NBS master cylinder Swap

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Supercharged111

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I swapped the 8600GVW calipers on, then GMT800 master. The truck stopped a little better, but it was not what I was after. I then swapped on hydroboost and matching GMT400 master cylinder and that was the effect I was after. The only part of my system that isn't factory matched is the 9.5" rear with smaller 6 lug drums, not the bigger 8 lug ones that would have come with the rest of the hydroboosted setup. I don't know that I'd even bother with vacuum brakes ever again. Or that POS 8.5" rear drum setup.
 

Knuckle Dragger

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1. Why are you posting brake issues in "Engine Performance + Maintenance"?

2. "NBS" and "OBS" are vague and ambiguous terms.

3. JB3 and JB5 brakes have the ****** leading-trailing shoe rear brakes. It's uncommon to find them properly adjusted, and therefor they rarely work properly.

4. Replacing a Quick-Takeup master cylinder with a non-Quick-Takeup master is a FRIGGIN' DISASTER. You will LOSE hydraulic advantage, and you FAIL to supply the low-drag calipers with a "gush" of low-pressure fluid to push the pistons up to the rotor. GM put STEPPED-BORE master cylinders on these vehicles FOR A REASON.

Photo 1. Note HUGE rear piston, and stepped-casting. JB3 has a 1" nominal bore with 32 or 36 mm (I forget which) rear low-pressure piston. JB5 & (I think) 6 have a 1 1/8 nominal bore with 40 mm rear, low-pressure piston.
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Some doofus removes a correct-but-failed master cylinder, installs an incorrect but working master, and thinks he's accomplished something. Then he tells other folks who also don't comprehend the concept of low-drag calipers and high-volume (stepped) master cylinder bores, and they make the SAME MISTAKE.

The JB3, JB5, and (I think) JB6 (well, xx3, xx5, and xx6) brake systems use low-drag front calipers, and NEED the quick-takeup (step-bore) master cylinder.


The problem here is you don't understand the hydraulics as well as you think you do. You certainly don't understand the "step bore" because you continually describe it's function incorrectly. The constant bulling from you on this subject is getting boring and I'm tired of it enough to respond. The large bore of the NBS master cylinder is able to fill the caliper chamber quickly and push the piston out just fine. Likely there is an increase in needed pedal pressure because the size change but it appears most folks are OK with that given the number of them that make the switch. GM has used low drag caliper a lot longer then this abortion of a master cylinder, there is no magic in it.

34+ years ago when I was in school for this stuff we were taught to understand the theory of what we were fixing so we could logically find the fault when it arose. We were taught almost all theory as opposed the model specific stuff taught now. I'm factory trained by both GM and Ford and with 30+ years as a professional mechanic, and more than that tinkering with hot rods and such. You seem like you have a good general knowledge but for God's sake cut out the loud mouth look at how smart I am stuff and calling people names because they don't capitulate to your opinion. Stuff like this is why most guys keep quiet and never offer advice. There is always someone with marginal knowledge that wants to argue about stuff they don't understand.
 

L31MaxExpress

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Will suggest this as well. The OBS master for vacuum boost in most applications has a 1.13" bore. Only exception I know of is the Police/Limited Tahoe with the larger front caliper pistons and it is 1.25". The NBS master is either 1.25" or 1.34" Means that the highest peak hydraulic pressure will come from the smaller OBS master. Properly functioning OBS master cylinder does have a spongy feel to it but for a given input pressure on the input pin will develop the highest pressure therefore give the strongest clamping force on the caliper and the strongest push on the wheel cylinders. Cannot argue with physics. Unless you add a larger diameter or multiple diaghram/chamber booster or a hydroboost for the 1.25" master you will not develop as much braking force.

As I have stated in multiple threads on this, a true emergency panic stop will make you trash the NBS master with quickness. I was rolling 75 mph with nothing in front of me on a 3 lane road one day, car entering the flow of traffic moved over into the center lane without looking to see what was beside him and was pit manuvered by the pickup truck in the center lane ahead of me that I was about to pass. The car ended up in front of me with its front bumper implanted in the left hand concrete divider. I was 2 feet on the pedal getting stopped and only just got stopped with feet to spare. Huge loss of stopping power with the 1.34" NBS master compared to the 1.13" OBS master when used with the stock vacuum booster and stock OBS pedal.
 
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Schurkey

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The problem here is you don't understand the hydraulics as well as you think you do. You certainly don't understand the "step bore" because you continually describe it's function incorrectly.
I'll accept that I haven't described the function of the step-bore FULLY. I have not described it INCORRECTLY. (See below)

If I have, point out what I'm getting wrong, so I can learn.



