Hello Witzend and thanks for the intro MF 135.
The shoes do appear to be dirty and would benefit from a clean-up with some sand paper. Make sure you don't breathe in any of the dust.
The wear that normally occurs is on the front upper, and lower rear face of the operating cam.
With the shoes fully adjusted check the position of the inner shaft arm when the brakes are applied. It should be perpendicular to the centre line of the tractor at maximun and NO more. When correct the arm (see photo) will be hanging rearwards slightly. As the brakes are applied full leverage is transferred to the shoes giving maximum braking efficiency. If the arm goes beyond the max point the leverage will start to drop off.
The cams are easily built up with weld and ground back. The new face can be left a little proud. Remove the brake rods and make sure that the clevises are free. Grind the worn area off and build up. Grind back to suit. Assemble the shoes. Check that the shoes are sitting 'square'. Use a set square off the half-shaft face to check this. Adjust if necessary by the screws in the backplates. Fit the drums and adjust up until they cannot be turned by hand. Using a piece of rope tie the pedals fully up to the steering wheel. Adjust the rods so that the pins can just be fitted. Slacken the adjusters about 5 or 6 'clicks and check that the drums are free and that there is one to one and a half inches of pedal movement. Final adjustment will need to be done on a hard surface.
Subject to the shoes not being glazed or contaminated the tractor should now stop on the proverbial (for us) sixpence....probably dime on your side of the pond.