Brake Issues - RPO codes

[Doppleganger]

The RPO codes on this truck says it has the JB7 brake system. Apparently it does not. I dont know if this is a case of someone swapping things out (doubt it but possible) or was one of those instances where GM used what they had on the shelf.

My rear drums are 11" not 13". My fronts look to be the bigger brakes, but I cant find any specs on what size the fronts are in JB6 or JB7. The GVW is 8600 so it should anyways. My master cylinder was the smaller plastic reservoir, but had yellow paint marker all over it like a yard part. The original owner's sht for brain's son bastardized this truck so bad I would not have even considered it had I known.

Now, in light of the RPO stating JB7, I found / bought an NOS GM booster and master cylinder for that brake (JB7) setup. New JB6 GM Pro MC's are cheap - boosters however, are NLA. I cant even find a PN to look for one. I am beyond p.o.'d with this whole truck at this point. Doesn't help the ********* doing my axles is going on month 9 now with the front.

So before I seriously consider parting this thing out and cutting bait, I have a few questions that hopefully someone has already done.

Can I use the booster and MC from the JB7 with the smaller brakes (I know the rears are, but not sure on the fronts)?

What is required to upgrade the rear axles to 13" and the fronts (if they also turn out to be smaller)? I would imagine its all bolt-on parts.

Can the rear axles be pulled without removing the diff cover? I know some do - some dont. No idea on this one.

Does anyone know the PN for the JB6 booster (if I cant use the one I have)?


Thanks.

 

[Keith Seymore]

Brake chart for '85 K truck JB6 vs JB7:

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[Doppleganger]

Thanks very much for that.

The PO's son butchered everything and swapping out axles would NOT surprise me. Dunno. The list is long with what was missing/incorrectly replaced/etc. And I dont know how much is 'they used what they had'. Have heard that alot.

So can I use the JB7 booster and MC with the smaller brakes until I locate and replace them with the larger ones? This truck is approaching the cut bait and part-out point. I cant keep reinventing and paying for every system 2-3x over.

Thanks.

 

[Blue Ox]

You don't say what rear you have, but it's probably the 9.5" 14 bolt semi-floater. To convert the rears you'll need backing plates, brake hardware, wheel cylinders and drums. You may be able to swap them without pulling the shafts, but if not it is a clip-retained rear and you'll have to pull the cover.

The fronts use the same size rotors with bigger piston calipers and D153 pads instead of the D52s. No idea how to tell about the booster, but it may be correct. The master has a bigger bore, so more pedal effort and a bit faster response if you use it with the JB6 parts, I guess. I'm not a fan of guessing at the engineering on brakes, so I'd be looking to do everything at once. It's not monumental, but you will have to do a little homework.

 

[Doppleganger]

Dont think its a full floater - but do think I would have to pull the axles to swap the backing plates.

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I suspected the front rotors were the same as there was never a variable PN for JB6 or 7. Great to know - thanks. I have been told that I can use my JB7 booster and MC with the 11" rears. Since I'm getting new everything for the fronts, I'm wondering if I install the larger calipers / pads, but keep the 11" rear drums for the time being, if that would create a problem? I would still replace the rears with bona fide 13" drums, but another day...another dollar.

The PO's son swapped and jury rigged literally everything he touched. I am not even ruling out these axles were swapped. The JB6 MC is plastic - like a car's - JB7 is steel. My MC was plastic but with a yellow paint marker on it from a yard. My alt said "86 Olds" on it. My tilt column would get the wheel stuck on the shifter. Intake and carb was both used/non-original. ALL wiring from the carb was cut/missing. 90% of the vac lines were pulled. A volly ball sized *** of solid core red wiring under the dash.....no gauges (except speedo) worked. No hazards worked. Heater core was bypassed. Overflow tank was MISSING / capped off. But the fact the GVW is 8600 alone says it had the JB7 larger brakes.

I've been hunting specs on these for awhile now - lots of info but none as far as interchanging MC's and boosters.

Thanks for the info......every little bit is priceless at this point.

 

[shiftpro]

That's the 9.5 semi floater. As I mentioned in my email you can pull the axles after you remove the wheels.
Hopefully someone can verify.. I have not actually done this before. Look at the end caps of the axle behind the wheel. Does it look like a full floater only larger? Bolts..? Sorry I'm confused the more think about it.

 

[Doppleganger]

'nuther angle. No idea if this helps.

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[Blue Ox]

It's the semi-floater.

 

[Craig 85]

This came up a while back on a different thread, but up until about a year or two ago I didn't realize GM made a light duty 3/4 ton in a Square body. I was aware of them in the early GMT-400s. This picture is from my 1985 sales brochure. The first K20 listing has a lower GVW and probably the smaller rear brakes.

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[Doppleganger]

The telling thing was my GVW sticker says 8600.....yet I have the small brakes.

As soon as I find out if I can simply bolt larger front calipers on (same backing plates?)....I will.

 

[Blue Ox]

The telling thing was my GVW sticker says 8600.....yet I have the small brakes.

As soon as I find out if I can simply bolt larger front calipers on (same backing plates?)....I will.

Have at it. The plates are the same.

 

[Doppleganger]

I found an article written by a company that makes the rear disc kits. On the larger subject of conversions, they suggest if one has the smaller brakes, to upgrade the fronts (booster/MC also) and the 11" rears will be fine for towing under 15,000 lbs.

Sounds doable to me.

THX!

 

[shiftpro]

'nuther angle. No idea if this helps.

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Ok I was wrong, there must be C clips like any other non floated axle housing.

I don't get what makes it 'semi-floating'. Time to pull up a schematic. This is one of those questions that has been bugging me for years and I never
unpacked it.

 

[Blue Ox]

Ok I was wrong, there must be C clips like any other non floated axle housing.

I don't get what makes it 'semi-floating'. Time to pull up a schematic. This is one of those questions that has been bugging me for years and I never
unpacked it.

What makes it "semi-floating" is that the outer bearings ride on the axle shaft. Only yhe inner half of the axle is unloaded. In a full-floating rear the bearings are contained in the outer hub and the axle bolts in, carrying no weight.

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[Doppleganger]

Are the housings the same size?