The Bernina 831 is an awesome machine! Be certain it's the foot pedal before you replace it. It just may be that the motor brushes need to be replaced. It may have the min/max button switched to min. Or the belt may be slipping. If it has the foot pedal with the carbon piles, it needs to have the capacitor replaced by a thermal switch. If it has the circuit board foot pedal, there's a potentiometer adjustment on the back. If the potentiometer is maxed out, you need to replace the capacitor.
The older Berninas take the same BERNINA bobbin case as the new machines -- not the $5 to $15 generic oscillating bobbin case. The Bernina ones have a hole in the "finger," for when you're making a buttonhole. You can get a generic Bernina bobbin from Brewer. The part # is 0015347200. They retail for about $42.00. They can take a standard 15 class Singer bobbin, but do better with a generic Bernina bobbin #0115367000. Bernina bobbins have 7 holes, Singer 15 class have 10 holes.
That machine will do a beautiful satin stitch.