brake maintenance Brake Bleeding Woes

[Louis]

I changed the seals in the master cylinders of my M20F, put fancy Teflon brake hoses on my brakes and changed the brake pads.  After many tries of pressure bleeding from the brake caliper up and from the master cylinders down I still had mushy brakes.  I know that there are many more Mooney pilots that have experienced the same problems bleeding their brakes.  I consulted some mechanic friends and after some fought after success the brakes work as they should.  This is what I found out.  I hope it helps you avoid the immense frustration of bleeding Mooney brakes. Note: My aircraft does not have brakes on the copilot side.

The brake line comes from the reservoir to a T above the master cylinders then to each respective master cylinder and then downward toward the brake calipers.

If each side is bled individually, an air bubble can shuttle at the T and not bleed out.

Air can also hide in the master cylinders.

After consulting the maintenance manual and some Mooney mechanics who agreed that it was at times extremely frustrating to bleed Mooney brakes, more that one offered this advice.

"If you pressure bleed from both the calipers up to the reservoir at the same time and jiggle the tops of the brake pedals during the process you have a very good chance of eliminating all of the air in system."

You can purchase 1/4 inch clear plastic tubing from many hardware stores along with a T fitting so that you can bleed both brakes at the same time with one pressure vessel.

If you have any other sure fire techniques for bleeding the brakes I am all ears but this one works great.

PS Remember to run a clear plastic line from the top vent on the reservoir to an empty brake fluid can so you can keep an eye on the bubbles and avoid getting overfilled fluid on the cockpit floorboard.

I will attach photos of the process later. 

 

Edited July 19, 2020 by Louis
elaborate on the brakes

[MooneyMitch]

Very interesting...... thanks for sharing.

 

[RobertGary1]

That last bubble is the hardest to get out. Try pulling jnto a 30 degree pitch up and pump the brakes. Worked for me. 
 

-Robert

[rangermb]

When we did mine a few years ago, had the same issue. We suspected that the cylinders under the pedals were hiding some air. We ended up raising the nose a bit while pressure bleeding from the bottom and the are now quite firm.

[Ragsf15e]

My mechanic pulled the tail down as well.  That seems to be a common trick.

[RobertGary1]

19 hours ago, Ragsf15e said:

My mechanic pulled the tail down as well.  That seems to be a common trick.

Easier to do in flight. 

-Robert 

[Andy95W]

20 hours ago, Ragsf15e said:

My mechanic pulled the tail down as well.  That seems to be a common trick.

 

51 minutes ago, RobertGary1 said:

Easier to do in flight. 

-Robert 

Agreed, but this method is much more difficult for the mechanic with the pressure pot.

[RobertGary1]

57 minutes ago, Andy95W said:

 

Agreed, but this method is much more difficult for the mechanic with the pressure pot.

I just did it by pumping the brakes at a 30 degree pitch. 
 

-Robert 

[xcrmckenna]

If I get this pressure system, I could put a tee on the pressure side, hook them up to both the bleeder screws press the brake peddles and make sure I don’t over flow the reservoir and pull the tail down and it should bleed the brake system?




Asking because I need to replace the seals in one of my calipers.


Sent from my iPhone using Tapatalk

[RLCarter]

This is my standard go to for bleeding, just a standard oil squirt can. One of the issues when bleeding a system is introducing more air as your trying to bleed it. If the system has been opened... master/slave cylinder rebuild, hose replacement, etc the system will be full of air. Pressure pots work well for forcing fluid up through the system but if the pot has too much pressure the fluid will move too fast allowing air to still be trapped. Any air that gets introduced during the “bottom up” method now has to travel throughout the whole system before exiting into the reservoir, the common error is having the bleeder valve open too much which will cause a venturi effect and suck air into the system. 

[RobertGary1]

You should suction bleed first to pull the goopy old fluid down first. Otherwise you’ll push it jnto the master cylinder valves. Then once the old goop is out pressure bleed it. 
 

-Robert 

[Louis]

The brake was too mushy in my opinion but the right brake worked OK for the last year. (every now and then needing an extra pump to get it to grab) Not optimum for sure.  This annual I was determined to get good braking and took apart the right brake cylinder.  I found that on the back of the valve (the thing that looks like a disk with a stem) has an O ring on the back of it.  If the O ring is not in good shape you may never get a firm peddle.  I called LASAR and otherwise could not locate a brake kit that had a new valve or an O ring that fit that valve.  It was explained to me that the O ring on the valve was part of the manufacture process and that you could not change it.  If anyone has a handle on an aftermarket source for the valve that would be awesome to pass on. Also if anyone has figured out a way to repair the valve I am all ears.  In the meantime I bent the spring steel washer spring so that it applied more pressure and the brake works for now.  But thinking ahead, the rubber on the backside of the valve will eventually fail completely and the valve will have to be replaced or the rubber O ring/seal will have to be repaired.  I have not heard of anyone who has repaired one yet. Any expertise would be appreciated.

[MB65E]

So I just went through this. A few times…

Plane parts sells the kit for the paramount V1-625 brakes. They supply the oring for the popit valve. However, the oring is pressed into the brass housing. That spring washer you mention cracks internally too. Finally, there are different thickness of retaining washers. I caution The Beech 33 drawings are not correct if you find them on the Internet. 

Bob at plane parts sells oring kits.

Bill Goetschel will rebuild your master cylinders for you. 
I was at my whits end with one of mine. I bought a refurbished one from Bill. There are a few tricks that are important. This is the only master cylinder I have ever worked on that is this tricky. Bill developed a way to rebuild the popit valve. He bought every paramount brake part that wentworth aircraft had. He has a good supply of parts and is well versed. He’s been rebuilding the cylinders for 4 years. 
I recently worked with bill and explained some things that could be improved upon. Please contact him. Awesome guy.

tell him I sent you his way. 

Bill can be reached at:

carolannegoetschel@yahoo.com

+1 (507) 581-9761

 

-Matt

IA, ATP

[Will.iam]

On 7/19/2020 at 8:53 PM, RobertGary1 said:

I just did it by pumping the brakes at a 30 degree pitch. 
 

-Robert 

Is that done with the gear up or down?

[Yetti]

I suck it down with a mity vac.    Then oil can 15-30 rapid pumps will force the bubble up.   Use a Turkey injector to get the extra fluid out of the reservoir.

[EricJ]

15 hours ago, Yetti said:

I suck it down with a mity vac.    Then oil can 15-30 rapid pumps will force the bubble up.   Use a Turkey injector to get the extra fluid out of the reservoir.

In the past I've had success with just the mighty vac, but the last time I did what you just described and we had to pump some up from the bottom after I couldn't quite get all of it with the mighty-vac.   I was watching the reservoir for overflow and there was a big bubble came out when we were pumping from the bottom.

I haven't had to use the tilt method yet, but my IA mentioned that he used that once and it worked and another time it didn't.