It looks like auto trans fluid.
Did you have to add to the PS reservoir and did you check the trans fluid level.
PS I believe have a separate cooler, but the trans fluid gets cooled by the radiator.
What ever the case you will need the line and probably the radiator. Rusted fittings can sometimes be loosened with heat , but it does appear that the radiator lower tank may be plastic. If it is plastic possibly a well equipped rad repair shop may be able to remove the lower rad tank and replace the cooler coil and fittings.
Of course the radiator ill have to come out to do this.