Brake Booster Vacuum Hose Check Valve (Manual Trans)

[fka]

Hi all

I've been investigating an intermittent rough idle. Normal idle at around 800rpm but dips occasionally and very briefly feels like it's about to stall. Both EVAP and breather check valves appear to be working OK, although the EVAP check valve did seem to 'flap' with light pressure but closed properly with a firm blow 

I have the version of the brake booster hoses linked below. Firstly does the 3 way "brake vacuum strengthener" contain a check valve on the line that runs from the charge air pipe? WIS shows a check valve between the charge air pipe and the valve separate to the 3 way "vacuum strengthener", so I'm assuming it must be somewhere??
Secondly, if this valve had failed would it cause a rough idle?

https://www.eeuroparts.com/Parts/11908/Brake-Booster-Vacuum-Hose-Manual-Trans-5230495/

Thanks
D

[fka]

I've just found this pic linked on one of the BMW forums, it's the same venturi type vacuum strengthener as used on the manual trans B2*5.
Pic actually answers my question, there's two check valves on the port that goes to the brake booster so to test I'd need to leave the booster end connected, disconnect the other two, block each end in turn blowing into the other to see if air is passing to the booster.

As opposed to trying to just suck on the end that goes to the charge air pipe 

There's little or no info about this on the Saab forums, so I'm guessing it doesn't fail too frequently.. Also I don't think it would constitute a boost leak if the check valves were to fail, it would just be pressurizing the servo and would be evident when braking..