Vintage 1986 Bridgestone MB-2 Lugged Steel Mountain Bike Repair Needed - $289 (Sebastopol)

Vintage 1986 Bridgestone MB-2 Lugged Steel Mountain Bike.
The chain doesn't sit when riding sometimes and I don't know enough about bikes to tell you what the problem would be caused by.
⚠️ This is not a bike that can be used without repairs.
Both brakes work but the back pads were worn out and removed but not changed out.
Price negotiable