method was to have someone pump the pedal down until pressure, then open and close the bleed screw on caliper (acting like a one way valve) then repeat.

It was painful process, but resulted in very solid brakes.

I am considering bleeding bottom up this time (e.g. pressure pot)

For a collection container connection, does anyone know the thread and diameter size of the plug in the brake fluid resevoir over the battery?

The Mooney part number is 850009-000 plug, filler

I would just measure it, except it is 4 hrs of driving to get to the plug.

Trying to make something with a barbed fitting so i can connect to a tube obviously.

Any help appreciated.

 

I have 2 batteries and the reservoir is on the rear firewall.  I removed the plug, took it to Lowes to only find a 1/4" to 1/8" sharkbite.  Then I found a brass 1/4" to 1/8" barbed fitting which seemed to be a better choice.  I plan to go to the hangar tomorrow morning and will post the fitting label.

[-M-]

23 hours ago, -M- said:

 

I have 2 batteries and the reservoir is on the rear firewall.  I removed the plug, took it to Lowes to only find a 1/4" to 1/8" sharkbite.  Then I found a brass 1/4" to 1/8" barbed fitting which seemed to be a better choice.  I plan to go to the hangar tomorrow morning and will post the fitting label.

 

[RoundTwo]

The following information is for my J with pilot side only brakes so I’m not sure if this will work for other models.

This past annual I went as far as replacing piston o-rings, so the brake system was completely open requiring a total bleeding. I was working by myself, so the “pump and squirt” method wasn’t going to work, and I didn’t have access to a pressure pot. The secret to this process is having both calipers OFF of the wheels allowing much higher volumes of fluid moving through the system.

SUPPLIES NEEDED:

1/8” hose barb as shown above with 6 or 8 feet of clear vinyl tubing

2 spring clamps large enough to squeeze caliper piston into caliper

Clear vinyl bleed tubing

2 fluid catch containers

Ladder or other suitable support, the taller the better.

1 quart of brake fluid

 

1. Place a large spring clamp onto each caliper to prevent piston movement and attach clear vinyl tubing to each nipple.

2. Remove vent plug from the brake fluid reservoir, fill it TOTALLY full and then install the nipple.

3. Place the container of fluid as high on the ladder as you can, start a siphon to draw fluid into the tube and then connect the tube to the nipple on the reservoir. This will give you a little bit of positive pressure, but not much. Most importantly, you’ll have plenty of fluid to ensure no air enters the system.

4. Remove BOTH spring clamps.

5. Hop into the plane and pump each brake pedal ONE FULL STROKE.

6. Hop out of the plane and check how much movement there was at each caliper. (Once you’re comfortable with how much each piston moves, you can SLOWLY increase the amount you pump each pedal.)

7. Pick up the first caliper and point the bleed fitting UP, crack open the bleed fitting, replace the spring clamp and quickly force the fluid from the caliper. Lock down the bleed fitting.

8. Move to the other caliper and repeat. NOTE: You’ll probably see that caliper #2 is now a bit farther extended than caliper #1, because it has been unclamped longer and gravity has forced it out a bit further. Once you’re comfortable with how quickly you can bleed #1 and get to #2, you can give #1 a bit more of a pump to help speed things up.

9. Repeat steps 5 through 8 until you no longer see any bubbles when you compress the pistons into the calipers. I was doing a total flush, so I continued until all old fluid was gone.

10. When your flushing is complete, replace the calipers onto the wheels, disconnect the fluid siphon and set the appropriate reservoir fluid level and give it a test.

As I stated above, this worked very well on my J but may not be applicable to other models.