TechWriter one the ones I have opened after finding the hidden door just have a little rubber like cap over the fill hole open that 1-2 PSI would let them vent I expect. If they did vent they would not draw air back into the cells I expect.
Any battery that you find needing water and lot of water, even the FLA type is SHOT from my experience. The six T-105's started drinking water two year ago in the GC. Before that they were good for 20+ miles. Now it is like two which is like four trips to the church so they are OK. Just trying to get to next Spring to get new ones. :)
Adding water to a AGM may get you a few more minutes of run time IF you have time to waste or want a project to mess with. :B