Low B-flat when hitting low B issue

Is it possible that you actually mean the low C# key and not B key? If that's the case the problem makes a lot of sense. If you really mean the B key then maybe just when you press both the B and Bb keys for some reason you don't press one of them (the Bb key) enough. If you press the Bb enough, adding the B key will do nothing that would create a leak. Unless pressing both the B and Bb key somehow makes your hand twist in a way that another pad doesn't close.

OK. There is an adjusting screw connected to the B key that is pressing on a part connected to the C# key. I assume you are talking about the Mark VI in your signature but I'm going by an alto I have here (not Mark VI) which is probably about the same. Press the C# and B keys seprately and together and you will notice how this mechanism works.

That adjusting screw is probably not screwed enough, which leaves the C# a little open if you press both the B and C# buttons. Most likely you need to screw that adjusting screw just a little more, but what you will notice if you look at this mechanism is that if you screw it a bit too much it will make the B not close with a light touch, and if you screw it even more it won't close at all. So it has to be exactly right, and preferably without double action between the screw and the part under it (which on some models is realistically impossible, but on good ones it should be ok).

Yes Gordon by "screw it a bit too much" I meant anything that would compromise the Bb. On some instruments it is possible to have both with basically no compromise (i.e. the C# key might close a bit softer with the arm than it is normally but still seal when playing Bb + C#).

Griff that is why I said "probably" I'm guessing you are right since I really don't remember how it is on Mark VI but like Gordon said same principle.

Decade, If it doesn't have the screw then it is most likely a worn cork or something bent.

I hope I could help.