This issue was discussed a couple years ago.  Rather than disassemble/clean, I just installed my backup unit (and a new trans. seal).  Now the backup is doing it.
I just wonder, what would happen if I stuck the spray nossel of a can of electronic cleaner into the solenoid drain hole (and sealed around the tube), filled the solenoid, let it soak a while, and then drain (?).

6 VOLTS/POS. GRD. NW INDIANA