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Having a soft brake pedal on any model is not safe nor is it a manufacturer issue. The mechanic telling you this sounds to be trying to get rid of you as he cannot fix the problem or possibly not doing something right. I will give you some things to check out then depending on the results I will hopefully be able to come up with a solution to help you out with your issue.
The first thing you need to do is to check the brake master cylinder level (I see you said you have no fluid loss but just need to get an idea of the fluid level, being I cannot see the vehicle it can be slightly tougher to diagnose). Is the level close to the max fluid level?
Next with the vehicle off, pump the brake pedal up until it becomes hard (if the pedal ever does become hard). Then with your foot still on the pedal, start the vehicle and note if the pedal drops fastly to the floor or not.
If you can pump the brake pedal up and the pedal becomes hard this will be a determining factor to tell if you have a hydraulic issue or possibly a faulty brake booster. If the pedal becomes hard then when you start the vehicle if the pedal drops this will indicate you have a vacuum issue which I will help you out in where to look next.
Also when the work has been done either time have you ever had a normal feeling brake pedal or have you always had the low pedal issue?
Also has the mechanic done anything else to the vehicle other then a master cylinder?
Please respond as soon as possibly so I can assist you to fix your issue! Thanks once again for choosing Just Answer!