pilot side brakes

[romair]

For the past few days I have been dealing with very soft brakes on the pilot side. There is significant travel of the peddal travel before any kind of braking occurs. Yesterday, I was doing the run-up, and the brakes would not hold when bringing the RPM to 1800. Co-pilot brakes are completely normal. Parking brakes work fine. No fluid that I can see on the floor. Any thoughts?

Thanks

Stefan

1989 M20J

[Seth]

Stefan-

 

Just so you are aware, most Mooney aircraft do not have co-pilot brakes.  It's was an option and often added after the aircraft left the factory by a local mechanic.

 

However, seeing that you have a 1989 bird, I think that's when the AT (though I could be wrong) was being produced, which is the advanced trainer (and M20J with dual brakes and a few other options, but not too fancy as it was meant for a flight school), and they may have been factory co-pilot brakes.

 

I am not familiar enough with the braking system, but the fact that you have co-pilot brakes raised a red flag in my mind that there may be an issue with the install or a fail point somehwere in the interlink setup between pilot and copilot brakes.

 

I'm sure others on the board are more knowledgable and will have a good response for you shortly.  I for one have wanted to put co-pilot brakes in but don't want to add to the useful load, add complexity for another fail point, and give the passneger a way to get in the way of flying (even though it would be nice to have the ability for the copilot to apply the brakes). 

 

Both my former 1967 F model and current 1983 M20J Missile 300 did not have and do not have co-pilot side brakes.

 

-Seth

[Cris]

Typically the brake fluid over time gets gummy and it affects the holding power of the brakes particulalry the pilot side as it is most used. Bleed the brakes and it should be fine. There is a thread or threads on the site that discuss the how to.

[johnggreen]

Stefan,

 

A while back, I had a problem with soft brakes on both sides.  I posted under the Bravo forum and you can read the info posted there.  As it turns out,, my problem was that the brake fluid had coagulated to a thick brown syrup.  It took my shop two tries to get the system completely flushed.  The first time they tried the "drain and refill" and it did NOTHING.  To their credit, they bit the bullet and immediately flushed the system which was a much bigger task.

 

As a response to my post, several other members reported the same problem and the same solution.  In a Mooney, particularly with co-pilot brakes, flushing the system and fresh fluid about every three years seems to be a smart practice.

 

Jgreen

[romair]

John

So that I understand this and not get double time billed by the mechanic...what is the difference between draining and refilling the system versus flushing the system...

thanks

stefan

[johnggreen]

Romair,

 

You can apparently "drain" the fluid until nothing comes out and still leave the coagulated fluid.  My mechanic told me how he flushed the coagulated fluid out, but I wasn't paying enough attention or didn't ask enough questions.  I just recall that it took him about 4 hours to get all the coagulated fluid out of the system.

 

Tomorrow, I'll try to remember to call him and ask how he got it done and post tomorrow night.

 

Frankly, I'm kind of surprised someone else who has experienced the problem also hasn't posted.

 

Jgreen

[Bolter]

On my J model, I believe the co-pilot brakes are factory.  If yours are plumbed the same, watch for the co-pilot side to get soft, next.  They are installed such that if there is a fluid leak somewhere, taking down the reservoir level, the pilot side brakes will suck air first.  When the level drops enough, the co-pilot side will get soft.  This will tell you there is a leak, which may not always be obvious with puddles or belly stains (mine was not).  Note that both left and right eventually went soft in my case, indicating a leak on the source side, not on the pressure side.  

 

I made repairs, starting with bleeding and flushing.  Each time, it seemed fixed, but came back in a few weeks or months.  I found leaky lines that we did not find the first time, and replaced those.  The last repair was replacing o-rings in the co-pilot brakes.  That has held for over a year (maybe 150 tach hours) without a hint of trouble.  The point being that your pilot side problems may be from co-pilot side issues.  Replace all the o-rings in the master cylinders if you are already getting to one side.  It is not a lot more effort, and takes care of a non-intuitive problem that you might not think of otherwise.

 

-dan.  

[fantom]

My right side brakes are original, from the factory. You need a good pressure flush every two or three years to avoid the softness you're experiencing. May be an inherent design flaw. Not a big deal and should take no more than two hours, assuming the mechanic knows what he's doing.

