A pressure modulator for a brake system is already known comprising: a primary cylinder with a servopiston; a secondary cylinder with a piston valve; and a guiding bore linking the two cylinders, in which a control ram is arranged. With this pressure modulator, guidance of the servopiston within the first cylinder is not exact. The new pressure modulator also comprises a primary cyclinder (70) with a servopiston (83), a secondary cylinder (72) with a piston valve (99), and a guiding bore (74) connecting the two cylinders (70, 72), within which there is a control ram (131). The servopiston (83) is provided at the extremity facing the control ram (131) with a lug (92) which protrudes into the guiding bore (74) of the housing (83). This ensures that the guidance of the servopiston (83) is precise. The new pressure modulator is especially suitable for hydraulic brake systems in motor vehicles.