No pressure from the Master cylander to rear breaks.

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I'm assuming you have fluid pressure to your front wheels. In a ABS system, the ABS unit acts as a brake proportioning valve (distributes fluid to all wheels). There is a possibility that ther is air trapped in the ABS unit! Ask your mechanic to bleed the abs system using a scan tool. The ABS pump solenoids have to be actuated by the scan tool before manual bleading can be performed.
Now if there is no pressure up in front. Then you may be right. It could be the booster but i'm sure your mechanic would have figured that out by now if it was!
If he tells you he does not have the scan tool, I suggets you bing it to the dealer & have them bleed the brakes. I'm very sure that's the problem!

Note: the previous reply by RASMATAZ is 100% correct provided the mechanic rebuilt the master cylinder himself or got a defective rebuilt unit. I'm assuming that he must have warrantied out the master cylinder atleast once. To come to a point where he is clueless!