Water coming up through sink and shower

[SBL]

My bath runs through a macerator to get the water to the stack but the sink and shower are attachd to the same pipe further down stream.
Tested it for the first time yesterday and I get spits coming out of the sink and shower waste when the macerator cuts in. I have to assume that the presure increase in the pipework when it starts pushes the air to do this.
Question is,,,is the only option to seperate the shower and sink from the bath pipework or is there some kind of pressure relief type of thing that could be fitted to prevent this?

Just when I thought it was finished

 

[Bahco]

I would separate alll the wastes and have them running into something like THIS

Get rid of the macerator too!

 

[SBL]

Unfortunately I can't get rid of the macerator due to the distance away from the stack.
I'll take a look at the system later to see if I can fot that into the system, I have the feeling that I may not have the room to do this.
At the moment I'm thinking to seperate and use a boss.