An air bubble shouldn't keep the switch from working; even with the bubble, the pressure is the same throughout the system.
But many of us have had similar problems with the TR3 style switch. The "dirt" on the contacts appears to be burned brake fluid. Silicone has a reputation for "creeping", and it seems like somehow it manages to creep into the cavity where the contacts are.
Not everyone has the problem, so it probably varies from switch to switch; but I've had several failures of both original switches and replacements from several different sources (including one sold by FLAPS for an old Ford pickup truck). I eventually got frustrated and converted to a mechanically operated switch instead. Since I didn't like having it exposed under the hood on my previous TR3A, I put it under the dash on the TR3.
The switch shown did malfunction after a few years, mostly I think because I had a MC leak and some of the DOT 5 ran down the pedal and found it's way into the switch. But I replaced that with a TR6 type switch (which also fits Stags) from TRF, and it has been working fine ever since.
I've also heard rumors that there is a Harley Davidson motorcycle switch that will fit in place of the original TR3 switch, and stand up to DOT 5. But I've never investigated any further than that.