This question really belongs as a comment to
but unfortunately my reputation is so low that I'm not allowed to comment.
has very helpfully explained that that the reason why the various optional settings for \linebreak don't work very well is because the values of low, med, and highpenalty are "a bit odd". I would use a less charitable term, personally. In particular, linebreak assigns a penalty of 301 out of 10000, which presumably explains why it is so ineffectual. I would like to change these settings to the values that would seem obviously more sensible, e.g., spread them evenly between 0 and 10000. Is this possible?
Why do I care? I'm extremely puzzled that the consensus is that use of linebreak options should be extremely rare. For me, if it worked properly, I'd be using it all the time. The reason is that I want to avoid breaking up possibly long inline math displays, and would like, ideally, to have linebreak decide to insert a linebreak, fill the preceding line and put the entire math display (enclosed in an \mbox to prevent breaking it up) on the beginning of the next line if and only if the math display, if left on the preceding line, would run beyond the regular right margin.
Perhaps there's a better way than linebreak of accomplishing what I need?