I went over to the airport and took the brakes apart again, this time putting them together with some special O ring grease that Jack has - I figure the problem here was lubrication, so extra lubrication should help. I also filled the pistons with fluid before putting them back in, and didn't press them back in until they were connected to the brake lines, so there shouldn't be air in the lower system. Jack brought his pressure bleeder, and we used that to push fluid in from the bottom, so that looked like it was done.
Tom showed up to buy some old 2 stroke Rotax parts I have, and we spent a while chatting - enough that walking backwards and forwards to Ken's hangar (so he could also buy some manifolds that Ken has) meant I got sunburn on my head :o(
I took the gyro out for a flight - I had thought about doing some circuits, but there were already 3 planes in the pattern, so I went and pottered about elsewhere. I came back when my head was getting sore from the helmet, so I was up around an hour. The brakes weren't dragging when I got back, so I'm calling those fixed for now :o)