Or worse... like the proportioning valve* is rusted.

(The expense and difficulty in replacement of this part alone is probably the most effective incentive to maintain a flushing habit.)

* if you google this, you'll find even the parts suppliers don't know what this is, displaying the octopus, or differential pressure sensor instead. The proportioning valves (2) are under the rear seat.
--
Art Benstein near Baltimore

“Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.” - John F. Woods