Thread: Intermittent vacuum loss - where to look?

I'm periodically losing the vacuum boost to my Perentie's brakes. First thing in the morning there is seemingly no boost; brakes work well braking from speed but coming to a halt there is no boost - a little hairy. After driving for a while the boost is evident and once present it seems to to remain thereafter. Next day, same situation and so on.
I was told first place to look is the flexible 1/4" flexible nylon tube which feeds the differential lock control valve fed from the steely vacuum pipe between the vacuum pump and the brake booster. I have run new tube through; but am planning to fit a 90-degree elbow at the vacuum switch (as per parts list) rather than the straight connector currently fitted. The old tube looked OK, no evident cracks or pinholes, just stiff, and when I pulled it off the vacuum pipe there was an audible pop of air which makes me think the tube and connections were sound. I pulled out the steel vacuum pipe itself, cleaned it up - no major rubbing or cracks evident - and reinstalled it. The short sections of rubber hose at each end of the vacuum pipe are fine.
I'm not holding my breath that the replacement tubing will fix the issue so I am thinking about where to look next. According to the service record the brake vacuum booster was replaced late 2011 so it's not that old, and I would have thought if there was a problem with the vacuum booster such as the diaphragm the boost would be absent all the time. No obvious hissing at switch-off, and obviously with the 4BD1 you wouldn't hear hissing when it's running. Next thoughts are maybe the check valve at the booster; failing that I might visit a mechanic I know will to check the vacuum at the booster with a vacuum gauge.
Any thoughts/suggestions will be appreciated.
Cheers, Dave

Appreciated Ash, but I'm in Newcastle north of Sydney - a bit far away for a borrow. If need be I will either drop in to a good mechanic I use for the things I choose not to do on my other cars and ask them to run a vacuum test, or I'll lash out and buy a basic tester kit.
I'd be interested to hear how you got on with your own vacuum problems. I'm happy to install a replacement brake booster if that is the problem; I'd just like to be able to trace out the problem first in case it's not the booster at all as it's not that old.
Cheers, Dave