A straddle packer has a multicomponent mandrel with an active mandrel component. A modular pressure cylinder reciprocates within a limited range over the active mandrel component. Pistons the modular pressure cylinder are respectively interconnected and cylinder walls of modular pressure cylinder are respectively interconnected. When fluid is pumped through a tubing string into the straddle packer, the pistons are urged in one direction along an axis of the active mandrel component while the cylinder walls are urged in an opposite direction along the axis to simultaneously compress spaced-apart packers of the straddle packer to a packer set condition.