Waste water tank

I know that water will back up from the waste tank when it gets full but lately we have been getting a gluggkmg sound when emptying the sink and even if the waste tank is virtually empty water comes up through the shower tray plug holes. I have had all the pipes off and there are no blockages.
I have read in previous posts about a blocked breather pipe. It I can’t find one anywhere on the tank.

Any ideas?

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.

I also had this problem a while ago. Near the top of the tank on the sidewalls are several small shrouded vent slots. I prodded them out with a piece of bent wire and it's been fine since.
Al.