I had a 170 in the shop once and i drained the hyd oil and I could not get the steering working. an old AC man told me to park it down hill and that worked. I did some checking and found that the compartment where the hyd pump is supposed to fill up with oil to some holes high up in the housing well above the dipstick level. the return line runs into the pump housing and the dipstick is one housing back. you probly have a bad seal on the trans input shaft and the oil is draining back out of the pump housing.
If you didn't want to fix the seal you could fill the pump housing thru the return line till it came up on the dipstick. the next morning you will show the oil way overfull but after running it awhile it will level out. Just thought I should say that I am not sure if a d21 is the same, but I bet it is
Good Luck Ed