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I searched pretty thoroughly before I posted and I found conflicting information. However it may very well be a 1 1/32" bore.

If it is, I wonder if it would be prudent to switch to 15/16" MC to improve my braking? Got the 10 3/4 inch Wilwood 4 piston caliper rakes all the way around, and at 140 mph and 3450 lbs braking ain't great.

Opinions?

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I have the big bore one in my Dart Sport (because I couldn't find the one I intended to use until later, both OE parts) and it works fine, no big deal. Compare the area of your caliper pistons to stock to see how much the Wilwood brakes change things. You've got more pistons but the total piston area may be more or less than stock. I bought an AC/Delco knockoff 15/16" from RockAuto recently for $14 plus shipping.

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I searched pretty thoroughly before I posted and I found conflicting information. However it may very well be a 1 1/32" bore.

If it is, I wonder if it would be prudent to switch to 15/16" MC to improve my braking? Got the 10 3/4 inch Wilwood 4 piston caliper rakes all the way around, and at 140 mph and 3450 lbs braking ain't great.

Opinions?

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I got the 15/16 bore master from Dr. Diff. I love the feel of it. Wilwood fronts, drum rear....manual.

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Same here, changed from the mopar large bore master cylinder. Much improved

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What do I mean by poor braking - I hit the pedal and the car slows but it just is like the brakes are fading. Does not feel like a lot of grab.

I have been running these brakes for about 15 years and the pads and rotors are original (not a lot of runs in that time period). Perhaps it is time for new rotors and pads - the rotors have some grooving - not sure if that is affecting braking or not.

However, I also question if a 15/16 inch MC might not be an appropriate upgrade.

I do have the Doc 15/16" MC in my 67 Coronet street car with 73 Duster discs up front and 10 1/2 drums in the rear. The braking stinks - currently trying to figure that one out. I can lock up the rear brakes pretty easy on dry pavement so the issue has to be in the front.

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Do you have a adjustable prop valve on the 73..
if you do which line is it on, front or rear.. is
the pedal hard to push with very little movement
to the pedal... if you go with a smaller master the
pressure will go up but the pedal will travel farther

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What is the bore size of a Mopar Performance P5249271 master cylinder?

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Do you have a adjustable prop valve on the 73..
if you do which line is it on, front or rear.. is
the pedal hard to push with very little movement
to the pedal... if you go with a smaller master the
pressure will go up but the pedal will travel farther

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Quote:

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Do you have a adjustable prop valve on the 73..
if you do which line is it on, front or rear.. is
the pedal hard to push with very little movement
to the pedal... if you go with a smaller master the
pressure will go up but the pedal will travel farther

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I do have an adjustable proportioning valve on the rear brakes. I did keep the original block which I believe is only a distribution valve not a proportioning valve - is that correct?

When I installed the discs had a larger bore MC and the pedal had little travel and was very hard. Its better with the Doc Diff 15/16" MC.

Plan to measure the pressure at the calipers and at the drums. In addition I plan on replacing the 73 A Body calipers with the larger piston 76 F body calipers. That should act like making the MC bore smaller without affecting the rear brakes.

Interesting that before I put the discs on the car, the brakes really stunk. I replaced the rear drums and shoes (10 x 2 1/2") but not the wheel cylinders.

Installed a cast iron E Body MC with a larger bore when I installed the discs. Braking stunk and pedal was very hard so I went with the Doc Diff 15/16" MC. Helped a little and pedal is better but braking still stinks.

With the proportioning valve set at about 50% my daughter spun the car out at the finish line because the rear brakes locked up. A couple of weeks ago I did some trials on dry pavement - stopping hard from about 10 mph. With valve fully closed the rear brakes would lock up every time and the fronts were acting almost like they weren't there. At 50% it was questionable and at 90% it seemed better. I left it at 90%.

Other than checking pressures and the larger piston calipers, any suggestions?

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Was the pedal pressure good now or is it still sorta
hard... you might want to put more pressure to it
so the fronts see more pressure... also understand
that the prop valve just controls the instant spike
if you jump on the brakes.. in a few seconds the pressure
will equalize on both sides of the prop valve(it
takes longer with the valve mostly closed) but it
will equalize.... also... on the master which line
goes to the front of the master(front or rear brakes)
EDIT
are you running skinny front tires... if so you do
want to do more stopping with the rear tires(more surface
area).. I put my prop valves on the front tires so
I dont skid them and blow them out on the hit