Hi

What are typical bore diameter master cylinders please to use on a book 750 MC Locost 1300cc Crossflow racer running Cortina front hubs and calipers and Ford Escort rear axle with drums please? I will be using twin Girling or Willwood master cylinders (integral or remote) on an OBP pedal box with a brake balance bar.

Just to get me going as I realise that this set up gives an opportunity to change bore cylinder as well as balance using the bar. My knowledge of old Ford Escorts and Cortinas is limited so not sure what to start with, either 5/8”, 7/10”, 3/4” or 7/8”.

Currently planning to use a cable operated clutch actuating mechanism on a Ford Type 2 gearbox (standard or Escort Sport) with a converter for cable operation on the OBP pedal box - if I were to switch to a different Ford gearbox (Type E, 2000E or Type 9) with a separate Ford or aftermarket bellhousing to suit hydraulic clutch actuation with a single Girling clutch master cylinder what would be typical bore sizes for the clutch master and slave cylinders please?

I am building a road legal hillclimb car similar spec to a 750 MC 1300cc Crossflow Locost racer.

Thanks
Mike

Hello Mike,

What pedal ratio does the OBP Pedal Box give you?

The main reasons behind master cylinder choice are;

1) Pedal effort: smaller cylinders will reduce the force required to slow the car. However calipers require more fluid pressure to produce the same torque as a drumbrake; so front and rear have different needs.
2) Fluid consumption: smaller cylinders will 'run-out' of piston travel when operating brakes that consume more fluid. Obviously large calipers consume more fluid (per mm of caliper piston travel) than drumbrakes (per mm of wheel cylinder piston travel)

[Edited on 5/8/24 by JAG]

Thanks Justin for a very helpful explanation of how to understand the problem. I’ll take a look at my pedal box and work out the ratios as you suggest.

Best wishes
Mike

Further help - maybe.

My car weighs 565 kg.

It has a Brake Pedal ratio of 3:1 and a Servo Boost ratio of 3:1.

I have M16 front calipers with 54 mm diameter pistons and 8" drumbrakes at the rear. Rear drumbrakes have 22.2 mm pistons in the wheel cylinders.

My car has a tandem master cylinder with a 20.64 mm (13/16" ) diameter piston. It has sufficient fluid reserves for the front calipers and rear brakes.

Tandem master cylinders reduce the pedal effort required (for the same piston diameter) but they increase pedal travel.

---------------------------------------------------------------------------------

No Servo will increase the pedal effort on your car. You can mitigate this by increasing your pedal ratio.

A twin master cylinder set up will increase pedal effort but reduce the pedal travel. You can mitigate this by increasing your pedal ratio.

You will probably need a much smaller diameter master cylinder on each circuit.

The biggest issue is making sure each of your twin master cylinders has enough piston stroke to keep operating the brakes even at very high pressure (during rapid braking).

[Edited on 8/8/24 by JAG]