I have isolated systolic hypertension of 150/70. All meds I've tried push the diastolic too low (<60). Which med lowers systolic but not diastolic?

None. Don't concern yourself with the diastolic number. It will come down with the systolic pressure. Some people have a diastolic pressure of zero and are fine. (the diastolic isn't really 0 - it's that our indirect method of measurement (using a cuff) isn't able to accurately measure true central diastolic pressure).