Slow running waste, no blockage, pressure or pipe setup?

[cuttingman]

Slow running waste pipe, but I am 100% sure pipe is clear. Seems to be a pressure issue or angle of pipes maybe not sufficient. In image sink and shower both drain slowly. If sink and shower used at the same time the non return valve causes water to sit in shower. If non return is removed using the sink the water will push up into the shower. Taking off the blanking cap caused an air pop and water seemed to flow better. Would an air admittance valve instead of the blanking cap be the solution? But worried water goes up this pipe and will come out of air admittance valve, I could extend the pipe to be same height as sink bend.

Maybe non return valve should be between sink and blanking cap? Alternatively (but a lot of work) the sink to have a new pipe running along the top of the current pipe and joined about halfway to the stack.

 

[Madrab]

Your length of pipe run looks to be at the max, what size of pipe is fitted? Should really be 50mm in that setup.

The shower also doesn't have a lot of 'head' when it's draining, given it's lower profile and with the NRV that's fitted, it will struggle to overcome the resistance so you also have to maximise the fall that can be achieved on the pipe run or give it it's own waste pipe to the stack.

Venting the run should help and yes you will need to raise the pipe up to the overspill level of the highest item, that being your basin.