Caliper Surprise

[Thatrebelboy48]

Let me guess: Your truck had the weakass JN3 brakes, pancake brake booster, and small master cylinder.

Now you've upgraded the calipers to the JN5 spec, but you haven't installed the JN5 master cylinder or booster, or the thicker rotors. The bigger caliper pistons need more fluid, you need the bigger master cylinder to supply it...and the bigger booster to maintain power assist. Thicker rotors (and the hubs to match) would be nice but not essential. Apparently, there's enough piston still in the caliper when the pads are worn-out.

The JN3 master has a 1" main bore, and a 32 or 36mm Quick Take-Up bore. The JN5 master has a 1 1/8 main bore, and 40mm QTU bore.

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The booster is similarly huge compared to the "pancake" booster.

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I got all my brake parts from the Treasure Yard, from an extended-cab truck that would have come with JN5 or JB5 brakes.

Schurky, I’ve looked through the forum pretty good and I see that you’re pretty knowledgeable with brakes and seem to know interchangeability pretty well too. As of right now, I’m stuck with the 1” rotors because of my drop spindle. My truck is ready for a brake change so I figured since I get a pretty good discount I could upgrade while I’m at it. I’m pretty sure it would work fine but I wanted to get your thoughts before I go for it: 1” rotors, 3.2” piston calipers off the 8,600gvw 2500 trucks, and the 1.125” master cylinder you mentioned below. My truck already has the bigger booster you’ve got picture below as well.

 

[AuroraGirl]

Let me guess: Your truck had the weakass JN3 brakes, pancake brake booster, and small master cylinder.

Now you've upgraded the calipers to the JN5 spec, but you haven't installed the JN5 master cylinder or booster, or the thicker rotors. The bigger caliper pistons need more fluid, you need the bigger master cylinder to supply it...and the bigger booster to maintain power assist. Thicker rotors (and the hubs to match) would be nice but not essential. Apparently, there's enough piston still in the caliper when the pads are worn-out.

The JN3 master has a 1" main bore, and a 32 or 36mm Quick Take-Up bore. The JN5 master has a 1 1/8 main bore, and 40mm QTU bore.

You must be registered for see images attach

The booster is similarly huge compared to the "pancake" booster.

You must be registered for see images attach

I got all my brake parts from the Treasure Yard, from an extended-cab truck that would have come with JN5 or JB5 brakes.

My dads 95 C2500 has the RPO for Hydroboost as well as a 2nd return capped off on the power steering pump reservoir, which means it SHOULD have hydroboostr Sadly by appearance it seems to have the JN3 or at least a criminially undersized master/booster in its place

He said the brakes are soft and im like i bet

 

[Schurkey]

I’m stuck with the 1” rotors because of my drop spindle.

What vehicle? 2WD or 4WD?

On the 4WD, the hub/bearing assembly is different between the 1" rotor and the 1.25" rotor. Swap the hubs/bearings, upgrade the rotors.

I don't know if the 2WD uses the same bearing sizes and geometry for the 1" vs. 1.25" rotor/hub. It'd be worth finding out, and if possible upgrade the rotors, too.

I wanted to get your thoughts before I go for it: 1” rotors, 3.2” piston calipers off the 8,600gvw 2500 trucks, and the 1.125” master cylinder you mentioned below.

The 8600 GVW trucks don't use a low-drag caliper, so you either need the 8600 GVW master cylinder, or don't go all the way to 8600 GVW calipers--stay with the (7200?) JB5/JN5, which are the same as JB/JN6.

My truck already has the bigger booster you’ve got picture below as well.

Someone swap it already? I would not have expected that big booster coupled to the 1" rotors.

My dads 95 C2500 has the RPO for Hydroboost as well as a 2nd return capped off on the power steering pump reservoir, which means it SHOULD have hydroboostr Sadly by appearance it seems to have the JN3 or at least a criminially undersized master/booster in its place

No JB/JN3 by '95. Even GM realized that they were too small; they quit using the "3" brakes in the early '90s--'91 or '92 perhaps.

A 6-lug 2500 should have JB/JD6, and an 8-lug would get JB/JD7, I'd expect.

But it all depends on the guy who swapped-in the vacuum booster, and what donor vehicle he used--or what parts he bought. Post photos.

He said the brakes are soft and im like i bet

As usual, verify all the brake components for wear, including the caliper pistons and guide-pins for sticking/seizing, the rear wheel cylinders for leakage, rear wheel bearings, all the friction linings, rear brake adjustment, etc. Then bleed the ABS with a scan tool, flushing ALL the fluid. See what happens.

 

[someotherguy]

What vehicle? 2WD or 4WD?

On the 4WD, the hub/bearing assembly is different between the 1" rotor and the 1.25" rotor. Swap the hubs/bearings, upgrade the rotors.

I don't know if the 2WD uses the same bearing sizes for the 1" vs. 1.25" rotor/hub.

I can answer that just by his statement on the drop spindles - he's got a C1500, early model with the non-HD brakes. The axle on the spindle is shorter so he's unable to use the 1.25" rotor. To use the 1.25" rotor he'd have to buy new spindles.

Richard

 

[AuroraGirl]

What vehicle? 2WD or 4WD?

On the 4WD, the hub/bearing assembly is different between the 1" rotor and the 1.25" rotor. Swap the hubs/bearings, upgrade the rotors.

