Waste water tank

I doubt the waste water set up on your much older Nuevo will be much different to mine.  I don't think my waste water tank has a breather or needs one.  When waste water enters the tank through one of its inlet pipes, air will escape from one of the other inlet pipes.  My inlet pipes are all at the top of the tank so if waste water bubbles up into the shower tray, the waste water tank must be full.  If water stops flowing from the waste water tank drain, it's easy to assume the tank is empty.  But this is not always the case.  On several occasions my waste water drain has become blocked, usually after a tank cleaning session.  When this happens, the easiest way of clearing it is to connect a mains garden hose to the open drain tap to give it a back flush and see what happens.

We had similar. There should be a vent on the top of the tank but it may be blocked with crud.  I just drilled a 10mm hole at the very top of one of the tank walls to provide an extra vent. It all depends how the installation is plumbed. If the shower waste and sink wastes are tee-ed together and the tank cannot vent then water will come up the shower waste. Our shower waste was separately plumbed, but the tank was still not venting adequately.