Re: [Amc-list] rear lockup problem still

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

This is getting into "not for the feint of heart" territory here! Apply
with caution, but note that I've some experience in all this with
various hot/street rods, and have discussed with other rodders who have
modified/designed their own braking systems. I don't know that I'm an
expert, but I'm knowledgeable enough to make some "use at your own risk"
suggestions. Remember, no information from the list is without your own
risk -- I'm not responsible for any advice you decide to take! Do some
more research THEN make decisions you can live with. End of "legal
speech"...

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

The Mustang PBR calipers are dual piston, but the pistons are much
smaller than the single piston AMC/Jeep calipers. Jeep XJ calipers are
the same 2.6" as late model AMC cars. Mustangs are:
93-98 Cobra - 38mm (1.5")
99+ V6 & GT - 44.5mm (1.75")
99+ Cobra - 40mm (1.575")
2000(+?) Cobra R - 36mm & 40mm (not sure how these are made and why two
piston sizes are listed)
The rear caliper for 94+ models is a Varga unit with built-in park brake
similar to GM rear calipers w/38mm piston. This would be a good one to
use for rear disk conversions. Different years use different rear rotors
-- either solid or vented. Six cylinder parts would be a great upgrade
for AMCs and be relatively easy to find in salvage yards. For more on
the calipers see
http://mjbobbitt.home.comcast.net/mustang/sn95brakes.html. 

Getting back on track here, the 38mm PBR brakes have a total of only 3.6
square inches surface area behind both pistons combined (1.8 per piston
-- rounded up from 1.766). The stock AMC/Jeep calipers have 5.3 square
inches (5.3066). Based on that the PBR calipers need less fluid, and may
not have as much stopping power. What helps is the fact that the power
is spread more evenly over a larger area though, so it's hard to say
which would actually stop better. More pistons aren't always better! I
think the Mustangs went to two piston brakes for better clearance, not
better stopping. There's a lot more to it than just piston and pad area,
type of pad and rotor makes a difference too. But based on this I'm not
certain the Cobra brakes are better than factory disks, though they are
certainly better than the drums your 62 originally had. The Mustang and
62 Classic are close to the same weight, so you should see an
improvement. 

The PBR calipers should work better with MORE pressure, not less. In
other words, they want a SMALLER diameter master cylinder. According to
this website, however, a 1" bore master should be just fine:
http://mjbobbitt.home.comcast.net/mustang/product.html (scroll down to
the conversion kit descriptions). 

Power brakes used a 1.06" master, non-power a 1", with the exception of
94-98 Cobras, which used a 15/16" master (for added pressure - 93 Cobra
used a 1"). No mention if the 94-98 Cobras were boosted or not. The 1"
non-power/1.06" power MCs is right in-line with what AMC did on 71-76
Matadors (1.06" non-power, 1.125" power).   

I don't recall what year and model your brakes are from. If you don't
think the front brakes are getting enough power, a 15/16" Cobra MC will
increase pressure and add a noticeable amount of stopping power. That
will increase power to the rear brakes also, making them lock faster
than they do now, making the adjustable proportioning valve even more
important. 

The Cobra used disks front and rear, so you'll need an in-line 10 psi
residual pressure valve for the rear drum brakes. You can get those from
any hot-rod shop. Better yet, get a set or rear calipers and rotors from
a Mustang. The rotors should fit your hubs, depends on the center
diameter. If they don't there's a variety of rotors to use -- Jeep XJ
4x4 front rotors will definitely fit (I've tried them for fit!), and any
number of Ford rear rotors can be measured. 

You'll ruin your brake backing plates, so make sure you can make a
bracket first! A bolt on bracket is doable, use the bolts for the
backing plate. The backing plate must be cut near the axle tube flange
as it holds the seal and axle in. You might be able to use the backing
plate as a mount, just notch it where the caliper needs to be. It's
strong enough -- the drum brakes put most of there force on the top
anchor when stopping the car. Depth of rotors will be the deciding
factor, and the calipers have to clear the wheels. If you used spacers
for the wheels you have there's plenty clearance, if not, 2" (roughly
the distance between the hub flange and backing plate) is probably not
enough. 

Relation of Master Cylinder Bore to Pressure
Bore Size    lbs on pedal    PSI out 
1-1/8           75            453 
1               75            573 
7/8             75            748

Pressure isn't linear, but there is about a 100 psi difference between a
7/8 and 15/16" MC bore (would be 87.5 psi if it was linear). These
pressures and pedal inputs are based on a 6:1 pedal ratio. I have no
idea what most AMCs use for a pedal ratio. I posted the chart to
illustrate the difference in pressures only. Actual pressure may not be
the same, differences are proportionate to ratio.


----------------------
Date: Tuesday, February 13, 2007 07:18 PM
From: d stohler <das24rules@xxxxxxxxx>

I'm aware of the pressure/volume differences.  A recap
tho. I have put the mustang pbr dual piston calipers
up front. The master cylinder is off of a jeep. I need
to find out the size of the bores on the wrangler. I
do not know what year it is from. I need to find out
what the bore sizes are on the mustangs also, see what
the difference is. It don't feel like the front is
getting enough pressure. Probably to small a bore for
the front calipers and to large one for the rear?

dave stohler

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