Mk IIB lead channel question

Anyone experienced the same on his Mk IIB?:
when I'm on the lead channel, lead master on 10, and put lead drive on zero, I get the clean sound bleeding through. In fact it is quite the same loudness as on the clean channel. When I slowly turn up lead drive, the sound actually gets lower and lower till it disappears at about 3-4 on the drive pot. Turning up more and more sound comes back and starts to act normal, gradually getting overdriven more and more.

Hi guys! nobody in here who can help or answer??

I'm not sure if I could answer your question but I would like to mentioned Mark I, II uses "cascading gain". The lead channel uses the gain (preamp tube) from the "rhythm channel". The lead channel is not independent from the "Rhythm Channel".