My vacuume thermostatic control unit is leaking antifreeze. I did take it out & replaced the o-ring gasket that seals the flared pipe coming from the firewall. Tried an o-ring from Osh hardware store the same size but it was not like the flat stock one. Put it back in & it still has a slight leak. Does anyone have any experience in repairing one of these units with a gasket or sealants? A new one is still available if it can't be repaired for around $80 but I want to know if this can be fixed first? Thanks for all replies in advance.

Maybe try the next size thicker o-ring?? Be sure the sealing area is clean, any little piece of crap will deform the o-ring and allow a leak.

a very, very light coating of silicone on the Oring might help it to seal.

Well the o-ring was thicker than the stock one & when it was tightened down, it took more torque to get it flush. It may have split from the tightness. I might put the stock one back in with some silicon sealer again if I can't find a flat rubber like the stock one at a plumbers supply place. Thanks for the tips guys.