This is in response to Kurt’s post at #35. Consequently, it’s lengthy, and I apologize for that. But I hope it shares some good information in a clear and concise way. Please let me know if you see any mistakes, and I’ll gladly edit it. Thanks again to Kurt for his great summary of the posts and his clear writing. He's an example to us all.
1) The first thing my new buddy, Kurt, mentions is extra pedal travel associated with the 15/16” (I got it right this time!) bore master cylinder. I think that was mentioned earlier as well, so we seem to all agree that it’s a trade off. The smaller the bore in the master cylinder, the less effort for the same clamping force, but at the cost of pedal travel. That should be true regardless of manual or power brakes. Perhaps one person’s description of “spongy” might be just the extra travel. I know Kurt clearly described what he meant by spongy above. I’m not going to try to define everyone subjective terms. I’m just pointing out that one person might conjure up a different idea of “spongy” or “low pedal” than what the writer intended when using the term. I’m just acknowledging the vagueries we all deal with when trying to describe a feeling, sound, smell, etc. Moving on….
(2) The next point Kurt brought up is '73+ A body calipers vs. F/M/J calipers vs. B-body calipers. This is a good point. Tom Condran’s book (and R. Ehrenberg’s Disco-O-Tech article, and I think Frank Adkins's book) discuss two different brake caliper adapters (brackets): slider caliper adapters and pin caliper adapters. Each has two sizes (large and small). Most people run sliders because that’s what most Mopar-parts-based disc conversion kits use. That’s not because those are better than pin calipers, they are probably not. I’m sure someone will argue that they are better. I’m not getting into that argument, but I will say that the slider design is easier to use in a conversion because of the F/M/J calipers that change the hose attachment points. This allows the '73-76 A-body knuckles to be run on opposite sides if there are clearance issues. I’m an A-body guy, so for my ’72, it allows just that. Consequently, it allows for the normally front-mounted calipers to be mounted to the rear of the wheel.
Kurt also mentions other posts discussing large calipers. Most likely, this is the difference between the 2.6” single-piston A-body slider caliper, from 1973-75, and the 2.75” single-piston A-body slider caliper, from 1976. As Condran points out, “the larger bore calipers require less fluid pressure for the same amount of force. “[F]luid pressure is constant in any closed system. Pedal effort is thus inversely proportional to the ratio between the effective piston areas of the master cylinder and caliper.” In short, for this discussion, the main difference between “A-body calipers” and "F/M/J-body calipers” depends on year of caliper. The 1976 A-body slider caliper piston bore is 2.75”, the same as on the F/M/J slider calipers. So pedal feel should not be affected. However, if the A-body slider caliper is for 1973-75, the bore is smaller (2.6”), so pedal travel and effort will be different.
(3) As for the chart Kurt shared, from =Performance Handling for Classic Mopars, Tom Condran explains that the chart shows two different all-Mopar brake kits (that Chrysler never used all together) consisting of parts selected from the universe of all Mopar brake parts for that era. You can run those kits on any A-, B-, or E-body. He suggests the pin caliper design. But, if you’ll notice, the photo is only of a partial page. The cutoff text below the chart explains that there is one option to the chart: to use slide-mount calipers. He notes to use “’76 A-body, ’76-79 2-door B-body, or ’79-81 R-body slider calipers (or off F/M/J if hose fitting and bleed screw are at the same end. Use brackets [(adapters)] from any B-, F/M/J-, or R-body, with rotors the diameter of your new ones, plus flex-lines in the right length.”
Condran has other charts in the book as well. For instance, there is a chart which shows a much easier disc conversion swap for A-bodies. That chart says to to use the sliding calipers I just mentioned. It has some other changes as well. But Condran says that the best (not the easiest) brake kits consist of the parts in the chart Kurt provided.
Condran also points out that some other changes might be necessary to use this ultimate kit. For instance, for a 1972-earlier A-Body, using this kit requires bigger upper and lower ball joints. This requires some kind of change to the upper control arms. Condran’s 1998 book discusses using the 1973-76 A-body upper control arms. The book was written when lots of the required parts were in junkyards, and Condran goes into detail about how to identify and remove parts from the junkyard, recondition them, and install them on your car. In contrast, some modern kits for sale use the cheaper option of an adapter to install bigger ball joints on the old style upper control arms.
Condran also explains the anti-swaybar interference if trying to run forward-mount calipers on pre-1973 A-bodies, so he suggests swapping out the K-member to a '73-76 and the correlating anti-swaybar. Doing that would allow you to run both calipers styles (sliders and pin) in front of the wheel, as designed. Every modern kit I see uses the slider calipers instead because certain ones allow for rear-mounting the calipers. However, Condran, Ehrenberg, and Adkins all warn against this due to issues with routing the flex lines. I am not certain how these rear-mount caliper kits address the flex-hose routing. Nonetheless, whatever their fix is, it only works for sliders; it does not work with pins. (CAVEAT: my research focuses on A-bodies, so I may be incorrect about modern kits not using pin-style calipers for B-bodies. But I don’t think I am. Please correct me if I’m wrong).
(4) As for the larger rotors (11.75” vs. 10.85”), the only difference is the size of the rotor and the size of the caliper adapter (bracket) needed. As I mentioned, there are two sizes for each of the two types (sliders and pins). The only other necessary change is to run at least a 15" wheel.
(5) Kurt noticed the chart calling for the 15/16” master cylinder for power brake setups. This is one place where Kurt misunderstood the chart, and it’s an easy mistake to make if one has not read the book. Early in the book, Condran advocates for manual breaks, explaining multiple ways to reduce the pedal effort required for disc brakes, without using a booster. As I discussed above, one of those ways was using a 15/16” bore master cylinder. In the chart, Condran is simply pointing out that the master cylinder, for a '73-76 A-body power brake car, has a 15/16” bore. So that chart is a parts list. He is suggesting that master cylinder for use on a manual brake/front disc car.
(6) Lastly, I was not aware of the disagreement with Erhenberg over whether the F/M/J knuckles alters geometry compared to the '73-76 A-body knuckles. I think I can solve that issue right here and now, though. Those who disagree should just do the math, because math is either right or wrong. The truth is either it actually changes the geometry or it doesn’t. I will add only one more thing: as financially poor as Chrysler was in the late 70’s, I doubt they would have spent money on new engineering and changing parts unless they had to. I’ll leave that there.
I hope that clears some things up in my original post.
EDIT:
Condran also notes that fresh, single piston calipers are difficult to bleed completely. If experiencing difficulty, pull the clips and dismount the caliper. Insert a 1" chunk of wood between the pads, elevate the caliper above the hardline "up-loop," and hold it sideways with the piston at the top. Bleed the brakes and shake it every few pumps.
As for brake feel, he adds that, disc brakes change the feel of the brake pedal. Once completed, the pedal becomes firm at least one inch above the floor. Stopped, the pedal travels further and is softer, but in service they begin working high in travel and, unlike with drums, you will seldom apply full pressure to the pedal. If the pedal still goes to the floor, check rear brake adjustment because automatic adjusters are often either frozen or ineffective. Then recheck pedal height.
