Just like humans, some properties are a lot more low-maintenance than others.Your entire mortgage payment does hit cash flow, but technically the principal repayment portion is not an expense (it is repayment of debt).  

Became an agent in real estate business 12 years ago and  a  principal broker and owner today.The income from transactions helps to have cash to do further deals.

Lol....I don't know what you mean about practicing real estate, however it's not illegal to be a principal within a real estate transaction on the buyer or seller side.  

Take a look at this thread:https://www.biggerpockets.com/forums/61/topics/124...I use an All Inclusive Trust Deed where the payment amount and interest matches the seller's principal and interest payments.  

I recall reading an article or blog post about an investor giving their child a wedding gift of pre-paying their first year's mortgage principal.  

Interest only or principal plus interest loan?

As Matthew Brill mentioned, there is a self-sufficiency rule for 3- and 4-unit properties funded with an FHA loan.  85% of the market rents on all four units need to cover your monthly payment (principal, interest, taxes, insurance, and mortgage insurance).

Well I am the principal commercial broker and owner.  

Add that to your $425 principal and interest payment, plus the $75 you budgeted for taxes and insurance, and now your cashflow is negative $25 per month.

Some mortgages require the payer to pay the property tax to the bank, along with the principal and interest payments on the loan.