Having a brake down
QI’ve built a brake system for my Austin-healey 3000: split front (discs) and rear (drums) using an in-line twin-circuit master cylinder and two remote servos. It failed its MOT with no braking action at the rear. The front was fine and the pedal felt hard. I’ve checked the shoe adjustment, bled the system and even rigged up an old single-circuit master cylinder – with which the rear circuit worked OK. My guess is that the pedal pushrod is not long enough to operate the rear part of the master cylinder. Do I need a longer pushrod?
Trevor Gammage, via email
ANo. The operation of a twin-circuit master cylinder is as follows. The pedal pushrod pushes the first piston, pressuring the first circuit. The pressure is equal throughout the system and not only operates the front brakes, but also pushes the second circuit’s piston forward in the master cylinder, operating the second circuit.
There is no physical connection between the pedal pushrod and the second piston. If the second circuit worked with your substitute master cylinder then it seems likely that an internal defect exists in the twin-circuit cylinder. Strip it and compare it to the diagram in your manual. If you rebuild such a cylinder, note that the first seal of the second piston must face the first piston – that is, it’s the opposite way round to the other three seals.