Chronic Brake problem with my daily commuter.

1993 Honda Civic LX Sedan 172K miles 1.5 liter

I was originally having a problem where the brake pedal would sink to the floor only after not using the brake for long stretches and when I first use the brakes after startups. So what I did was replace the Master Cylinder with a remanufactured part from Kragen's. After replacing and bleeding the system, it would still do the same thing. I replaced the front pads and rotors about 6 months ago, so I decided to replace the rear shoes since they have not been replaced in a while, still no help. I do not see any leaking brake fluid and the level always stays constant. So I sent it to a local repair shop and they adjusted the rear drum brakes and bled the brakes. The pedal firmed up a lot when I picked it up. But by the next morning the pedal moved almost down to the floor again. So I brought it back into the shop and they said that I shouldn't have used a remanufactured part and replaced it with a new Honda part. Again after picking it up the pedal was very firm and very little pedal movement before the brakes engage. But then again the very next morning the pedal goes almost to the floor and even when pumped the brakes would not firm up. The pedal would drop almost to the floor and when pressed hard it would stop the car but it seems like the rear brakes is only working because they would lock up and the car would not slow down very fast. The repair shop said to be bring it back in and they will look at it again. So this is now the 3rd time I brought it in and they said the new master cylinder was defective and they replaced it with another New Honda part for free. When I picked the car up again the pedal feel was great. But then the same thing happened the next morning. The pedal went almost to the floor and had to be pumped to engage. Driving around town the pedal feel was good. But when driving more that 10 minutes on the freeway without using brakes and the pedal will drop again way down and would have to be pumped to get good brakes again. I will be bringing this back into the shop for the 4th time over 2 weeks and I am beginning to think the mechanic might be overlooking something. I do trust this repair shop as I have used them several times in the past and have always been helpful even when I just asked questions about fixing things myself. I apologize for writing such a long question but I wanted to convey all of the details. If you can help give an answer to the problem I am having I would highly appreciate it.
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