What is the normal diastolic pressure while exercising?

Less than 80: Diastolic blood pressure should stay the same or decrease when you do aerobic exercise like brisk walking or running. A normal diastolic blood pressure is less than 80 mm hg.

Less than when not: It's an interesting aspect of exercise physiology that the diastolic blood pressure generally falls while the systolic blood pressure rises to exercise. This is a response to the body opening up more blood vessels while simultaneously increasing the amount of blood being pushed by the heart. Generally speaking, your diastolic blood pressure should fall with exercise.

Probably 90's: I typically see high 80's to mid 90's in patients during stress tests. Higher than that would be unusual.