I agree with Joe and these are just open vents with a cover. No check valves or seals other than the sealing ring at the threads. If it foams up into the vent, it is something other than the vent unless it is itself clogged. The replacement shown is very much like the vents on both my rear axle and my auto transmission on my '68 280SE. Your original is just like the vent on the manual trans case on my '60 220SE W128. Both I have taken apart.
I really do not see why the old vent can not be reused. I did just that on my 220SE and just re-plated the parts. Good as new. The replacement vent is just a cheaper version of the old one. Less parts but does the job. On the replacement type vent, you can easily pry the top off and inspect to see if clogged. Also with the replacement type, check that you can blow air into it with the cap held down. If not, it could be a bad vent. I had a new one like that and sent it back to the Classic Center a few years ago. It should have a small groove that allows air to pass even if the cap is held down.