[johnggreen]

Romair,

 

As promised, I asked my shop how they flushed the coagulated fluid out of the system.  They said that they went to the brake valve which in my Bravo they accessed by removing the belly pan.  At that point there is apparently a valve, parking brake???.  They removed the lines there and at the brakes themselves and injected mineral spirits through the lines using air pressure.

 

Jgreen

[jetdriven]

The line from the reservoir goes to the parking brake valve, which is located right under the pilot side pedals, before the master cylinders.  It is nothing more than a shutoff valve, trapping high pressure fluid from the master cylinders from releasing the brakes.

 

From the parking brake valve the line splits, and goes to the two master cylinders. The cylinders sit horizontal under the floor. We have a 1977 M20J with the Paramount master cylinders, but others may be similar.  The principal problem with these I can see, is the ports are on the side of the bore instead of the top. You get air in those things and you need to remove them and bench bleed them, or loosen the fittings, rotate them 90 degrees, and hand stroke them to purge the air. Jacking the plane and sitting the tail on the ground is an alternate method suggested by Dan at LASAR.

 

Another thing is there is a high point in the brake line in the wheel well. It traps air in it, and from our experience, you cannot flush it out with a pressure pot.  Breaking this union fitting open in the wheel well under pressure will allow the air to escape.

 

We let the fluid all run out under gravity last year at annual inspection time, and refilled it. The brakes have been totally crappy and spongy ever since. This year we put new o-rings in the master cylinders, bench bled them, chased the air out of the high point in the gear well, and installed them under pressure with fluid running from the caliper (and running down my arm, in my shirt, and down my back).  I think we have it fixed real good, but stay tuned.

[N201MKTurbo]

If you look through all the manuals there are about 5 different parking brake valve arrangments, I don't think they ever got it right.

 

Use chocks...

[romair]

I ended up doing the simplest thing first, which was bleeding the brakes. Now they are as good as before if not better. Not sure how long it will last, so stay tuned. It is still quite weird why the copilot brakes were completely fine while I had no brakes on the pilot side...

[kortopates]

I ended up doing the simplest thing first, which was bleeding the brakes. Now they are as good as before if not better. Not sure how long it will last, so stay tuned. It is still quite weird why the copilot brakes were completely fine while I had no brakes on the pilot side...

Not weird but an indication that the air in your system was in the Pilot master cylinders or the line between them and the co-pilot master cyclinders. In the dual brake system, the pilot and co-pilot master cylinders are plumbed in serial with the pilot being furthest upstream or further away from the brakes. or to put it another way, the output fluid of the pilot master cylinders in plumbed to the input of the co-pilot cylinders. Like Dan explained above, your issue could have been from the fluid reservoir getting low and allowing air into the pilot master cylinders, yet no air had yet gotten further downstream to interfere with the co-pilot cylinders which then delivers fluid to the brake calipers. (The parking brake shuttle valve is between the pilot and co-pilot  master cylinders.)

[Alan Fox]

I ended up doing the simplest thing first, which was bleeding the brakes. Now they are as good as before if not better. Not sure how long it will last, so stay tuned. It is still quite weird why the copilot brakes were completely fine while I had no brakes on the pilot side...

I am in the buisness of repairing cars , and it always amazes me how experienced techs can overthink things...........9 out of ten times it is something very simple.........Fuse , tires, battery, etc etc .......7 out of those are usually misdiagnosed as something more complicated...... Then the poor customer gets the excuse that it needed that too.......   You always start with the basics...... Good job.....

[romair]

Paul - I was not able to find the schematics for the brake system, but if indeed it is arranged the way you are explaining it, then it surely makes sense. Thanks!

Stefan

[kortopates]

Stefan - see your IPC for illustrations of brake system plumbing - you'll have to put it together from a couple of drawings.

[RJBrown]

I am in the buisness of repairing cars , and it always amazes me how experienced techs can overthink things...........9 out of ten times it is something very simple.........Fuse , tires, battery, etc etc .......7 out of those are usually misdiagnosed as something more complicated...... Then the poor customer gets the excuse that it needed that too.......   You always start with the basics...... Good job.....

So true