It's not the convertor. Does the master cylinder have brake fluid in it? Sometimes a brake line fails and it sprays the fluid and doesn't show much of a wet area. If the fluid level is low, add some and have an assistant pump the pedal while you watch underneath. You'll probably find a leak. There is always a chance the master cylinder itself has failed, but they usually cause the pedal to drift down (fade) at stops before it just quits working.