Abstract: A pressure controller of the type using pulse width modulated apply and release solenoid valves (80, 82) connected to an outlet port (74) of the controller overcomes the technical problem of poor control stability by using a digital control loop (98, 100, 76, 78) which utilizes stored bias characteristics representing the variation with pressure of the value (n.sub.o) of the pulse width modulation signal at which each valve initiates flow (n.sub.o). These values are determined in a characterisation step conducted with the controller off-line. The values may be modified during on-line operation of the controller by monitoring the stability of the controller or other signals representing the opening and closing of the valves.