When I first started transition training on the plane in August 2000, we suffered a brake failure on the ramp at Marco Island. This was caused by a caliper piston o-ring failure. The failed o-ring was replaced.
Since both o-rings had originally been installed in the braking system in 1995, it was likely that the other o-ring would also need replacement.
The day before a scheduled flight to Taos, I found that the other o-ring was indeed starting to fail. I found that the brake dust around the caliper was wet, and removal of the caliper showed a leaking o-ring.
I removed the piston, and replaced the o-ring. I replaced the piston (note this for future reference) and then re-assembled the brakes to the wheel and bled the brake. I tried the pedal and all seemed well.
The next morning, Marsha and I loaded up the plane and prepared to depart. While sitting in the plane, I pressed both brake pedals several times to make sure that all was well. On the third press, the brake pedal on the right sank to the floor. That was the brake that was repaired...yesterday. Fluid was all over the ramp under the wheelpant.
How had this happened? Quite clearly something was not right, and since I was the person who had performed the repair, it seemed that I had done something wrong.
I found a mechanic on the other side of the field, and he agreed to come and look at the brake. We removed the caliper from the wheel, and pulled the assembly apart.
The mechanic smiled. It was a "d'oh!" moment (aka Graham You Idiot).
The o-ring rides in a groove in the piston. This groove is machined near the bottom of the piston, so that the piston can project a long way from the caliper base surface. However...I had replaced the caliper piston in the caliper bore upside-down. Now the o-ring was only 1-2 mm below the "top" of the piston. As soon as I pressed the pedal enough to move the piston that 1-2 mm in its bore, the o-ring was exposed and the piston partially popped out of the bore. When we took the caliper apart, the piston was partially out of the bore at an angle.
Well, no real harm. We cleaned up the fluid leakage, replaced the piston the right way up, bled the brake, and we were on our way 2 hours later.
After this, the checklist was amended to include a brake pedal function test before moving the plane under its own power.
Make sure that you check your brakes carefully as part of preventitive maintenance and as part of the pre-flight. On these planes, the brakes not only stop the plane, they are the steering system. Brake failure on landing or rollout will leave you with limited or no steering, and that could easily ruin your day.