Papershruiken

I replaced my Longshot's plunger head with a hereticorp/CS style plunger head. The plunger head and plunger tube were both lubed with white lithium grease from Liquid Wrench. When I first fired it, ranges were 90-100 feet. But within five shots, the range had dropped ten or fifteen feet. When I opened it up, I found that the rubber washer seal was TOO good: It had pushed all of the lube down to the end of the plunger chamber. The lube had also bunched up around the rubber washer. I don't want to revert to an O-ring seal, but I'd prefer not to open the Longshot every five shots and cover the shaft in lube. Has anyone else ever encountered this problem?

I didn't think pictures were necessary by the way I described it, but if anyone needs to see them let me know.

P.S.- If it really matters, I'm running an AR-15/LS spring combo.
Thanks in advance.

Okay. So I'm looking for an air valve for a potential automatic homemade. Obviously I've googled it and I can't find what I'm looking for, so this is the next best thing. Anyways...
I need a valve that releases air when a desired pressure is reached. It's similar to a pressure release valve, but those wouldn't direct all of the airflow into a barrel or something similar. Any ideas?