HELP! Do I pay rental HELOC debt or acquire more units ?

[Todd Powell]

I am new here today everyone. I have flipped 25 or more units until I saved and now have many off market buy and hold rentals, four 4 plexes, and 50/50 ownership with my son @Jason Powell on a 5 plex and 8 plex. I also have ownership in a 72 unit building near Oregon State University. 

I have read many books that say leverage and borrow as much as you can for real estate. Namely, as much as you can borrow will equate to more net worth in real estate. But contrary, paying down debt is safer some say. To be transparent, most of my deals were below market off market deals, and in major distressed condition and below market rents. The issue at hand is that I owed $500,000 in interest only HELOC LOANS as I leveraged my paid off house, and borrowed my house funds as down payments and massive rehab costs. I now have the HELOC down to a $421,000 balance on a 15 year draw period and a .25% point under prime rate.

I currently have roughly $12,000 cash flow on my units, so should I buy another "deal" or just pay down this HELOC? I guess I don't care as much about my first mortgages, but I am concerned about the RE market and rising interest rates.

I would love your input and expertise here as I am 51 years old and trying to make retirement decisions. Thanks for reading and any advice you can extend !