Just because it's worked for you previously doesn't mean it's legal.
I've seen a part 61 raided in Florida and all those on waivers and B1s were "offered" the ability to depart the USA within 24 hours or go through deportation proceedings - I'll give you once guess what they chose.
The law is extremely clear and they don't hesitate to enforce it.
(7) Enrollment in a course of study prohibited. An alien who is admitted as, or changes status to, a B–1 or B–2 nonimmigrant on or after April 12, 2002, or who files a request to extend the period of authorized stay in B–1 or B–2 nonimmigrant status on or after such date, violates the conditions of his or her B–1 or B–2 status if the alien enrolls in a course of study.
The term course of study has nothing to do with maintaining 18 hours of study etc etc - that is the requirements to fulfill the M1/F1 continued status.
By contrast, if you are having one week of flight training on an M1, and then visit your relatives for 3 weeks, you might have a problem, because the M1 is supposedly for a "full course of study", which you then are not attending.
This is well within the privileges of a student visa. You may enter upto 30 days prior to your report by date and if you complete your course of study you also have a 30 day grace period.
At the end of the day it's down to your discretion and I know which way I fall given the consequences I first hand witnessed.