MC upgrade question

[thegawd]

you can tell if your sensor is bad just by unplugging it and then driving.

In fact I bought my sierra off a good buddy for practically free. he kept chasing what he thought was something out of balance. after new tires, balanced multiple times and even having the driveshafts balanced he gave up and bought a brand new truck, actually a ram with a car engine.... LMAO and sold me the sierra and told me, "I dont know man,sometimes I'm just driving along when the truck just randomly jerks left or right!" I'm like dude, that's not a vibration! I looked it up and found out about this EVO sensor and asked him if he change it and he had no idea what I was talking about. he said the truck was unsafe to drive but I said we should be able to unplug that sensor and if the problem persists we'll find out right away and it didnt act up at all. Hell I passed my wife on the way home cuz she was driving to slow. when I got home I blasted into the field and did like 20 donuts before parking it. my buddies wife wouldnt let him do donuts in his new truck and they were arguing by the time they pulled in. LMAO.

my buddy couldnt believe he gave up on that truck over a sensor.

it's still unplugged..... 6 years later. lots of people do this and forget all about it but apparently that puts the pump into full blast all the time. well, this truck is a farm truck and I really dont car. if it starts going down the road I might swap in an older power steering pump but there would have to be one more reason for me to do that like it blew a line or whatever...

YUP they are both a PITA! LOL

 

[Schurkey]

Sure, both of the old Square body rigs that I had, the '76 and '80, and my '97 Sub all had/have MCs that have 2 different dia. pistons in them.
The big piston runs the rear drums and the small piston runs the front discs.
The '76 and'80 had JB8, the Sub is JB7.

Interesting that the "big" bore is used for the REAR brakes, when the Step-bore "Quick Take-Up" master cylinders use the big bore for the fronts.

The Square bodies used iron masters, not aluminum. This is an issue with my terminology--I've made the assumption that "step-bore" equals "Quick Take-Up", and maybe that's not true. The master cylinders I'm referring to have three chambers--the giant one at the rear providing low pressure, high volume, (the reason the casting has a step in it) plus the normal high-pressure "primary" and "secondary" chambers.

What I take away from this, is that I gotta quit using the phrase "Step-bore" and start using "Quick Take-Up" or "Three chamber".

Your '97 with JB7 brakes also does not have Low-Drag calipers, and does not have a 3-chamber master cylinder. My '97 K2500 also has JD7 (same brakes, Hydroboost instead of vacuum boost) and does not have a three-chamber master cylinder.

Here's a link to a JB8 master cylinder for a '76 1-ton. It's an iron cylinder with no Quick Take-Up provisions, and there's no obvious step in the casting. Unfortunately, NAPA does not list the actual bore size(s).

www.napaonline.com/en/p/NMCP2135?impressionRank=4

https://www.napaonline.com/en/p/NMCP2135?impressionRank=4

The JB5, JB6 uses the same Master Cylinder and front calipers. Part numbers are the same from GM dealership.
Please see attached image.

Yes, exactly right. The only real difference between JB5 and JB6 is at the rear, not the front. So the master cylinder is going to be a step-bore / Quick Take-Up / three-chamber design suitable for Low-Drag calipers.

JB5 has the horrible leading/trailing 254mm (10 inch) rear drums, JB6 has the 11.x Duo-Servo rear drums. Maybe there's a difference in the combination valve between the two. Maybe not. I don't know.

Do you have JB5 brakes (common on 1500s) or JB6 brakes (rare on 1500s)? If so, you have low-drag front calipers. Those are not compatible with the so-called "NBS" master cylinder.

 

[Gibson]

Your '97 with JB7 brakes also does not have Low-Drag calipers, and does not have a 3-chamber master cylinder. My '97 K2500 also has JD7 (same brakes, Hydroboost instead of vacuum boost) and does not have a three-chamber master cylinder.

I used wrong terminology too, my '97 Sub has JD7, not JB7, it does have hydroboost, sorry.
I agree with you, using a MC made for 4 wheel discs on rigs with drums in the rear isn't so great.
I "think" they only used low drags on the 1/2 tons, right?

 

[Schurkey]

I used wrong terminology too, my '97 Sub has JD7, not JB7, it does have hydroboost, sorry.

No problem.

I agree with you, using a MC made for 4 wheel discs on rigs with drums in the rear isn't so great.

The bigger problem is not that it's intended for 4-wheel discs. The main issue is the bore size--it's larger than the high-pressure section of the correct master cylinder. It's smaller than the "third chamber" "Quick Take-Up" section of the correct master cylinder.

So it provides approximately the same amount of fluid per normal brake stroke (I have not worked the math, this is merely a guess) but it does it by throwing away the hydraulic advantage of a smaller-bore cylinder coupled to a high-volume, low-pressure third chamber. Which--maybe--is no big deal if the booster is working properly. It's a VERY big deal if the booster fails.

The so-called "NBS" (I really hate that term...) master cylinder is UNSAFE when used on the GMT400 with Low-Drag calipers.

I "think" they only used low drags on the 1/2 tons, right?

I'd expect low-drag calipers on any of the 6-lug trucks, unless they're ordered with a "heavy duty" or "trailer towing" brake package.
For example, JN3--JN5--JB5 brakes standard on half-ton pickups, with the horrible leading/trailing shoe rear drums.
JB6 standard on half-ton Tahoes/Blazers/Suburbans which is the same as the JN5/JB5 up front, but with the good Duo-Servo rear brakes. (maybe this varies by year...I've heard of Tahoes having JB/JN 5 Leading-Trailing rear brakes.)
JB6 standard on 6-lug "light duty" 3/4 ton pickups.

Far as I know, the low-drag calipers and the three-chamber master cylinders were NOT available on any 8-lug or 10-lug GMT400-series vehicles. Those trucks got JB/JD7, JD8 or JF9 brakes.

 

[Erik the Awful]

Here's a link to a JB8 master cylinder for a '76 1-ton. It's an iron cylinder with no Quick Take-Up provisions, and there's no obvious step in the casting. Unfortunately, NAPA does not list the actual bore size(s).

1.312"

Cardone | Brake Master Cylinder

CARDONE Remanufactured Master Cylinders are re-engineered, built and tested to match O.E. performance. Each unit features new, O.E.-quality seals and cups to ensure leak-free, long-lasting performance

www.cardone.com

I looked up the application on NAPA's site, then looked up the part by application on RockAuto, then searched the Cardone part number on Cardone's site. The margin of error in that research path is significant, but it looks right.

 

[Schurkey]

1.312"

Cardone | Brake Master Cylinder

CARDONE Remanufactured Master Cylinders are re-engineered, built and tested to match O.E. performance. Each unit features new, O.E.-quality seals and cups to ensure leak-free, long-lasting performance

www.cardone.com

I looked up the application on NAPA's site, then looked up the part by application on RockAuto, then searched the Cardone part number on Cardone's site. The margin of error in that research path is significant, but it looks right.

That sounds reasonable.

However, it's claimed that the bore is different for the primary vs. secondary piston. I would have expected the same bore all the way through. I've never seen an iron master cylinder that wasn't a simple, smooth bore all the way from front to back...but then, it's uncommon for me to rebuild a master cylinder any more. I certainly haven't "seen everything".

 

[Gibson]

I gotta say, about the 2 square bodies I had, that I was like the 3rd>4th owner, they had high milage, and who knows what the previous owners did.
The '80 was from Canada.
I never changed MCs on them, just assumed they were "stock" as I got them.
So I easily could be wrong about them.