The monovalve is inboard of the battery on the firewall under the diagnostic port.

- remove 2 philips screws behind the transmission dipstick
- move the diagnostic port out of the way
- remove 4 flathead screws on top of the monovalve
-- it might help to disconnect the 1/2" OD hose above the monovalve
--- go easy because the fitting is plastic
- lift the monovalve cartridge and top plate out of the housing
- note the order of washers as you remove the cartridge
- installation is reverse from this point

The amount of coolant I lost didn't even register in the reservoir. Maybe there's a pocket of air in my block now.

The monovalve usually fails through tearing of the rubber seal on the valve or the rubber diaphragm.

[added on 03 Jan] I've added a monovalve album to http://photos.yahoo.com/tabijan for some visuals to go with these descriptions for future reference on monovalve cartridge repair and/or replacement. Corrections are always appreciated.

Sixto
91 300SE
81 300SD