A pastilha de freio precisa ser trocada.

A is the definite article: the. It refers to a specific brake pad that both speakers can identify, or to the brake pad being discussed in context.

Uma would mean a / one, and it sounds less specific.

So:

• A pastilha de freio... = the brake pad...
• Uma pastilha de freio... = a brake pad...

In practical situations like mechanics, Portuguese often uses the definite article where English might also use the.

Pastilha de freio is the normal Brazilian Portuguese term for brake pad.

Breakdown:

• pastilha = pad, tablet, small plate/piece depending on context
• de freio = of brake / for braking

So literally it is something like brake pad.

In Brazil, freio is the usual word for brake.
In European Portuguese, you may also see travão.

Portuguese often uses noun + de + noun where English uses a noun as an adjective.

So:

• pastilha de freio = literally pad of brake
• natural English = brake pad

This pattern is extremely common in Portuguese:

• sapato de couro = leather shoe
• escova de dentes = toothbrush
• luz de freio = brake light

So de here is just part of a very common way of linking nouns.

Literally, it means:

• precisa = needs
• ser = to be
• trocada = changed / exchanged / replaced

So word-for-word, it is roughly:

needs to be replaced

This is a very common structure in Portuguese:

• precisa ser feito = needs to be done
• precisa ser limpo = needs to be cleaned
• precisa ser consertado = needs to be repaired

Because the sentence is built in a passive meaning: the brake pad is the thing receiving the action.

• A pastilha de freio precisa ser trocada.
  = The brake pad needs to be replaced.

If you used trocar directly, the structure would usually need a person as the subject:

• O mecânico precisa trocar a pastilha de freio.
  = The mechanic needs to replace the brake pad.

So:

• precisa ser trocada = the brake pad needs to be replaced
• precisa trocar = someone needs to replace something

Because trocada agrees with pastilha, which is:

• feminine
• singular

In passive constructions like this, the past participle agrees with the noun.

So:

• a pastilha ... trocada → feminine singular
• o freio ... trocado → masculine singular
• as pastilhas ... trocadas → feminine plural
• os freios ... trocados → masculine plural

This agreement is very important in Portuguese.

The article tells you: a pastilha.

In Portuguese, nouns have grammatical gender, and the article shows it clearly:

• o = masculine singular
• a = feminine singular

So if you see a pastilha, you know the noun is feminine.
That is why the participle also becomes trocada.

No, not in standard Portuguese with this meaning.

That would sound wrong because a pastilha de freio is not the one doing the replacing. It is the thing being replaced.

You need the passive structure:

• A pastilha de freio precisa ser trocada.

Or you can rephrase with a different structure, for example:

• É preciso trocar a pastilha de freio. = It is necessary to replace the brake pad.
• Precisamos trocar a pastilha de freio. = We need to replace the brake pad.

Yes, trocar is very common and natural for replace / change in everyday Brazilian Portuguese, especially with parts, components, clothes, money, and similar things.

Examples:

• trocar o óleo = change the oil
• trocar o pneu = change the tire
• trocar a bateria = replace the battery

You could also hear substituir:

• A pastilha de freio precisa ser substituída.

That is also correct, but it sounds a bit more formal or technical than trocada.

Precisa ser trocada most directly means needs to be replaced.

Depending on context, English might translate it as:

• needs to be replaced
• has to be replaced
• sometimes even must be replaced

But by itself, precisa is usually closer to needs to than to a very strong must.

If Portuguese wanted to sound more strictly obligatory, you might also hear:

• deve ser trocada
• tem que ser trocada

It would be:

As pastilhas de freio precisam ser trocadas.

Notice all the agreement changes:

• a → as
• pastilha → pastilhas
• precisa → precisam
• trocada → trocadas

This is a great example of how several words in Portuguese change together.

Yes, it is completely natural and standard.

It sounds like something a mechanic, inspector, or car owner might say.

Other natural alternatives include:

• A pastilha de freio está na hora de trocar.
  more conversational, less formal
• Tem que trocar a pastilha de freio.
  very common in speech
• A pastilha de freio precisa ser substituída.
  slightly more formal/technical

But A pastilha de freio precisa ser trocada is absolutely normal.

In Brazilian Portuguese, lh is a special sound, similar to the lli in million for many English speakers, though not exactly the same.

pastilha is approximately:

pahs-CHEE-lya

Very roughly by parts:

• pas = like pahs
• ti = often sounds like chee in many Brazilian accents
• lha = a soft lya sound

So the whole word sounds close to:

pahs-CHEE-lya

Also:

• freio sounds roughly like FRAY-o

So the whole sentence is approximately:

A pahs-CHEE-lya dji FRAY-o preh-SEE-zah sehr troh-KAH-dah

That is only an approximation, but it helps with the main sounds.

Because freio is a noun inside the expression pastilha de freio, and it does not need to agree with pastilha.

The gender agreement happens with words that modify or refer directly to pastilha, such as:

• the article: a
• the participle: trocada

But freio remains freio because it is just the second noun in the phrase pastilha de freio.

So:

• a pastilha → feminine
• de freio → unchanged

Yes, if the context is already clear.

If everyone knows you are talking about the car’s brake pad, then A pastilha precisa ser trocada can work.

But without context, pastilha by itself can mean other things too, depending on the situation. For example, pastilha can refer to a tablet, lozenge, or small piece of material.

So pastilha de freio is clearer and more specific.