Soft pedal

If you have ABS then you need an advance scanner to bleed air out the ABS hydraulic pump. If you do not have access to a scanner then try to rent one from your parts store. You may need to take it in to a shop to have t hat procedure done?

You have to understand that GM vehicle will always have a soft pedal when the engine is running but you will find that the vehicle stops well before applying that much pressure. You should have a high and hard pedal with the engine off.

If there is actually a problem stopping the car with a low pedal, then bleeding with a scan tool would be suggested.