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Brake bias, caliper BORE size and PAD recomendations... #1233067
05/14/12 02:11 AM
05/14/12 02:11 AM
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Granite Bay CA
Kern Dog Offline OP
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Today my brother in law and I finished swapping a 904 into his Duster that was a 4 speed. On the spirited test drive, we ended up "testing" the brakes as well. Years back I swapped in a front disc setup from an 88 5th Avenue. The rear brakes are 10 inch drums. The brakes on this car would rival any new car. Non power master cylinder, but the pedal feel is great and this sucker just stops as if I threw out a grappling hook. What the heck?
The brakes in the Charger have been a pisser. In February I added frame connectors and torque boxes. I had to reroute the front to rear brake line. The brakes were bled afterward, but they have given me trouble ever since. Since 2006, the system was: 12" Cordoba rotors, 75 Dart calipers in the front and the 10.7" rear disc brake kit from Dr Diff. I also used a power booster with a master cylinder from a 75 Dart. Braking was excellent except at low speeds since the idle vacuum was so low. I thought that switching to a NON power master cylinder would result in improved braking. It IS a little better, but after driving the BILs Duster, I have MUCH room for improvement.
I've read in Mopar Action that the slider calipers have a tendency to allow the pads steel core to DIG into the sliding mechanism, causing the braking action to be radically compromised. I'm pulling the wheels tomorrow to get a look, but the point of this post is.... What relationship does the caliper bore size have to do with brake effectiveness? This Duster flat out STOPS better than any other A body that I've ever driven. It beats the Charger by a long shot. Its probably better that my 2007 Ram, and I've always been impressed with the truck. The only difference on the Duster is the calipers. I thought that I heard that the calipers on the 76 A bodies as well as the FMJ cars had different sized pistons. Does anyone know the specs on the different single piston calipers? I am tempted to swap in a pair of these later calipers if there are gains to be had. I really need the Charger to have brakes that equal the engine!
Thanks, Greg.

Last edited by Frankenduster; 05/14/12 04:34 AM.
Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233068
05/14/12 02:33 AM
05/14/12 02:33 AM
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Factory Dart calipers do have a smaller piston ,so less pressure and could be a pad issue also.Try swaping your buddies brakes onto your knuckles for a test?

Re: Brake bias and caliper BORE size. [Re: pro451bee] #1233069
05/14/12 02:46 AM
05/14/12 02:46 AM
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Granite Bay CA
Kern Dog Offline OP
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THATs funny. He wouldn't object, but I dont want to jinx his car. It works, so I will let those sleeping dogs right where they are.

Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233070
05/14/12 04:34 AM
05/14/12 04:34 AM
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Kern Dog Offline OP
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Whenever I have doubts about interchange, I go to www.RockAuto.com. That site is great. I just look for part numbers that match and it really clears things up.
Their site shows that the 73-75 A body cars used front calipers with 2.6" pistons. The 76 A body and ALL FMJ cars used calipers with 2.75" pistons. The Raybestos line showed those specs. Since the calipers and hoses can physically interchange, I'm inclined to just go ahead and get a pair of the 2.75 bore calipers.

Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233071
05/14/12 07:54 AM
05/14/12 07:54 AM
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Marysville, O-H-I-O
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well, a larger piston in the caliper gives you more clamping force for the same size master cylinder, so long as your master cylinder has enough stroke volume to operate the pistons without bottoming out.

I just put big brakes on my dakota, went from 10" drum brakes in the rear to 4-piston Brembo calipers on a 13" rotor. on the stock master cylinder, my pedal would go to the floor without generating any real braking effort, and I had to step up to a larger diameter master to get enough stroke volume for the big rear brakes.


I think I read on one of Andy's posts that it's not about pad size, but rather, it's all about caliper clamping force. the size of the pad only determines how fast the pad wears.


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Re: Brake bias and caliper BORE size. [Re: 70Cuda383] #1233072
05/14/12 09:03 PM
05/14/12 09:03 PM
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Kern Dog Offline OP
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Heres a few more tidbits:
I pulled the wheels to get a look at the calipers. The channel in the caliper mount that the caliper and inner pad ride upon had a few indentations. I read about this in Mopar Action. Rick wrote that this area is problematic, and that a recess here could cause the pads to hang up. I took a flat file and milled down the area until it was smoothe. The pads move free now. I drove the car and noticed no improvement.
On the way home, I stood on the brakes harder than ever. It finally locked the REAR wheels. I haven't been able to get a tire to skid in months. Now with the rear skidding, it appears that I need the bias to shift towards the front. The larger piston calipers should help, right?
Oddly, the car also stops BETTER in reverse than going forward. I don't understand THAT at all. I bought 2 Raybestos 2.75 calipers and new pads. I'll get them on tomorrow and report back.

Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233073
05/15/12 02:09 AM
05/15/12 02:09 AM
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Kern Dog Offline OP
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Does anyone have a suggestion for caliper piston bias for 4 wheel disc applications?
Like many people here, my cars are a sort of "Mr Potato Head" creations. Lots of parts from different cars. A stock bias may not be right for a lowered car with a cornering suspension. There isn't the same degree of brake dive with the stiffer suspension, so in theory, the rear brakes are less likely to skid due to rear end rise.
The stock setup for a 75 A body disc/drum car is as follows: 2.60" caliper pistons and 15/16" drum wheel cylinders. This is also with 1" bore NON power master cylinder.
MY car has 2.60 front calipers and what appears to be 1 7/16" rear calipers. (Dr Diff Cobra brake kit) It worked great with the power setup except at low speeds/low vacuum.

Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233074
05/15/12 09:27 AM
05/15/12 09:27 AM
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Marysville, O-H-I-O
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I always thought you wanted 1000-1200 psi at the caliper for a disc brake system. as far as front/rear bias, if you can't get it dialed in perfectly with just piston sizes, I would err on the side of caution, and go bigger on the rear, then use an adjustable proportioning valve to dial it back so they lock up evenly. you NEVER want to put any kind of prop valve or pressure limiting valve on the fronts!


Also, car suspension can greatly effect brake bias. if you have a soft or worn out suspension, all your weight will shift forward, the nose will dive down, and the rear end will get very light. this will cause the rear to lock up before the fronts as well, even if the brake system is evenly matched.


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Re: Brake bias and caliper BORE size. [Re: 70Cuda383] #1233075
05/15/12 10:03 AM
05/15/12 10:03 AM
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No too sure your disc/drum master cylinder is the best choice for an all disc system. Lack of volume for the rear calipers.

Also, if you flipped the spindles for sway bar clearance make triply sure the bleeder valve is at the high point of the caliper bore, otherwise you will trap some air and you will have lousy brakes.


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Re: Brake bias and caliper BORE size. [Re: Supercuda] #1233076
05/15/12 10:09 AM
05/15/12 10:09 AM
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Quote:

No too sure your disc/drum master cylinder is the best choice for an all disc system. Lack of volume for the rear calipers.

Also, if you flipped the spindles for sway bar clearance make triply sure the bleeder valve is at the high point of the caliper bore, otherwise you will trap some air and you will have lousy brakes.




I thought I read somewhere in one of his threads that he has an aftermarket, disc/disc master with a 15/16" bore?


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Re: Brake bias and caliper BORE size. [Re: 70Cuda383] #1233077
05/15/12 10:21 AM
05/15/12 10:21 AM
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went baack an reread this thread, only mc he mentioned is from a 75 Dart, dunno what he has in it for sure


They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
Re: Brake bias and caliper BORE size. [Re: Supercuda] #1233078
05/15/12 10:49 AM
05/15/12 10:49 AM
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Quote:

went baack an reread this thread, only mc he mentioned is from a 75 Dart, dunno what he has in it for sure




yea, he said it in his other thread, I went back to verify that I did infact read it in his thread...so many brake threads on here right now!

this one he posted in the Q&A forum...


https://board.moparts.org/ubbthreads/show...e=0#Post7206072


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Re: Brake bias and caliper BORE size. *DELETED* [Re: Kern Dog] #1233079
05/15/12 11:21 AM
05/15/12 11:21 AM
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In sixty-five I was seventeen and running up one-o-one I don't know where I'm running now, I'm just running on Jackson Browne-Running On Empty
Re: Brake bias and caliper BORE size. [Re: runinonmt] #1233080
05/15/12 01:02 PM
05/15/12 01:02 PM
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he's got 4 wheel discs


They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
Re: Brake bias and caliper BORE size. [Re: Supercuda] #1233081
05/15/12 03:06 PM
05/15/12 03:06 PM
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Kern Dog Offline OP
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I've been pretty confused with these brakes, I forgot to mention in this post that I currently have an aluminum 15/16" 4 wheel disc master cylinder on the car. It stopped BETTER with the power booster and a factory 75 Dart disc/drum master!

Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233082
05/15/12 03:31 PM
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The power brake m/c for a 75 Dart looks to be 15/16".

the manual one is 1 1/32".

Either your new m/c is flaky or you need to upsize it.


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They say there is always the exception that proves the rule.
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Re: Brake bias and caliper BORE size. [Re: Supercuda] #1233083
05/15/12 04:16 PM
05/15/12 04:16 PM
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Polson, MT
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Sounds like he had an iron 15/16" bore master with power brakes.

He now has an aluminum 15/16" bore master with manual brakes.

All else being equal, a larger bore master cylinder will DECREASE line pressure at the calipers.

Re: Brake bias and caliper BORE size. [Re: Supercuda] #1233084
05/15/12 06:48 PM
05/15/12 06:48 PM
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Kern Dog Offline OP
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Quote:

The power brake m/c for a 75 Dart looks to be 15/16".

the manual one is 1 1/32".

Either your new m/c is flaky or you need to upsize it.




I bought the aluminum master cylinder from Dr Diff. He has the best rep in the business, so I trust his judgement.
Regarding the bore diameter of the stock stuff, it appears to me that you may have your spec's backwards. Wouldn't it make more sense for the power applications to use a LARGER bore since they have assistance to push it? Just an idea. I haven't measured the iron power M/C yet.

Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233085
05/15/12 06:50 PM
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I looked them up at Rock Auto, assuming they had it right I simply posted what they spec'd.


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They say there is always the exception that proves the rule.
Don't be the exception.
Re: Brake bias and caliper BORE size. [Re: Kern Dog] #1233086
05/15/12 08:41 PM
05/15/12 08:41 PM
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'73-'75 power brake equipped A-bodies used (smaller) 15/16" bore master cylinders because the booster linkage yields a mechanical disadvantage.

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