300E A/C- Odd pressures

[csnow]

1986 300E.

Ambient= 90 degrees

System pressure was low. Lowside 10, Highside 90.

Added r-12 until sight glass was clear (28oz.) Highside read around 180 at that point. Paused to let system equalize...

Now, Lowside is -22psi. That's right, NEGATIVE 22.
Highside is 140. Compressor kicks in fine, but even when I shut the compressor off, the pressures stay the same for some time. After about 20 minutes, they do start to equalize (low=+10, High=130).

Ok, so I guess something must be clogged, right?
Where should I start?