Leaking under steering column- oil like substance leaking when parking brake is applied- 1978 thunderbird

Modulator valve is on the transmission.it has a diaphragm in it that ruptured and allows fluid to be drawn into the intake manifold
The emergency brake has a vacuum operated release mechanism . It's getting the transmission fluid from the intake manifold
Replace the modulator.
This here sounds like your possible issue. Though I would still check the transmission fluid itself. If your transmission fluid is brown it means the fluid is burnt or very old. Possibly meaning burnt bands or worn clutches. It could very be the only reason your car is still moving is because of the amount of particles floating in the fluid is allowing the clutches to still engage. I would inspect the transmission as well when you have the modulator checked.
It’s the modulator. My transmission was empty and my engine was burning the fluid and sucking it into the vacuum lines. I’ll get t fix and we’ll go from the but is just like to say THANK YOU being a member of this family of thunderbird owners are amazing. Thanks for the help- Gabe
Wondering if you found your fluid leak. Years ago I worked on 78 Tbird with same problem. This car had factory cruise control and was leaking transmission fluid at the coupling on the cruise control cables. It is a 2 piece cable.