The constant bulling from you on this subject is getting boring and I'm tired of it enough to respond. The large bore of the NBS master cylinder is able to fill the caliper chamber quickly and push the piston out just fine. Likely there is an increase in needed pedal pressure because the size change but it appears most folks are OK with that given the number of them that make the switch.
1. I need to repeat myself because folks keep starting new threads giving the same CRAPPY advice.

2. Someone modifies their truck, (hopefully) knowing the change in pedal pressure that occurs. And they're good with that. Then they let their wife or kid drive it, and the spouse/kid has no concept of how much force they're going to need to apply in a panic stop.

These non-step-bore master cylinders are borderline UNSAFE for the unwary driver. The larger bore size is NOT matched to an increase in booster power.

GM has used low drag caliper a lot longer then this abortion of a master cylinder, there is no magic in it.
1980, or thereabouts. First I remember them was on GM FWD "X-Bodies".

34+ years ago when I was in school for this stuff we were taught to understand the theory of what we were fixing so we could logically find the fault when it arose. We were taught almost all theory as opposed the model specific stuff taught now. I'm factory trained by both GM and Ford and with 30+ years as a professional mechanic, and more than that tinkering with hot rods and such. You seem like you have a good general knowledge but for God's sake cut out the loud mouth look at how smart I am stuff and calling people names because they don't capitulate to your opinion. Stuff like this is why most guys keep quiet and never offer advice. There is always someone with marginal knowledge that wants to argue about stuff they don't understand.
OK, here's the theory:
Requirements
1. Pedal effort has to be kept reasonable so that old ladies can drive the vehicle. Vehicles with larger master cylinder bores tend to have more-powerful brake boosters to compensate for the lack of hydraulic advantage.
2. Brake drag has to be minimized to meet CAFE requirements, and to be competitive with "official" EPA fuel-economy estimates.
3. The brakes still need to stop the vehicle without excess pedal travel.

Solutions
Calipers are then designed so that the main, square-cut seal retracts the piston farther than previous designs. Provides a small "air gap" between pad(s) and rotor. These calipers are known as "low-drag" units. Fuel economy improves...a little.

Downside of this is that it takes additional fluid movement from master to calipers to push the pistons out, so that the pads are back in contact with the rotors. It's the opposite to retracting the pistons an extra amount.

Therefore, to effectively and safely use "low-drag" calipers, a special, stepped-bore master is required, having three pressure chambers instead of the previous two chambers--a primary and secondary piston as usual for a dual-circuit master cylinder, plus a third, larger-bore chamber specifically to accommodate the needs of the low-drag calipers.

The larger-bore piston provides fluid at low pressure/high volume.at the beginning of the brake pedal stroke, that pushes the pistons out without requiring a long pedal stroke. The usual primary and secondary pistons are moving, too, but since the brakes haven't actually applied, (caliper pistons still traveling outward, rear wheel cylinder pistons still moving against return-spring force) they're still under low pressure, too. Therefore, initial pedal pressure is low, and pedal travel is reasonable.

Result: Lots of fluid provided primarily to front calipers, gets the pistons moved out with little pedal stroke and little pedal effort.

Once the pistons are out, so the pads are against the rotor, the high-pressure section of the master cylinder (the smaller, 1" or 1 1/8" bore) provides the higher pressure needed to force the pads into heavy contact with the rotor to apply braking force. Because the hydraulic advantage of the high-pressure part of the master cylinder hasn't been SCREWED UP by using an oversize main bore, pedal force is still reasonable. Since the only fluid movement is to account for caliper flex, pad compression, hose ballooning, seal distortion, etc., pedal travel is reasonable.

The "two-stage" Quick Takeup master cylinder has advantages that a single, larger-bore master cylinder can't hope to match. It takes the two-stage master to achieve large fluid volume required by low-drag calipers, while NOT requiring high pedal effort or long pedal travel.


^^^^What part of this is incorrect?





So, yeah, some doofus tries to re-engineer the brake system using incorrect "magic" parts, without having the understanding of what the compromises are, or WHY GM built it the way they did--and then broadcasts this silliness all over the internet.
 
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Supercharged111

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My theory is that the factory stuff causes excessive pedal travel, over centers, and eventually the pin on the brake pedal starts travelling up and not forward. My vacuum brakes were always good to a point, then the pedal turned to a rock no matter how the brakes were bled, adjusted, etc. The hydroboost pin is higher up so has less arc in its travel and I have to wonder if that isn't part of it.
 

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My theory is that the factory stuff causes excessive pedal travel, over centers, and eventually the pin on the brake pedal starts travelling up and not forward. My vacuum brakes were always good to a point, then the pedal turned to a rock no matter how the brakes were bled, adjusted, etc. The hydroboost pin is higher up so has less arc in its travel and I have to wonder if that isn't part of it.
I haven't looked at the geometry of the pedal/pushrod.

