Learn Swedish: Bromsen måste justeras snart. - The brake must be adjusted soon.

This is the s-passive form! After a modal verb like måste (must), you use the infinitive form of the main verb. To make it passive (meaning the brake receives the action rather than doing it), you simply add an -s to the end of the infinitive justera (to adjust), giving you justeras (to be adjusted).

Where is the Swedish word for "be" in "must be adjusted"?

You don't need one! The -s ending on justeras does the job of "be" all by itself. While you could technically form a passive using the verb bli (to become/be) and say måste bli justerad, using the s-passive is much more common, natural, and concise in Swedish.

Why is it bromsen (singular) when in English we usually say "the brakes" (plural)?

Bromsen is the definite singular form of en broms (a brake). In Swedish, it is very common to refer to a vehicle's braking system in the singular (bromsen), whereas English speakers tend to default to the plural ("the brakes").

Can I put snart earlier in the sentence, like Bromsen måste snart justeras?