Honda Repair: Brakes, honda accord, quess

Question
I have a 93 Honda Accord LX that we bought used about a month ago.  We are having problems with the brakes. The brakes will work fine or seem to then out of the blue you will have to pump the brakes in order to stop or while you are sitting at a stop the brakes sink, especially on a hill.
We took it to a service center and they couldn't duplicate the issue and wasn't sure if it was the master cylinder or the ABS Module. Since it wouldn't do it for them.  We had them flush the brake lines just in case.  They said they had never seen a car with so much air in the lines.  It didn't help.  There are no leaks that can be seen.  Any quess as to what might be the problem before we start replacing parts and spending lots of money.  

Answer
Most likely it will get worse as the temp gets warmer outside.  Master cylinder is going bad internally.  You will need to replace the master cylinder.  I would recommend it before you have to do a panic stop and nothing happens.  Common problem with these cars.