Exactly as Gambit wrote...if, once you get the car moving, there is no evidence of slipping or shuddering, then you should be looking at that master cylinder...first stop would be the fluid level and if it shows a low level, check for leaks in the line somewhere. Don't always assume the worst...we've all seen people spend lots of money to try and cure a simple glitch.
Please share your findings with us...other people may be helped by your experience. As far as I'm concerned, that's what this forum is all about.
Good luck!