I have a thought about that....
I second what rayjay said - my mother's 1961 Kenmore 70 was used most of it's life with All because of its low sudsing properties, which were especially needed in our house in Michigan since we had a household water softener. Occasionally we'd use something else, with an accompanying suds lock. Just occasionally though.
My thought though is about the size of the tub drains on machines. I suggest this because there are certain Kenmores that I have never experienced a suds lock in, and others that seem to be more prone. Those that don't seem to have this problem have large drain openings.
Standard capacity Kenmore belt drives, and the 1981 and newer large capacity belt drives use a narrow tub outlet hose, which is only about one and one quarter inches wide. Water has to swirl through it for some time for the tub to empty, the turbulence of which churns up more suds in addition to what's already there.
On the original 18lb. machines that were made into 1981, their outlet hoses are massive in comparison to the originals, and at least twice as wide in diameter. The machines use the same pumps, etc. but I'm betting that this outlet hose GREATLY reduces turbulence in the water as the machine drains, plus it allows suds already formed to fall into the drain and pump out.
We replaced our '61 Kenmore with a '74 large capacity, and like changing a light-bulb, I don't think we ever saw a suds-lock again, until my mom got her 1983 Kenmore, which has the original size tub drain/outlet hose, then occasional suds locks again.
I wonder if this is coincidental or a result of the outlet diameters?
I've seen locks in DD machines as well. I got annoyed with my 1993 direct drive Kenmore, which I've only used occasionally, but it would suds lock without any visible suds in the wash water. I learned later that these have narrow diameter tub drains as well. My DD would lock on the same amount of detergent that wouldn't phase my 1986 belt drive at all.
Gordon