I think the pronunciation may be a bit complicated if you want to hear the professional explanations. Many Turks even don´t know that there are many rules about the changing vowels, the correct diction. I even don´t know, but one of my friends had diction classes, and he explainted the shape of speech/pronunciation of the professional speakers, for example speakers on the tv.
One of these complicated rules are about the vowel "e". As you asked in your question, -ecek has this type changing -e- sound. I personally don´t think that is soo much important to learn by reading, but just hear how the people speak, you can get it better.
As an example, poem readers say"gidicee(ð)z" instead of "gideceðiz". Of course, that is pretty hard to write these things by referencing the same language. You are trying to learn how to pronunc and I am writing the same Turkish letter in the different word. But just as to be trying to simple;
pronounced - written
geliceeðz - geleceðiz
yapýcaaðz - yapacaðýz
yapabiliceeðz - yapabileceðiz
Other examples can be "deðil". We never pronounce it as it is written. It always shorten and say "diil". another word like this: "kaðýt" but pronounced "kaat"(k goes lighter)
For the future tense -ecek/acak; it may be even more shorter but that doesn´t mean it is correct and/or recommended. Like this "yapçaz". If you talk so fast, you probably say "yapacaðýz" as "yapýcaz" or "yapcaz" or even "yapçaz".
But as a foreigner, no one has to be perfect to know every single professional speaker rules.
That´s exactly what I was looking for. 
)) yapacaðým