Good afternoon
First I do not believe in anything mechanical fixing itself !!
My conundrum started on my return from my winter home in Mexico at the end of April
I departed for the long return flight to my home in western Massachusetts with a perfectly effective right brake. I made 3 stops prior to the problem appearing on a dicey (severe down pours) into Pine Bluff Arkansas. The right brake was a bit soft but seemed to work. As I landed and applied the brake it seemed to get softer. The next morning I returned to the plane to find that the pedal went to the floor. No sign of leaking fluid anywhere near the brake itself. or near the master cylinder as part of the pedal. A clear no go.
I was able to find a mechanic that had a gadget that allowed him to fill the master cylinder from the brake and bleed it at the same time. The pedal was rock hard. I tried pumping it and it stayed hard as a rock but I was clearly concerned as mechanical issues do not fixt themselves. My only thought is that there may have been an issue with the "O" rings. I planned my next fuel stop at an airport with a very long runway, just in case. That stop was uneventful. Pedal still rock hard. Last leg back to home airport was uneventful as well. I had the mechanic at my home airport trace the brake line completely from brake to pedal and found no issues (I expected them to find a pin hole leak). Since then I take a few flights and brake has held.
I am still concerned.
Mechanical things do NOT fix themselves !!
Any thought or additional trouble shooting ideas ?
TIA
Richard

Richard, when I had a brake issue it was a pinhole that although it took a while to find, the evidence was clear under the floor. Sounds like that is not your issue if you've already looked under the floor and not seen fluid. I've heard there can be an issue with the seals in the parking brake valve assembly than can allow you to leak fluid from one side to the other. I'm not sure if the brake would have to be set for that to happen. But an idea for something to look at, anyway, and consistent with not seeing leaking fluid but still having it go flat.

Hi Richard,

When it was time to change my brake pads, my mechanic recommended changing the
O-Rings. They apparently dry out over time, as you’d expect and will seep fluid. They’re relatively inexpensive and I usually heed whatever my guy tells me. I had noticed a small amount of fluid near the calipers before they were changed. Nothing since. I’d suggest that since there has been a loss of fluid, that you start there. Have’em change your pads too if it’s been awhile - I mean, it’ll all be apart anyway.

Best!
John

Find the parking brake valve and look at it. That is the problem we had with a 421C. It would show up as a gradually weakening brake until it failed. It can mask leaking until you figure out that there is a puddle of fluid under the floor that hasn't leaked out yet. One more reason for a white/light colored belly...

I don't remember where it's at in the 340. In the 421C it's under the pilots feet right under the rudder lock.

Thanks for the idea
Was there any evidence of fluid near the valve or on the bottom of the hull (inside) ?

Yes, there was a little fluid around the valve and below it, but I believe most of it had drained to lower points under the floor. It seems like I found it at the next annual and cleaned it up then. There was some drainage on the belly near the rubber diaphragm covered drain holes. It had a very dark blue belly so it wasn't immediately noticeable.

Hard to tell what else you can do if you’ve had the lines inspected thoroughly. Old brake lines, leaking parking brake and worn masters are a known problem on the twin Cessna line. A couple owners have solved their occasional soft pedal with new master cylinders as they can go soft when tweaked at a certain angle, turning, not depressed in a linear fashion etc. Just info I’ve collected over 30 years flying them.

thanks for the thoughts

I am not super familiar with how the 340 is built but on my Aerostar with the master cylinders in the cockpit where it is pressurized and the reservoir in the unpressurized nose, a slight air leak in the master cylinder (which will not allow the brake fluid to leak) will allow the pressurized cabin to blow the fluid back into the reservoir and thus make that master cylinder to be low on fluid and not work after a flight with pressurization.

Thanks,
I never considered that !!!!

There is no separate fluid reservoir on the 340. Just the two cylinders at the pedals, the lines to the calipers, and the parking brake assembly.

I'd vote parking brake orings...

In addition to what Bill Lawson said, bad o-rings at the brake can cause fluid to weep out when the airplane is pressurized. The Twin Cessna's that I've worked on just had a cap on top of the master cylinder that is often missing. Once you pressurize the cabin, you also pressurize the brake system by default.

That's why I was taught in a pressurized airplane to check the brakes on final and make sure they are not flat.

Thanks
I have already changed the O rings

In flight while pressurized play with the parking brake valve...
See if it goes soft...