Combine cucumbers, onions, and bell peppers in a large bowl. Add pickling salt, then pour in enough cold water to cover vegetables. Let soak for 2 hours. Thoroughly drain in a colander.

Inspect six pint-sized jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until relish is ready. Wash new, unused lids and rings in warm soapy water.

Pour vinegar into a large, heavy stainless steel pot; add celery seed and mustard seed. Stir in sugar and bring to a boil; continue stirring until sugar is dissolved, 2 to 3 minutes. Add drained vegetables and return to a boil; this could take up to 10 minutes. Remove from the heat.

Pack relish into hot, sterilized jars, filling to within 1/4 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe the rims with a moist paper towel to remove any residue. Top with lids and screw rings on tightly.

Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.

Remove the jars from the stockpot and let rest, several inches apart, for 12 to 24 hours. Press the center of each lid with a finger to ensure the lid does not move up or down. Remove the rings for storage and store in a cool, dark area.