The original had no sustain control, and the boutique versions have no sustain control. Let's spend $2. Don't do this on the original :).

The effect contains a little man (OK, two transistors and a diode) who knows when you hit the strings hard, and turns down the volume on your guitar accordingly. There is a volume booster after that, thats how it works. So we adjust the amout of cotton wool in the little man's ears.

Replacing the cotton wool (the 1.5k resistor)  with a 10k "Sustain" pot will vary the compression effect, with minimum compression is maximum volume. Maximum volume may be a liitle loud, since now the volume booster is flat out. You can reduce the value of the 220k resistor for less volume boosting. Or increase it if you actually want distortion. So for less compression you turn the new "Sustain" knob for more cotton wool (resistance), and turn the volume down. This can be an extra preset pot, or add an external Knob. A big volume boost with a little compression and a little distortion could be useful for the Big Solo.

Now, you could replace the 10k resistor with a 10k pot, and make a distortion compressor which cuts bass as you turn up the gain, or replace the 220k resistor with a 1 Meg pot for a distortion compressor which does not cut bass as you turn up the gain. But it's fine as is.