I don't know if the 2WD uses the same bearing sizes and geometry for the 1" vs. 1.25" rotor/hub. It'd be worth finding out, and if possible upgrade the rotors, too.


The 8600 GVW trucks don't use a low-drag caliper, so you either need the 8600 GVW master cylinder, or don't go all the way to 8600 GVW calipers--stay with the (7200?) JB5/JN5, which are the same as JB/JN6.


Someone swap it already? I would not have expected that big booster coupled to the 1" rotors.


No JB/JN3 by '95. Even GM realized that they were too small; they quit using the "3" brakes in the early '90s--'91 or '92 perhaps.

A 6-lug 2500 should have JB/JD6, and an 8-lug would get JB/JD7, I'd expect.

But it all depends on the guy who swapped-in the vacuum booster, and what donor vehicle he used--or what parts he bought. Post photos.


As usual, verify all the brake components for wear, including the caliper pistons and guide-pins for sticking/seizing, the rear wheel cylinders for leakage, rear wheel bearings, all the friction linings, rear brake adjustment, etc. Then bleed the ABS with a scan tool, flushing ALL the fluid. See what happens.

I have a potato for a camera at this time but next time i go over ill take my old LG since it has a great camera. But its a 8 lug yeah.

You must be registered for see images attach

Hmm maybe it was JB7 not JD7... well,.. i must have misremembered because I saw a return port on the pump and probably assumed it based off that.WEll, either way, the pancake sized vacuum booster was scary

You must be registered for see images attach

You can see my potato at work here, but ill get more pics of what I saw.

 

[Thatrebelboy48]

What vehicle? 2WD or 4WD?

On the 4WD, the hub/bearing assembly is different between the 1" rotor and the 1.25" rotor. Swap the hubs/bearings, upgrade the rotors.

I don't know if the 2WD uses the same bearing sizes and geometry for the 1" vs. 1.25" rotor/hub. It'd be worth finding out, and if possible upgrade the rotors, too.


The 8600 GVW trucks don't use a low-drag caliper, so you either need the 8600 GVW master cylinder, or don't go all the way to 8600 GVW calipers--stay with the (7200?) JB5/JN5, which are the same as JB/JN6.


Someone swap it already? I would not have expected that big booster coupled to the 1" rotors.


No JB/JN3 by '95. Even GM realized that they were too small; they quit using the "3" brakes in the early '90s--'91 or '92 perhaps.

A 6-lug 2500 should have JB/JD6, and an 8-lug would get JB/JD7, I'd expect.

But it all depends on the guy who swapped-in the vacuum booster, and what donor vehicle he used--or what parts he bought. Post photos.


As usual, verify all the brake components for wear, including the caliper pistons and guide-pins for sticking/seizing, the rear wheel cylinders for leakage, rear wheel bearings, all the friction linings, rear brake adjustment, etc. Then bleed the ABS with a scan tool, flushing ALL the fluid. See what happens.

It’s a 2WD single cab short bed. The RPO codes are gone unfortunately but the truck was originally a V6 5 speed truck. When I bought it, it was already sbc/th400 swapped so it wouldn’t shock me at all if the booster had been swapped.

As far as bearings go, the inner bearings ARE different from the 1” and the 1.25” rotors but the outer bearings are the same. Therefore, the wheel seals are also different. I’m assuming this is why you have to specify whether you have the 1” or 1.25” rotors when ordering your spindles. If I hadn’t already bought drop spindles for the 1” rotors, I’d definitely swap to the thicker 1.25” rotors. It’s something I’ll do in the future when I go to Wilwoods but it won’t be right now unfortunately. (college is expensive)

I did some measuring earlier and found that from the hat of the rotor to the outside edge of the rotor is 2.125”. The 8,600GVW calipers have 3.2” diameter pistons in them. I assumed that the 8,600GVW calipers wouldn’t work but after looking at some more parts I see that the caliper pistons are: 2.5”, 2.9”, and 3.2”

If the distance from the hat of the rotor to the edge of the rotor is less distance than the smallest caliper piston diameter…does that mean the entire piston diameter doesn’t squeeze against the face of the rotor? Doesn’t seem very efficient to me but I could definitely be wrong. I’m attaching a photo of what I’m measuring and calling rotor hat to rotor edge…

 

[Schurkey]

Braking surface vs. piston diameter means nothing. The piston applies force to the pads, not the rotor. The pads contact the rotor with whatever clamping force the piston/caliper provides.

 

[Thatrebelboy48]

Braking surface vs. piston diameter means nothing. The piston applies force to the pads, not the rotor. The pads contact the rotor with whatever clamping force the piston/caliper provides.

Gotcha! After seeing that the smallest caliper had bigger pistons than the distance I measured, I figured it wasn’t a relevant measurement.

I just done a bit of research and it looks like the 8,600GVW master cylinder is a 1.25” bore BUT that’s using the 13” rear drums. Mine still has the 10” drums (I’m not swapping anything in the rear because it’s all less than a year old and will be getting swapped to Wilwoods as well in the future) so I’m torn on whether I’d use the master cylinder for the 8,600GVW truck or not.

I guess I’m just going to do the 2.9” calipers with the matching 1.125” master cylinder until I get the Wilwoods.

 

[GoToGuy]

Use what the K2500 7200 , has they have great stopping power.