Re: [Amc-list] changing Master cylinders from Wagner to Bendix

[Tom Jennings <tomj@xxxxxxx>]

On Wed, 25 Apr 2007, Jay wrote:

> The Wagner unit is obsolete and next impossible to obtain even though I was
> able to get a rebuild kit from RockAuto thanks to Matt Haas.

As pointed out by others, the Wagner has (or possibly, or
probably has) different bore diameters for front and for
rear. The bendix's are the same front and rear as far as I know.

Therefore, if you switch to a same-bore master cyl, you will
radically change the braking force balance front-to-rear.

If you could look up the master cyl bores you could probably
find a slave cylinder bore for the rear (or the front) so that
the ratios are preserved.

With systems where the master cyl bore is the same for front and
rear (all AMC Bendixes as far as I know) then the front/rear
hydraulic pressure ratio is the ratio of the master vs. wheel
cylinder:


 		master		wheel		ratio (master / wheel)
front		1"		1"		1.00
rear		1"		.8125" (13/16)	1.230

front to rear ratio is (front / rear)		0.813
front to rear ratio is 		0.813

... for this car.

Because the Bendix master is the same front and rear, all you
need to worry about here is the ratio of the wheel cylinders
front and rear (the master bore balances out of the equation;
you can ignore it).

The factory front-to-rear ratio is almost always perfect.

You can change wheel or master cylinders as long the front to
rear ratio is the same -- that will maintain braking balance.
If you make the master cyl bore larger, the pedal pressure will
increase and stroke decrease; make the msater bore smaller the
opposite. As long as front to rear ratio is OK the car will
otherwise brake fine.



The Wagner's are different front to rear. I don't know what
the bores are, the data below is FAKE, it just illustrates
the solution:


WAGNER CAR WITH WAGNER CYL
 		master		wheel		ratio (master / wheel)
front		1"		1"		1.00
rear		.875" (7/8")	.9375" (15/16)	.933

front to rear ratio is (front / rear)		1.072

... for this (fake) car. YOu ahve to include the two master
bore diameters to work out the front to rear ratio.





OK, so now you change the master cyl such that it's 1" bore
front and rear. You need to change a wheel cylinder to preserve
the front to rear ratio. Let's do the rear wheels:

WAGNER CAR WITH BENDIX CYL
 		master		wheel		ratio (master / wheel)
front		1"		1"		1.00
rear		1" (new)	????		.933

front to rear ratio is (front / rear)		1.072



We need to find wheel cyl diameter to preserve that ratio to keep
balance.

 	wheel= ratio / master

 	wheel= .933 / 1

 	wheel= .933" diameter

15/16" is .9375". That makes a .4% error. Close enough! Assuming
that a 15/16" Wagner type wheel cyl is available.

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