If I recall, at least on the faulty automatic only PL I had, it had separate aperture blades, but those only worked when you wanted to use a flash attached. Otherwise in program mode, it used the leaf shutter as the aperture.
I am guessing at least for the ML, in manual mode, the leaf shutter worked similar to an XA that has both the leaf shutter and separate aperture blades.
I have an MB that works (actually two because I bought a BGN one on KEH that also ended up working..), but it is only Aperture priority. It has been fairly reliable, with the three or so rolls of film I've run through it. I had more issues with the mercury battery powered models like the EL, GL, and PL.