On Tuesday, April 7, 2020 at 5:29:59 PM UTC-7, wrote:
The Tost hydraulic brake cylinder mounted to my stick failed on my last flight. I've emailed Tost, but so far no response, which certainly might be virus related and therefore understandable.

The brake worked fine before takeoff but before landing there was zero resistance when squeezing the lever. I'm 99.9% sure its NOT an issue with air in the system and just to check I flushed from bottom to top and no change and no bubbles exposed.

I suspect that if there is a check valve inside the piston of the cylinder that it has failed open. There is a swishing sound when the lever is squeezed as if fluid is being pushed through an orifice (no external leaks found) so this matches my hypothesis.

If you have actual knowledge of the internal design of this hydraulic cylinder, particularly if you've taken one apart, please message me directly.

Thanks,

Mark

This should be obvious, but firstly check your brake pads to be sure they are not worn. Replace if they are.

Second, inspect all fittings and lines for any evidence of a leak. This happened to me on my ASH2e; inspecting the brake line between the reservoir and the brake shoe required pulling the engine.

Third, completely bleed the brake fluid from the brake and measure how much you get. This video may help:
https://www.youtube.com/watch?v=qOtrnsNo8K0

Forth, you will need to put new brake fluid in the lines. Obviously, you will need the exact type of fluid specified by Tost - do not use any other brake fluid, certainly not automotive brake fluid. You will need a way of pressurizing the fluid; Schleicher recommends a stand similar to water ballast filling, but I think what the guy in this video used is better:
https://www.youtube.com/watch?v=bZlq87VXybU
Depending upon where your reservoir is located, you may need an assistant to tell you when to stop filling.

There is no certain way to know that there is no air bubbles in your brake line, so you must go thru this process.

Good luck!

Tom