Help with lack of progression in brakes

[sabbot]

The brakes on my elan +2 are great but not progressive. It is fitted with 2 rebuilt Girling mk2b Servo units. The problem I have is that the brakes work but the retardation isn’t proportional to the pedal pressure. Gentle pedal pressure slows the car slightly and then as I gently press the pedal further all of a sudden the brakes come on quite hard. I’ve checked for vacuum leaks, taken the servos apart to check for fluid leaks and sticking pistons and can’t find anything wrong. Any and all suggestions would be appreciated. - thanks