Itis very important to not only trust your instruments but know how they are powered.Some instruments are vacuum driven such as DI/Hsi and AH whilst others are electric such as the turn and slip.It is quite common,however,to have an electric HSI.I had a quite stressful experience in our group rockwell commander 114 in IMC a couple of years ago.The AH failed but fortunately I was on top of the cloud cover at the time.I looked at the vaccuum gauge which indicated a pump failure with the dolls eye clearly visable (the engineers found later the vaccuum pump had sheared).
By understanding the instruments I knew that the HSI was still functioning and turn and slip O.K.also.Obviously ASI is not a gyro or altimeters and VSI.
The next step was to descend through approx 2000ft of cloud and get visual without getting disorientated and using ASI and VSI for pitch and turn & slip and HSI for direction i.e.keeping out of a spiral dive.I set the aircraft up in a 500 ft descent which would take 4 minutes approx to get visual.
Things worked out but if I had lost the AH whilst in solid clag I may have followed it if I had not noticed the vaccuum gauge.
All good fun eh??