There is no hard and fast rule about the ideal brake master cylinder sizes to use in your pedal box. It is rather down to driver preference. However, as a guide: if a driver wants a pedal feel that he can modulate, then he should choose a larger size, or if he wants instant strong pressure, by stamping on the pedal, then choose a smaller size. (Obviously, some caution is needed, as instant strong pressure in the wet or on loose surfaces would probably cause the discs to lock up, which can cause lack of control). On top of this you must factor in the balance operated by the bias pedal adjustment.
What is also important is the weight and type of vehicle, plus type of use, together with the front to rear balance.
FWD (Front wheel drive cars), especially light weight vehicles, will experience more weight transfer when braking. So it probably needs a smallish bore for the front – say start with 0.625” and for the ideal balance between the front & rear for FWD choose a larger bore at the rear, and ideally at least 2 x sizes different, so 0.70” or 0.75” at the rear, with the 0.625” Front.
RWD (Rear Wheel Drive), depends on spec. and conditions, but there is likely to be less weight transfer during braking, so a difference between front and rear of 1 x size usually.
4WD (Four Wheel Drive), depends on spec. and conditions, but there again likely to be less weight transfer during braking, compared with FWD, so a difference between front and rear of 1 x size usually.
The table below shows the range of bore sizes available and shows the % step up from the previous size and line pressure based on 50 lb pedal load and a 5 to 1 pedal ratio.
|Master Cylinder Size mm/in.||14.00/
|Area sq in.||0.2386||0.2739||0.3068||0.3436||0.3848||0.4418||0.5185||0.6013||0.6903||0.7854|
|% step up from previous size||N/A||14.8%||12.0%||12.0%||12.0%||14.8%||17.4%||16.0%||14.8%||13.8%|
|Pedal Load lb.||50||50||50||50||50||50||50||50||50||50|
|Line Pressure p.s.i. (single cylinder)||1047.76||912.72||814.87||727.61||649.61||565.88||482.17||415.75||362.17||318.31|
For the clutch the need is not the same, (generally full action and off), so perhaps start with 0.70” and adjust, as above, if necessary.
If there is a hydraulic handbrake, start with similar to the rear pedal box cylinder, and go smaller if more urgency is required!