STM Mixer insert - Pre/Post fader

[Music Manic]

I've just finished my mixing session today and realised that when I turned down my Master fader
the signal decreased in the insert effects which affected my Compressor settings.
I thought that the fader would be set to post inserts so that the signal wouldn't lose its integrity and the effects would do their job relative to the input signal.

Am I missing something but shouldn't inserts be pre-fader? Especially, when it comes to compression the threshold settings would become redundant because of the signal level affected by the fader.

Is there a pre/post fader button that I can't see?

Thanks