I have noticed that the brake booster leaks a small amount of air into the manifold. This is more apparant on the sidedraught configuration which I have now. I'm not sure whether it's allowed to leak a little or not, but I replaced my old one with a new (used) one and has checked the hoses and pipe too. The new one is still leaking a bit, however, so I have now decided to restore my old one.
There's a procedure described in the workshop manual, and I have managed to take it apart - it's not that difficult - but before I go on much further, I'd like to know if any of you guys have been through the job.
Here's a photo of the parts. As you can see, the internals look very well despite the rusty external appearance.