Брать often use like an imperfective verb. # Каждый гож я (что делал?) брал с собой собаку в отпуск. Every years I took witn me a dog on holiday.
Взять always use like a perfective verb. # Я (что сделал?) взял собаку с собой, поэтому я не смог взять жену. I've taken a dog with me, so I couldn't take my wife.
There are some trouble witn understanding of imperfective and refective verbs. :)

брал= some continuous in some cases. For example 

1) Джимми постоянно брал гитару у своего брата

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Example that I noticed in google right now: "Что можно взять с собой в дорогу?" Again, here "взять" can be substituted with "брать".
And whatever verb is used, i just don't understand from the phrase itself, do they mean repeated action, i.e. use 'single action'-phrase in the sence of 'single typical sitiation': что можно взять с собой в самолёт, когда летишь в Америку? - or do they really mean single action (e.g. 'tomorrow').
Or if it was 'брать' - would they mean 'repeated' or 'continuous in the meaning of single action'?

That's after 'можно'. With another modal verb: хочу (I want) one always specifies exactly what he wants: брать (repeatedly), брать (to be in process of taking, pretty exotic case:-)), взять (in order to have it). No possible substitutions.

So, you see, infinitive brings us another bunch of sbtleties.
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I don't want tothink furhter and deeper, i'm just afraid of what i can find)

Generally, without a prefix брать is imperfective aspect and взять is perfective.
I.e. the former is used when you describe either 1 continuous or 2 repeated/usual acton.

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But in <em>imperative</em> the diference becomes too subtle. At the mometn I haven't figured out what it is, but in most situations you may substitute возьми with бери.

Of course there're situations where imperfective is necessary: ...всегда бери...

And there situatios when you do mean <em>single</em> action, but without the context it's not clear enough that you don't mean <em>repeated</em> action. As in the advertisment: "возьми в дорогу бутылочку Hershey''. In this situation возьми is preferable.

In the sense of repeated action "бери" sounds awkward if the set of situations to which this recommendation is applicable isn't specified. In the phrase itself: "всегда бери..." "Всегда бери ххх когда едешь в Иралндию..." Or in the context:
- Что делать, если...?
- Бери ХХХ.
- Какое пиво лучше брать в ...?
- Бери ХХХ.

The difference is only in their use, which I can't explain until I sleep it off. =)