References to the construction of the cistern of Modestus can be found in Libanius' Letters.
Letter 251 already speaks of "the abundant reservoirs", even though Crow et al. comment that the letter most likely dates to 360-362), which predates the constrtuction of the cistern of Modestus (363), alledgedly the oldest in the city. It may therefore also refer to another cistern. Furthermore, despite its high altitude location (Region XI in Notitia), it predates the completion of the Valens line.
Letter 827 is directly addressed to Modestus. It speaks of a certain Elpidius which Libanius recommends to Modestus: "...if - Poseidon granting - he [Elpidius] makes you [Modestus] well-watered (katarrytos)."
The Consularia Constantinopolitana reveals that the cistern was finished "In the time of the consuls Valentinian and Victor". This can be dated to 369.