Complex sentence: Pumps may work even if there is nerve or vascular damage preventing a natural erection, but excessive or prolonged use (hours) will cause permanent harm as over-enthusiastic pumping can burst blood vessels and form blisters.
Split into simpler sentences: Pumps may work even if there is nerve or vascular damage preventing a natural erection, but excessive or prolonged use (hours) will cause permanent harm. Pumping must be done very carefully to avoid serious injury: over-enthusiastic pumping can burst blood vessels and form blisters.