Toss the cucumbers and dill in a large heatproof glass, stainless steel or ceramic bowl.
In a small saucepan, combine vinegar, water, sugar, salt, mustard seeds, peppercorns and red pepper flakes. Bring to a low boil, stirring until sugar and salt dissolve.
Pour the mixture over the cucumbers and stir until well combined.
Let sit at room temperature until cooled.
Place in a large jar and store in refrigerator for up to 2 weeks.