newbie: access buttons from a running loop

I'm new to builder and I have the following problem. I'm a gegographer
working with a erosion model that I decided to give an interface with
BCB3. Basically the model is a big loop with massive calculations. I
tied this loop to a speedbutton and it runs. Once it is in the loop I
cannot stop it: the program does not react to OnClick of another
speedbutton for instance. The "run" speedbutton remains pressed. Do I
have to "release" it first or something, giving the focus to the stop

thanks !