Hi everybody! First post here.
I just completed swapping a 14 bolt SF rear end into my '92 Blazer, along with the bigger 11" drum brakes. This did several good things--better brake feel, better rear brakes--and one bad thing--the rear brakes now overpower the front. That got me seriously studying up on brakes. I thought I'd pass along what I've learned, as this affects front brake upgrades as well. Some of what I'll say has been posted already, some hasn't.
First, let's clear up some misconceptions.
1. Better brake feel does NOT necessarily mean better brakes. The only way to measure the real effect of your upgrades is to measure stopping distance under a variety of conditions and speeds. Also, and more importantly, you need to consider the possibility that your upgrade has made your truck unsafe. Several of the popular upgrades I've seen will in fact destabilize your truck under certain conditions.
2. It is highly unlikely that upgrading just the front or rear brakes will improve your braking. You have to look at your brakes as a system, with a number of components working together. You can improve your brakes, but you have to improve the brake system, not just one component.
OK, let's look at each component and see how it affects the system. I'll leave the combination valve--arguably the most important component--until last. While you're reading this remember that a hydraulic system--our brakes--is in effect a system of levers. If you don't remember that, it's sometimes hard to visualize what's happening. That leads to misconceptions.
Master Cylinder. Obviously, this transfers force from your foot and the booster to the brake lines. By pushing with your foot, you apply pressure to the MC. The bigger the MC piston, the more fluid the MC supplies. However, as you increase piston size, you also DECREASE the force you can apply to the brake pads. Remember, the MC acts as a lever. A larger piston decreases your leverage. The brakes will feel firmer, because it's harder to apply force to the brake pads! That's why manual brakes use smaller pistons in the MC, and Hydraulic boosted brakes use larger pistons in the MC. Vacuum brakes are in the middle. Wait though, there's more!
Master cylinders do more than just supply pressure. They have piston sizes that are compatible with the pistons in the brakes. MC's designed for Disc/Drum brakes may have a "residual pressure valve" for the rear brakes. (If not in the master it will be either in the Combination valve or in the ABS pump. Older cars had a separate RP valve. It's almost always in the MC in newer vehicles.) This valve is needed because it takes so much more fluid to operate drum brakes as opposed to discs.
ABS pumps. Everyone knows the basics, but ABS pumps vary as to what functions they perform. The only way to know what your pump does is to study up on your particular module.
Disc/Drums. There's a piston in each of these, sometimes more than one. The bigger the piston, the more force can be applied to the brakes and the more fluid it takes to work. Disc brakes require more pressure than drum brakes. That's why the pistons in disc brakes are so much bigger than the piston in your wheel cylinder. Disc brakes are also much more resistant to brake fade. Different types of drum brakes have differing levels of braking force/brake pressure. Our 10" drum brakes are called "Leading/Trailing brakes while our 11" drum brakes are "Duo Servo". Duo servo brakes are far stronger. That's why the 11" brakes are much stronger than the 10" brakes even though the 10" brakes have much bigger wheel cylinders. (The larger diameter of the brakes adds 10% as well, but the duo servo adds more.) That's why the 11" brakes are a great upgrade for our trucks. They need less fluid and pressure to operate than the 10's, so brake feel and force are both improved. But you have to change more than the rear brakes to see an improvement. By themselves, the 11" drums will dangerously destabilize your rig. That's why you should change the combination valve if you upgrade your rear brakes.
Before we talk about combination valves, lets talk about the perfect brakes. If your brakes are perfect, best braking comes when all four wheels are very close to skidding, or even in a 5% skid. (Wheel speed is 5% less than vehicle speed.) Also, the rear brakes MUST NOT skid BEFORE the front brakes. Once the tires are skidding beyond 5%, a lot of braking force is lost. If your rears skid, all of your braking is at the front and your vehicle will want to swap ends. Very bad. If your fronts skid first, your vehicle will want to stay front end forward. Very good. So it's obvious we want our front brakes to be stronger than our rear. How much stronger? That varies with braking force. The harder you brake, the more weight transfers to the front wheels. So, when breaking harder our fronts have more traction, our rears less. This makes it more likely for the rears to skid, so we have to decrease rear brake pressure to compensate. That's one of the jobs of the combination valve. You might be thinking "Hey, if I really beef up the fronts I can improve my braking without destabilizing my truck." Sorry, but no. It's true that you won't destabilize your truck, but because you've thrown off the balance of the system your stopping distance will likely increase. Your brakes will "feel better", as partial braking will improve. But stopping distance in a panic stop will increase because all four wheels are not braking at their optimum level. Also, your front brake wear will accelerate.
