Because the uni-directional vent only vents air as a result of the pressure build up within the bearing housing, it is creating an atmospheric condition that is different from that in and about the constant level oiler. The same concept holds true for the expansion chamber. The diaphragm in the expansion chamber is taking up the expanding air in the bearing housing, however, this air is contained and is creating a different atmospheric condition in the bearing housing from that at the constant level oiler. In these situations, a good easy conversion for the customer is to use either the closed system Opto-Matic or Watchdog oilers. These can be used with the uni-directional vents and/or expansion chambers because they are vented to the bearing housing.
Because the bearing housing is closed off to atmospheric pressure, this pressure,in the form of expanded air, needs some place to go. The seals of the bearing housing are made to withstand small amounts of pressure. By using an expansion chamber, you are providing a place for this expanded air to go, causing the seal not to breathe. If the seal were allowed to breathe contaminants could get into the system.