Hugh wrote:
>However, this is not what we are doing with my 1958. We are running two
>lines independently of each other. One line goes to the front brakes, the
>other, connected to the front pot, goes to the rear. Any proportioning will
>be done by the MC itself. The pot nearest the fire wall is smaller and the
>lines attached to it are smaller in diameter than those going to the
>rear.The theory is that the front brakes, responsible for 70%, or
>thereabouts, of
>stopping will get applied fractionally sooner and with slightly higher
>pressure.
Hugh, I am sure you have looked into this more than I (since I am fortunate
enough and all 68's have dual circuits) but here is the "engineering"
perspective. The diameter of the lines has little or nothing to do with
the pressure applied to the brake cylinders. The reason is that the
flowrate of the brake fluid is negligible, and there are no pressure losses
due to flow friction. This may be counter intuitive, but that's how it
is! The distance that the pressure signal travels from the MC to the brake
cylinder is also almost irrelevant. The pressure travels at the speed of
sound in that medium (brake fluid) which is too damn fast for the length of
the relatively short lines, even for a car the size of an Imperial! You
are right though, the front brakes need a lot more braking "power"
(especially if its disk brakes, since disks need far higher pressure to
operate). In your case (drums front and back), this is probably achieved
by using larger diameter brake cylinders on the front. So, even though the
line pressure may be the same front and rear, the brake force on the front
drums is higher. That may be why the front pot is larger.
>I am becoming quite leery of the new set up simply because, without the rear
>brake line attached and its aperture on the MC blocked, we have achieved
>phenomenal braking on the front wheels. However, if a small leak in a rear
>wheel cylinder can prevent pressure being achieved on the front at all,
>what's to stop that from happening again, if another leak occurs? That must
>mean there is some kind of link between the two pots.
Hugh, the way I understand this is as follows (if I am wrong, please
someone correct me). The dual master cylinder is basically one long
cylinder with 2 pistons connected in series. You push the first piston
with the brake linkage (the one at the back), and the pressure that builds
up between the two pistons (that energizes the front brakes [I think, I
need to look at my car!]) pushes the second piston (at the front) that
energizes the rear brakes [which pot energizes which brake is opposite from
your post, but you may be right). There is no direct mechanical link
between the second piston and the brake linkage, other than a spring. So,
if the forward cylinder looses pressure (due to air in the system or
whatever) both systems will be affected, because the air in the forward
cylinder will compress allowing the brake pedal to sink to the floor. Same
if the rear cylinder looses fluid. However, since at least one system will
maintain fluid, by pumping the brakes you may eventually build some
pressure to stop the car and not crush into the back of the Dodge truck!
D^2
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