Typically, excess pedal travel on 1/2-tons is from the horrible leading/trailing shoe rear brakes being hatefully out of adjustment. At least, that's the first thing I check, and inevitably they need adjusting.

Air trapped in the RWAL or other ABS can certainly be a cause of excess pedal travel, too.
 

Supercharged111

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I haven't looked at the geometry of the pedal/pushrod.

Typically, excess pedal travel on 1/2-tons is from the horrible leading/trailing shoe rear brakes being hatefully out of adjustment. At least, that's the first thing I check, and inevitably they need adjusting.

Air trapped in the RWAL or other ABS can certainly be a cause of excess pedal travel, too.

I'm with you there, it's just that mine were always good to a point with vacuum when properly bled/adjusted but now they're just good period. And of course stronger. I've had the 9.5" since 2011.
 

L31MaxExpress

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I'm with you there, it's just that mine were always good to a point with vacuum when properly bled/adjusted but now they're just good period. And of course stronger. I've had the 9.5" since 2011.
Wait a second...Your 9.5 had different rear brakes? The one I had on my Express van that came off a 6 lug 2500 van had the same exact rear brakes as the 8.5. I eventually welded up the axle flanges, had them redrilled 5 on 5, and reused the stock drums. Initially I also had to swap the backing plates onto it from the 8.5 to replace the ones on the 9.5 that were heavily rusted. The 7100 lbs 5 lug and 7300 lbs 6 lug vans use the same brake parts except drums and rotors. Same pads, same shoes, same calipers, same wheel cylinders, same hardware and same backing plates. The 9.5 even had the same style bolt on flange for the driveshaft as the 8.5. It was almost a bolt-in swap save for the conversion to 5 lug. I actually first swapped to 6 lug front rotors and ran 20x8 Titan wheels with IIRC 265/50R20s. When I got tired of them rubbing the front tires on turns I pulled the axles and had them redrilled, swapped the front rotors back to the stock ones and put the OEM 15" wheels back on.
 
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Schurkey

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Wait a second...Your 9.5 had different rear brakes? The one I had on my Express van that came off a 6 lug 2500 van had the same exact rear brakes as the 8.5.
The 8.5 in my '88 K1500 had "10-inch" leading/trailing shoe brakes. I removed that axle for a six-lug K2500 9.5" axle, which had Duo-Servo rear brakes, and larger-diameter as well. Needed a conversion U-joint on the driveshaft. Might have been a "Precision 447", but I don't remember for sure.

The way I understand it, the JB3 package had the small booster, 1" bore QTU master, and small pistons in the calipers acting on narrow rotors. 10" leading/trailing shoe rear brakes.

The JB5 changed "everything" except the rear brakes, which were still the crappy leading/trailing shoe, 10" diameter. Bigger booster, bigger-bore 1 1/8" QTU master, bigger pistons in the calipers acting on wider rotors of the same diameter.

JB6 is the same as JB5 but with the larger, Duo-Servo rear brakes.

But this may be different in different model years, or between Van, Suburban, Pickup. I don't know.
 

L31MaxExpress

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The 8.5 in my '88 K1500 had "10-inch" leading/trailing shoe brakes. I removed that axle for a six-lug K2500 9.5" axle, which had Duo-Servo rear brakes, and larger-diameter as well. Needed a conversion U-joint on the driveshaft. Might have been a "Precision 447", but I don't remember for sure.

The way I understand it, the JB3 package had the small booster, 1" bore QTU master, and small pistons in the calipers acting on narrow rotors. 10" leading/trailing shoe rear brakes.

The JB5 changed "everything" except the rear brakes, which were still the crappy leading/trailing shoe, 10" diameter. Bigger booster, bigger-bore 1 1/8" QTU master, bigger pistons in the calipers acting on wider rotors of the same diameter.

JB6 is the same as JB5 but with the larger, Duo-Servo rear brakes.

But this may be different in different model years, or between Van, Suburban, Pickup. I don't know.

Well that might explain it. Even the 1/2 ton Express vans are JB6. 11-5/32 by 2-3/4" drums. They also used the same u-joints and driveshaft regardless if it was a 4L60E or 4L80E. Same rear u-joint too regardless if it was a 8.5, 9.5 or 10.5". Only the front yoke was different for 4L60E (yoke was about 12" long) and the 10.5" removed the rear bolt-on flange replaced by the u-joint straps. From memory I believe it had 1350 u-joints. Driveshaft was a 5" aluminum job.
 
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