I see you've been waiting two days for a reply. I read it earlier but I can't answer some of your questions. What I CAN suggest is the brakes are already strong enough to lock up the wheels, so going to anything larger isn't going to give you more stopping power, including when pulling a trailer. What you will get is larger rotors will build up heat at a slower rate, and that leads me to an important point you should be aware of. A few decades ago, I had a car with a rotor I had cut a total of 1/8" under the legal limit, and under the other rotor. It never caused a problem, however, ...
On the newer vehicles, trucks in particular, two different size rotors will cause a horrendous brake pull when they get hot. Dodge had a 32 page service bulletin about this, and in fact I was involved with three of those trucks. One was a truck the owner had just bought. The previous owner sold it for this elusive problem shortly after having a brake job done. The other two were lemon law buybacks that would have been solved if the mechanics had just followed the service bulletin. Those rotors were about 1 1/4" thick, but on one truck, one rotor was.020" thinner than the other one. On the second truck, one was.007" thinner! That's the thickness of two sheets of paper, but it caused a very hard brake pull after about a half dozen fairly hard stops. The customer's truck got two new rotors. On both lemon law trucks, I cut the thinner rotors to true them up, then cut the thicker ones to match within.001". It's important to note the speed of cut has to be identical too. That results in the same number of grooves, and therefore the same pad-to-rotor contact area. This brake pull never shows up right away or during normal braking. It results from repeated hard stops such as when coming down a steep hill. On the trucks I was involved with, all of them stopped perfectly straight for seven hard stops on the same hill I used, then they pulled real hard on the eighth stop just a few seconds later. The cause was a change in the coefficient of friction that took place at the higher temperature, and since the rotors were different thicknesses, the thinner one heated up faster. By the way, that 32-page service bulletin had us checking everything under the sun from the alignment to replaced suspension parts, then they finally presented the solution on the last page or two.
The next thing to keep in mind is every vehicle has a proportioning valve in the brake hydraulic system that everyone forgets about. That limits the amount of fluid pressure to the rear brakes to reduce rear-wheel lockup. Most trucks and minivans have a height-sensing proportioning valve at the rear axle because they can have such a wide variation in loading. When cars are lowered or trucks are raised, that changes the amount of weight transfer to the front, and that changes the calibration needed in that valve. Lawyers and insurance investigators know all about altered ride height and they will use it to convince a jury that you were partly at fault for the crash when the other guy ran the red light because you were less able to avoid it, and they will be right.
You could run into the same problem with altered brakes. I'm less of an expert on potential lawsuits related to altered brakes, so the best I can share is you will no longer have the carefully-designed-in front-to-rear braking balance. A skidding tire has no traction, so when one does start to skid, you have to let off the brake pedal a little to keep it in traction. Three tires were not yet stopping to their fullest potential, but you let off the brakes a little. That is proof the stopping distance was longer than it could have been.
The first issue I mentioned was heat buildup from pulling a trailer. The second one has to do with the 3500s experiencing rapid front brake pad wear. Front pads should wear out about twice as fast as the rear shoes or pads. We were told that if there was a 3500 that was experiencing rapid front pad wear, we could increase the rear wheel cylinder diameter by 1/16". If there was a truck that was constantly heavily-loaded, we could increase the diameter by 1/8", but there was a hitch. Chrysler never used that size so the replacements had to come from a GM dealer. Those were factory-approved modifications to solve a problem under certain conditions, so there was no threat of a future lawsuit. The original wheel cylinder diameter was the result of those calculations that led to balanced front-to-rear braking.
Some people will tell you to switch from drum rear brakes to disc, but again, that isn't going to gain you any stopping power. On trucks and most older cars, drum brakes are of the "duo-servo" design which means the front shoe's only job is to apply pressure to the bottom of the rear shoe. This is referred to as a "self-energizing" brake and it's real effective. Drum brakes worked great for stopping heavy cars and trucks. The drawbacks are it's harder for heat to escape and they they take a long time to recover after driving through deep water. Disc brake pads are always in contact with the rotors, so they squeegee off water, and since they're in the open, heat dissipates better. Their drawback is they are not self-energizing so a lot of pedal pressure is needed. This is where a power booster becomes necessary. The main reason you'll find disc brakes on the rear is calipers present less moving mass compared to shoes, so they respond better to the pulsing action of an anti-lock brake system.
As for the parts interchangeability you're asking about, I can offer two suggestions. First, I use the Rock Auto web site for reference quite a bit. You can look up parts and compare the part numbers from various manufacturers for both applications. If the part numbers are the same, obviously that part fits both trucks, but when they're different, you don't know why. One could use a ball joint with a larger tapered stud, or it could simply have an additional threaded hole for a brake hose bracket. The next is to visit a Dodge dealer's parts department to see if they offer an off-road upgrade kit. They will also be able to tell you which parts were available as part of an optional brake system. This is where the off-road modification kits are nice. Those will include the proportioning valve, calipers, and wheel cylinders as a matched set, and they will include a master cylinder or specify if a different one is needed. Normally the master cylinder's bore size is a trade-off between pedal pressure and pedal travel. A larger cylinder diameter will move more brake fluid with less pedal travel, but you'll need much more foot pressure to stop the truck.
You DO have to change the master cylinder when changing from drum to rear disc brakes. Drum systems have a residual check valve in the port in the master cylinder. That keeps about ten pounds of fluid pressure on the lip seals in the wheel cylinders at all times. Ten pounds of fluid pressure on a caliper would keep it applied and dragging.
My daily driver is an old rusty trusty '88 Grand Caravan with the optional heavy duty brakes. I use it to pull a tandem axle enclosed trailer that's bigger and heavier than the van to the nation's second largest old car show swap meet, (next week), and I have never even bothered to hook up the trailer brakes. I can't hit 70 mph due to the wind resistance, but stopping has never been a problem. I can't imagine your truck having a problem pulling my trailer. What kind of braking problem are you trying to overcome?
Thursday, July 2nd, 2015 AT 5:12 PM