That little bulb, a.k.a. sight glass, looks at what is going out the waste or discharge port. At least on my Hayward multi-port valve. It's used to determine when the water is clear in backwash mode. When you're on FILTER then the sight glass should be a non-event and out of the flow circuit. Quite likely it will drain out. On the pressure gauge, if they freeze they will become innacurate. Cheap replacement, probably for $10 or so...not sure as I never had to buy one.
Al