Radiator/coolant question
 

This is my first post to the FRF :)

I believe I'm losing engine coolant somehow, because I had it flushed and exchanged a few days ago, and the reservoir is empty.

Long story short, I bought some coolant to add and see if I could find the leak. I didn't see markings on the reservoir for "Hot Full/Cold Full" so I added it directly into the radiator :( I did run the vehicle for about 20 mins after adding, and everything seemed to be okay...

Is this bad? I found an owners manual online that advises against this, but doesn't say why.



*Truck in question is a 98 Ranger XLT 4x4, 3.0 6 with 89,500 on it.