I use thick bleach (the cheapo loo kind) around once every couple of months in my machine and have done for many years and machines now, I squeeze a bit round the seal at the bottom where it gets black and wipe a bit more all around the seal in the folds, then i do a longest hottest wash. Its never been a problem with foaming or anything else. If there is a smell of bleach in the machine when i am done then I wash something like teatowels in there.