What would happen if you slowly crushed a live brain?

It depends on what part of the brain you are "crushing."

When we do brain surgery we often do exactly what you describe. That is, we open the skull and often remove part of the brain. However, in our case usually the part we are removing is diseased or malfunctioning to begin with.

There are areas of the brain that are known as eloquent and those that are considered "not eloquent." This is an overly simplistic but educationally instructive way to classify things. Eloquent brain regions are those regions that when disturbed will cause an obvious and devastating neurological injury. Examples are primary motor/sensory cortex, broca's area, wernicke's area etc. Noneloquent areas are those that can be damaged or resected and the sequelae are less obvious. For example, you can remove quite a bit of frontal lobe before you notice a problem. However, an appropriate and sensitive behavioral test/instrument may still pick up the deficit.

Damage to the brainstem and thalamus is more devastating and even small injury to this area can cause major deficits and changes in the level of consciousness. Injury to different levels of the brainstem produces different level of consciousness. Again, it really depends on the precise location and extent of injury and there is no single answer to your question.

As the brain itself has no pain receptors the patient would not really feel much pain even if you caused major deficits. However, again it depends on the location. You could theoretically irritate thalamic sensory regions and cause chronic pain syndromes but your lesion would have to be pretty precise.