Long time, no post. I wish I could say the delay was due to my making lots of progress and not having time. The truth is much closer to not having much to share.
I was able to get out and test some modifications this weekend. The good news is that the urethane o-rings seem to be working. I did a long term pressure test with N2O to see if they would remain sealed for several days. Also, after the testing this weekend they showed no signs of damage.
The main point of this weekend's testing was to test a new configuration with the propane/spark ignition with the goal of eliminating the initial pressure spikes. The new configuration seems to have reduced the tendency towards hard-starts. Unfortunately, the ignition sequence which was quite reliable before is no longer. I spent quite a bit of time just getting it to light at all and stopped testing without arriving at a reliably repeatable sequence.