That joke doesn’t translate well into French because "fermer les freins" is not idiomatic in Belgium French (nor France for what I know…).
So, while you probably would be understood, it will definitely sound weird and therefore something you intent as a funny remark could be misinterpreted and judged agressive.

As much as I like puns, I can’t think right now of a funny remark you could have done.

To answer more generally to your question, assuming "fermer les freins" would be idiomatic, a correct way of saying this would be:

Tu dois/Vous devez ouvrir les yeux et fermer les freins, et pas le contraire !

or

Tu dois/Vous devez ouvrir les yeux et fermer les freins, et pas l’inverse !

While logic fans out there might argue that "inverse" and "contraire" are not exactly the same, both sentences will be understood equally from where I see it (we’re not in some math demonstration…)

As already mentioned in Laurent S.’s answer, "fermer les freins" does not exist in French. And I cannot find an equivalent to make the pun work.

The correct verbs for the brakes are "utiliser" or "appuyer sur". Or more simply and more commonly, the verb "freiner". For instance, the following sentences have the same meaning:

• "Tu dois freiner."
• "Tu dois utiliser les freins."
• "Tu dois appuyer sur les freins." (This last one sounds less correct.)

Sometimes you can also find "serrer les freins", which can be used as "to brake strongly", but this is uncommon and slang. Same remark for "planter les freins" or "être debout sur les freins".

If you really REALLY want to keep your intended pun, you could try the following:
"On ne peut pas conduire en économisant ses yeux et ses freins…"
(one cannot drive while saving his eyes and brakes…)