Don't listen to these Shimano lovers about upgrading unnecessarily. If you like the Strokers and want to keep them, you just have to give them a little love. I'm still using Stroker Ace's and they do a commendable job.
You probably don't need piston seals yet as there are a few things you can try to improve your bleed method.
1) remove the bleed nipple from the caliper, clean up the tread and put one thin layer of plummets tape around the thread. It's fiddly but try and make it clean and tidy then reinsert.
I've found this was where small amounts of air was entering the system when I bled my brakes. Also when bleeding, only turn the nipple out 1/8-1/4 a turn, that's all it needs.
2) when pushing brake fluid into the caliper, I unbolted the caliper from the frame so I could twist it around a bit. I found that air bubble got caught in the caliper and wouldn't come out unless I turned it upside down and on the sides whilst squeezing fluid into it.
3) after you have all the air out of the caliper, you need to turn the lever upward on the bars so as to get any residual air out of the lever. You can also flick the lever back to the bars and release and bubbles usually come out then as well.
If you do steps 2 and 3 thoroughly and don't be scared about going through a bit of fluid, you should get them right. I haven't had to bleed my Stroker Aces in several years and I'm not having any issues with them. I had Trails before them and they were very good too once they had a good bleed.
Hope this helps you. If not, bring your steed up to Newcastle and I'll have a crack at it for you.