How to: NBS master cylinder swap for firm brake pedal

[Crookedaxle]

I read through this thread--five pages worth.

I understand very little of it.

I do recognize there's some mistakes, flawed logic, and outright bad information here 'n' there.

What I HAVE NOT seen is WHY the "NBS" master cylinder makes an improvement. What is different in the design?

Here's what I know: There's a one-inch bore master cylinder, and a one-and-an-eighth-nch bore master cylinder, in Quick Take-Up and standard varieties. There's at least three sizes of caliper pistons, and at least three sizes of rear wheel cylinders (drum brake style.)

Using a larger master cylinder bore on the smaller pistons will reduce pressure, increase volume, and raise the brake pedal. Smaller master cylinder with larger pistons will increase pressure, decrease volume, and drop the pedal. Basic hydraulic theory. It's equivalent to gear ratios and torque/speed exchange. In this case we're exchanging fluid pressure for fluid volume by changing master and slave piston sizes.

There's at least two sizes of vacuum power booster, a single-diaphragm and a larger, dual-diaphragm. The bigger booster provides more assist, which makes for lower pedal effort--or allows a larger-bore master cylinder to equal the pressure of the smaller-bore with approximately the same pedal effort.

And making things even more complex is the addition of ABS and low-drag calipers.

Low-drag calipers need a huge influx of fluid at low pressure from a "Quick Take-Up" master cylinder to move the caliper pistons to the rotor, then the high-pressure section of the master cylinder applies the braking pressure that forces the pads into the rotor. If the low-pressure section of the master is faulty, the pedal will be low. If some idiot installed a non-quick-takeup master cylinder on an application with low-drag calipers, the pedal will be low. If the calipers are sticking but not yet frozen, the pedal will be low.

Quick Take-Up master cylinder has a stepped casting, and a huge rear seal/piston--36 mm or 40 mm depending on whether the main part is 1" or 1 1/8".

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If the ABS unit is faulty, the accumulator absorbs fluid volume, and the brake pedal will be low.

Far as I can tell, none of the improvement you guys are finding is due to "NBS" master cylinders, Maybe it's just due to you finally getting all the air out of the hydraulic system, maybe it's due to replacing faulty masters, or faulty calipers or faulty ABS units with parts that actually function properly. .And whatever doesn't directly bolt up just creates more work for you.

But what the hell do I know? I may be old, but I'm not done learning stuff.

For the record, I really despise "NBS" and "OBS" and "NNBS". As soon as GM releases the next in the series, "NBS" changes. At least "Squarebody", "GMT400" and "GMT800" retain their meaning over time.

What I HAVE NOT seen is WHY the "NBS" master cylinder makes an improvement.

I agree with your post wholeheartedly. For me it was worth it just to have the different reservoir.

 

[Crookedaxle]

I made the swap last night. I got a used one out of the boneyard for a 2003 2500 thinking it will be the large bore diameter for hydroboost. The inside ID where the wire circlip is measured 1.277" with a dial caliper which translates to 32.4mm, the same as the one I took out. So do I have the 34mm or 37mm version? I wanted the large diameter one since I plan on converting to hydroboost in the future.

I knew my back brakes were the issue ever since the first time I went to use the emergency brake when I got the vehicle that locked up and broke on me. I got underneath last night and got the star wheel adjusters broke loose with some penetrating oil and blows from a punch and cranked it out almost 1/4". I'll continue to crank it out manually little by little while it wears in the shoes as time goes on. I imagine they are only hitting on a small contact patch for now until they wear in to the new adjustments made with cranking out the star wheel adjuster.

I've got a line on a 1 ton SRW axle out of a 3500 van that I plan on putting in over the coming weeks. That's why I'm not concerned about a residual valve or fixing the emergency cable.

 

[Jglew82]

I made the swap last night. I got a used one out of the boneyard for a 2003 2500 thinking it will be the large bore diameter for hydroboost. The inside ID where the wire circlip is measured 1.277" with a dial caliper which translates to 32.4mm, the same as the one I took out. So do I have the 34mm or 37mm version? I wanted the large diameter one since I plan on converting to hydroboost in the future.

I knew my back brakes were the issue ever since the first time I went to use the emergency brake when I got the vehicle that locked up and broke on me. I got underneath last night and got the star wheel adjusters broke loose with some penetrating oil and blows from a punch and cranked it out almost 1/4". I'll continue to crank it out manually little by little while it wears in the shoes as time goes on. I imagine they are only hitting on a small contact patch for now until they wear in to the new adjustments made with cranking out the star wheel adjuster.

I've got a line on a 1 ton SRW axle out of a 3500 van that I plan on putting in over the coming weeks. That's why I'm not concerned about a residual valve or fixing the emergency cable.

If you plan on converting to hydroboost, you'll need a gmt400 hydroboost M/C. The 800 MC won't fit the 400 HB unit. I'm planning to go hydroboost pretty soon as well, but I'm sticking with GMT400 parts to do it.