Brake Advice - Master Cylinder Choice

[YellowPinkie]

I'm needing to upgrade the front brakes on my all drum 66 Mustang.

I've found some discs to fit the front and I'm sorting out some calipers.

I need a small dual circuit master cylinder/servo combo to suit the new disc/drum setup - any ideas? Does the volume of the master cylinder matter? Obviously, I know if I've got some massive 6 pot calipers I couldn't expect to operate them with something off a Ka, but I'll be looking for an OEM single pot caliper (probably). Do I need a master cylinder that is a from disc/drum setup - or will a disc/disc unit operate my stock rear drums?

Thanks for your help.

[Hilux]

Yes the master cylinder size has a major bearing on their efficiency for thats what offers a mechanical/hydraulic advantage alongside the pedal leverage ratio. A larger servo will also add assistance.

Google `pedal boxes` and `bias bars` also

[flemke]

To add a bit to Hilux's points:
- The master cylinder piston travel has to be compatible with the rest of the system. If you don't have long enough travel, you can bottom out the piston, or not push enough fluid.
- The piston diameter will directly affect required pedal pressure.
- The collective areas (pi x radius squared) of all the caliper pistons determines how much fluid you have to shift. This in turn will influence master cyl diameter, master cyl piston travel, and pedal ratio.
- If there is a meaningful change in the distance between the axle and the centre (along the radius) of the pad, relative to the equivalent distance with the drum & shoe, your braking torque will change. The longer that radius, the less pressure is required.
- This is all before we get into the question of the relative efficiencies of discs v drums.

Bottom line is that all the parts have to be coordinated. AP are very helpful. The need to know a bunch of data about your specific car: weight on front axle, weight on rear, CoG, wheelbase, max speed, etc.

Start out by calling and asking for someone in road cars/technical:
http://www.apracing.com/info/contact.asp

If you've no experience and no guidance, you're crazy to change a key variable in your braking system without making sure beforehand that it's all going to be compatible.

[YellowPinkie]

flemke said:

If you've no experience and no guidance, you're crazy to change a key variable in your braking system without making sure beforehand that it's all going to be compatible.

Damn right! I understand the science, just not how it's applied in the real world of bits on the shelf.

The bit I don't understand is on a diagonal split disc/drum setup, where the master cylinder chamber needs to shift the volume for, say, the front left caliper, but less volume for the right rear slave cylinder. I assume this is where the proportioning valve comes in. On a Fiesta I used to have, there was a proportioning valve connected to the rear suspension - as the nose dipped under heavy braking, the rear would rise operating the valve reducing pressure to the rears. It worked really well.

On a front/rear split I assume the volumes of the separate chambers of the master cylinder are different...?

[flemke]

YellowPinkie said:

On a front/rear split I assume the volumes of the separate chambers of the master cylinder are different...?

You can vary that with different MC piston diameters or a bias bar, but, before you get there, differences in F-R braking torque (radius from axle to centre of pad) and pad swept area (how much disc surface the pad is biting into) will affect the balance.
Seriously, call AP Technical, ask for someone who does road car applications. I think that Tom Ferenc is still there, that was his area. His number should be 0247 688 2719
Alcon are supposed to be extremely helpful as well, but I myself have not dealt directly with them.

[YellowPinkie]

Good Advice, will do.

[160steve]

You could also try John @ Performance Braking (01600 713117), he worked for lucas/girling for many years as a development engineer and his knowledge of historic braking systems is second to none!