Isamu had a similar problem with his pedals in Rave Racer and it turned out
that the pedal needed to be calibrated in the service menu of the game.
The Service button is set to F2 by default in MAME, there is probably also a Test button that comes into play.
Look at the button mappings in MAME to see what they are.
The test button and other buttons that the game had (view, start, etc) will be used to navigate the service menu and select stuff, but I can't tell you which button does what.