Tony,
If the pedal arrangement on your '51 is the same as my '50 the pivot pin (the bolt with a cap on the end you wrote about) goes through the master cylinder bracket -this is on the other end of the pivot pin to the nut you've undone.
To remove the pedal assembly, as well as the big nut you've already undone, you will need to undo another 4 or so bolts that hold the master cylinder bracket to the top and bottom of the driver's side chassis rail. Once you've undone these other bolts this large bracket that holds the pedals will come away along with the brake master cylinder as a single sub-assembly, then you can easily slide the pedals off of the pin once you have removed the retaining clip that sits in a groove on the pin at the opposite end to where the nut fits.
Stuart.
