Parking Brake not working = no fluid!!

[Boilermonkey]

Awhile back I learned an important lesson.   In most GA aircraft the parking brake is not like a car's.  It is not a separate linkage to engage the brakes, but a system that locks the pressure in the brake lines to keep the brakes engaged.   Something they don't talk about in flight schools: you must have the brakes engaged before you set the parking brake!!!  Otherwise, there's no pressure in the system to lock the brakes...hence students going off the run-up pad because they don't understand systems.   I learned that lesson by studying the systems, but not before another student ran into the grass!!!

Ok onto my lesson...I could not get the parking brake to work on our M20C.   I thought it must be a valve or a linkage from the panel to the valve.   The toe pedals engaged the brakes just fine...so what could be the problem?   Then the toe pedals started to feel mushy, uh oh!   Pop off the cowling to look and the brake reservoir is empty!  The issue turned out there was a bad O-ring in one of the cylinders, so we were leaking fluid.  We had enough to engage the brakes from the toe pedals, but not enough to keep the parking brake engaged.

 

Moral of the story....if the parking brake won't engage...it is probably the first sign of a fluid issue...glad we found the issue and it was easy to fix.

 

 

[carusoam]

Lesson #2...

The brake fluid and the flap fluid share the same reservoir...

One of them stops working before the other.

If you run out of brakes, how well were the flaps being pumped down?

Or do you have the electric flaps...?

Best regards,

-a-

[Bob_Belville]

BM, I'm pretty surprised that you had not discovered how the parking brake works long before you were low on fluid. If you do not have the brakes pressed down hard pulling out the parking brake rod does nothing - even with full fluid. Do you set the parking brake for starting and for run-up? I think that's standard check list line items. FWIW, my John Deere riding mower's parking brake works essentially the same way as my Mooney's.

[carusoam]

Lesson #3 ...

The parking brake is valve.

when the valve is closed, whatever pressure in the line is locked in.  

Stand on the brakes, to put pressure in the line.  Lock it in, with the valve....

If the seals are old, worn, or missing.... it's probably a few dollars in parts to get it to work properly.... OH the valve.

My C's parking brake never worked.   Craig wasn't flying his Mooney yet... and like Mr. Monkey, my flight training didn't include use of the parking brake....

The Power of MS...

Best regards,

-a-

[Guest]

25 minutes ago, carusoam said:

Lesson #2...

The brake fluid and the flap fluid share the same reservoir...

One of them stops working before the other.

If you run out of brakes, how well were the flaps being pumped down?

Or do you have the electric flaps...?

Best regards,

-a-

Not in all cases.  If you have electric flaps, as many Mooneys do, hydraulic fluid level will only effect the brakes.

Clarence

[carusoam]

Doc, we are in complete agreement...

I added a line while you were typing.  You quoted my post with the added line, freshly in it! 

I remember the flaps not going down, but the brakes still working... not sure how solid that memory was...

It is great to be able to borrow your memory on MS.

Hey, that leads to Lesson #4...   what is involved with an OH of the parking brake valve?  Is that just a couple of O-rings in a hard to reach spot?

Best regards,

-a-

[Boilermonkey]

I knew how the system worked at about 10hrs of training when the other student went in the weeds a long time back.  Surprised it wasn't taught in the discovery flight ;-)

We have hydraulic flaps and brakes.  The parking brake was the first indicator, the mushy toe brakes the second.  We put it back in the hangar and found the o ring before the whole system went down.  The flaps worked fine, so there was clearly something left in the system.

 

[Skates97]

2 hours ago, Boilermonkey said:

Awhile back I learned an important lesson.   In most GA aircraft the parking brake is not like a car's.  It is not a separate linkage to engage the brakes, but a system that locks the pressure in the brake lines to keep the brakes engaged.   Something they don't talk about in flight schools: you must have the brakes engaged before you set the parking brake!!!  

I'm there with you. I did't know that you had to push the brakes for the parking brake to work and if it was mentioned during my training I don't remember it. I just looked back at the checklist for the planes I trained in and the first step of the start up and run-up checklist was "Parking Brake Set." Even with the parking brake set I always had my toes on the brakes before starting the plane. And,  after taxiing to the run-up area I already had the toe brakes engaged when I set the parking brake because I had just stopped us,  but I never put it together that having the brakes on was required for the parking brake to work. It has also always been my procedure to stand on the brakes during run-up just as a backup in case the parking brake failed. (In fact, if the engine is running and I'm not moving I have my toes on the brakes, just in case).

After I got my Mooney I had it in the hangar, pulled on the parking brake and got out. I leaned against the wing and it rolled a little so I thought, "That's a bummer, the parking brake doesn't work." It was after that I learned that you have to have the brakes on before pulling on the parking brake, turns out it works just fine.

[carusoam]

Either way...

Trusting the parking brake or a tie down to hold the plane during a run-up can lead to big surprises...

 a new C152 owner at My Airport broke a tiedown, and pivoted around the other tiedown until he hit a hard object with his spinner...

Probably the shortest ownership period on record...

Doing the run-up is a high level, multi-tasking activity.  If you are solely looking at the instruments, there is a huge chance you won't recognize the plane moving.

Best regards,

-a-

[Yetti]

Lesson #5 - Never trust the parking brake for anything.

 

Lesson #6 - OH of the master cylinder is 3 orings is probably one of the easier cheaper tasks on the Mooney

[Guest]

The parking brake in a Mooney is a hydraulic line lock.  On older models the valve was between the reservoir and the master cylinders, on newer models it is between the master cylinders and the wheel cylinders.

On many of the Cessnas many of us learned to fly in the brakes were applied and the handle pulled, locking the master cylinders in place, some of the time.

Clarence

[EricJ]

15 hours ago, M20Doc said:

The parking brake in a Mooney is a hydraulic line lock.  On older models the valve was between the reservoir and the master cylinders, on newer models it is between the master cylinders and the wheel cylinders.

On many of the Cessnas many of us learned to fly in the brakes were applied and the handle pulled, locking the master cylinders in place, some of the time.

Clarence

I was gonna remark about the Cessna parking brake method, that I could never decide whether that's brilliant or cheesy.