The combination valve--often incorrectly called the proportioning valve--balances (proportions) your brakes and performs several other functions. Ideally your front and rear brakes are balanced at low to medium brake force, but weight transfer in a panic stop ruins that balance. The combination valve starts cutting pressure off to the rear as MC pressure increases. The exact level of pressure is fine tuned to specific vehicles. That's why there are hundreds and hundreds of different combination valves. The combination valve also cuts the first 20 or 30 psi off to the front brakes in a disc/drum setup. That allows the drum brake to activate at the same time or slightly before the fronts. Drum brake shoes are further from the drum than the front pads are from the rotor, so the drums need the first bit of pressure. The CV also acts as a warning switch, activating a warning light if you lose pressure in the front or back brakes.
There's a whole crapload more to cover, but that should be enough to get the idea across. Randomly upgrading a portion of your brake system will un-balance your brakes and increase stopping distance. Your brakes may subjectively feel better, but objective testing will almost certainly show increased brake distance. Worse, you may destabilize your truck and end up in a crash.
So, how do we improve our brakes? Experts with a thorough understanding of brake systems and dynamics can design brake systems that may suit their driving better than a stock setup. You and I can not. I could write for pages and pages and pages why not, but that would be boring. Now, I absolutely am not recommending that anybody modify their vehicle. I am not an expert, and I would never suggest that anybody make any changes whatsoever. I can tell you what I did, what you do is your business. So, what did I do? I decided to change my brake system to match better brakes available from the factory. The engineers already did all the testing, why not take advantage of that? With a lot of research, I found that the MC, the ABS, and the front brakes are identical on JB5 brakes (10" drums) and JB6 brakes. (11.15" drums.) What's different? The drum brakes--obviously--and the combination valve. Assuming all vehicles with 4 wheel ABS, both the SUV's and the pick ups show the same brake components for both JB5 and JB6. So I changed my combination valve to the JB6 valve--part # 15650150. The 11" drums came with the 14 bolt rear end. The result? I have a factory stock brake system with much improved brake feel and performance. (The 11" brakes have a smaller wheel cylinder so less fluid is required. That reduces pedal movement and improves feel. The duo servo type brakes improve braking capability even with the smaller wheel cylinder.) My brakes are now nice and solid and less pedal pressure is required to brake. Before I had to jam both feet on the pedal to max brake. Now moderate pressure with one foot can get all three ABS pumps cycling. BTW, the 15650150 valve is ONLY for 1992-1994 model years and four wheel ABS. Later years use a different CV that is readily available. I'd still want the CV from the appropriate year for JB6 brakes.
Now you guys should know that all the lawyers will tell you modifying your truck is evil. They're pretty smart so they must be right; whatever you do don't modify your brakes. But for someone that just wants to look at a 15650150 combination valve there is a problem. There is not a single new 15650150 left in the country. There's a TSB out for Heavy duty trucks, I forget which GVWRs are affected, to install a 15650150 in place of the factory CV. The engineers screwed up and the factory CV causes poor braking. The 15650150 valve fixes that according to the TSB. (Something to think about if you have a HD truck with bad brakes...) Anyways, GM sold all of their 15650150 valves. Luckily for us, JB6 brakes are common on 1/2 ton Suburbans. Find a Suburban in a wrecking yard, verify JB6 on the placard in the glove box or by the VIN--or visually confirm the 11" drum brakes. (Internal diameter on the drum.) Then get yourself a 15650150. Bear in mind I'm not the only one that knows this as the CV on these rigs is one of the first things to go. People are scooping these valves up for their HD trucks and for 11" brake swaps.
I have no idea if you can add 11" brakes to a 10 bolt rear end. (I did find a 10 bolt with JB6 brakes specified on one parts page, so it may be doable.) I do know that swapping in the 14 bolt semi floater rear end is a piece of cake and a huge upgrade in strength and brakes. Finding a 14 Bolt SF was easy and cheap, finding a 15650150 CV took a lot of research--now done--and a lot of calling around, searching through wrecking yards, and internet searches. If I thought I EVER MIGHT do a 14 Bolt swap I'd find me a 15650150 CV now. They're going to be hard to find as the years go by. I bought three so I'd have two spares down the road. BTW, the braking was unstable with the original CV and the 11" brakes. The rears easily started cycling the ABS well before the fronts. Very bad. With the JB6 specified combination valve? Very, very, nice. HUGE improvement over JB5 (Stock).
Again, the 15650150 valve is ONLY for 1992-1994 model years and four wheel ABS. Later years use a different CV that is readily available. I'd still want the CV from the appropriate year for JB6 brakes.
Anyways, that's my take on brake upgrades. I don't think the NBS master cylinder swap is a good idea unless you match systems. The whole system should match or you have no idea how your brakes will perform under an infinite number of possible conditions. (Four wheel disc would be a nice upgrade anyways! Oops, wait. Don't modify anything. It's wrong.) Just because brake "feel" is improved, that is no indication that braking is improved or even safe. A MC intended for a disc/disc system will have major incompatibility with a disc/drum system. ABS may prevent a spin/roll in most conditions, but not all. Plus, do really want to be one blown fuse away from a major crash?
