I can truly believe those glands can produce a quart of spit a day. I've had the one under the tongue that's right under the jaw area, or feels like it when feeling for it with my tongue, anyway, that gland has literally squirted my mirror when I open my mouth wide. Now I thought that was the only one but you make it sound like there are more. Maybe I produce 2 quarts then since I appear to have that squirter.
I can't imagine how painful getting that one cleaned out might be. That's one tender area there.