Quite a few factors contribute to determining the master cylinder size
needed. Weight balance of the car and other factors determine the
amount of braking desired in the front vs. rear. Rotor size, pad
material and caliper piston area determine the PSI of hydraulic pressure
required to cause the desired braking in each circuit. Master cylinder
piston area determines the amount of mechanical force required on the
piston to produce the desired hydraulic pressure. And of course
mechanical leverage determines the amount of force needed on the pedal
to produce the required mechanical force on the master cylinder piston.
Depending on the equipment used, the required pressure can be quite
different in the front and rear circuits. Large rotors and calipers with
four big pistons in the rear will not require much pressure to do alot
of braking. If this is used with stock calipers in the front, alot more
hydraulic pressure will be required in the front than the rear. A
larger rear master cylinder will be required to keep the balance bar
reasonably centered. With a single piston master cylinder, the front
and rear circuits must be designed to produce the desired braking force
with the same hydraulic pressure. Doing the same with a dual master
cylinder will allow the use of two identical cylinders. Wilwood offers
most of their calipers with a wide range of piston sizes to help in
balancing the system. However, if you need to change the hydraulic
ratio of an existing system I am sure one master cylinder costs less
than two calipers.
Increasing the hydraulic advantage by reducing the master cylinder
piston area or increasing the caliper piston area or increasing the
mechanical advantage will decrease the force required on the pedal.
Going too far can make the pedal feel spongy. I once worked on a 550
Spyder replica that had "spongy" brakes. There was no problem locking
up the brakes but it felt like there was air in the system. I finally
gave up trying to bleed non existent air from the system and figured out
that there was so much mechanical and hydraulic leverage that the
calipers were over powered and flexing with little pedal effort. I
reduced the mechanical leverage in the pedal by a large percentage and
fixed the problem. Still plenty of braking without too much pedal
effort and now a nice firm pedal. A larger master cylinder would have
done the same thing. Of course if you go too far in that direction you
will not be able to stomp hard enough on the firm pedal to stop the car.
Richard Good
Good Parts
rob wrote:
>Can anyone explain why a brake system with a balance bar and duel masters
>would have different size masters ie a 5/8 bore for the front and a 3/4 bore
>for the rear also can anyone sujest the correct size masters for a TR6 with
>stock front brakes and rear disc brakes with a 4 piston willwood caliper.
>Thanks rob
>_______________________________________________
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