from what I can tell the 10” have one long spring straight across the top (under the cylinder).
True. The spring and the adjuster mechanism/star wheel are just under the wheel cylinder, near the top of the brake assembly. The anchor block is at the bottom.
Generic leading/trailing shoe brake:
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By deduction the 11” have two springs, one on either side canted.
True. The primary shoe has a spring from shoe to anchor pin; the secondary shoe has a spring from the shoe to the heavy-wire link near the anchor pin. The heavy-wire link goes around the anchor pin, the bottom hooks around the self-adjuster mechanism. The star wheel is at the bottom, between the two shoes.
Generic Duo-Servo photo:
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Or pull the wheel and the drum and measure???
Best idea. Really, you shouldn't need to remove the drum--the maximum (safe) diameter of the braking surface is cast or stamped into the outside of the drum somewhere; if you can find that (and it's still readable) you'll know without taking the drum off.
Metric measurement of or near 254mm = "10 inch" drums with leading/trailing shoe design--the crappy brakes.
Imperial measurement of 11.yyy inches = 11.x drums with Duo-Servo shoe design--the good "6-lug" brakes. [Marvelous. I'm not allowed to use "x-x-x" to designate three decimal places of diameter. I have to change it to another letter, or forum software thinks I'm being naughty.]
Of course, the 8-lug trucks have still-bigger rear drums of about 13-inch diameter.
I’m wondering what all it would take to go from 10” (what I have on my 95 Tahoe Base) to 11” as it looks like everyone stocks parts for the 11” and not the 10”.
1. I thought the Tahoes got the 11.x Duo-Servo rear brakes along with Suburbans. I thought only the pickups got the crappy leading/trailing shoe drums. I guess I was wrong.
2. I'm really surprised you're having trouble getting parts for the 254mm ("10-inch") brakes. They should be more-common than the 11.x-inch jobs.
3. You need "everything" from the backing plates out to the drums. Find a "donor" axle in the Treasure Yard, pop the cover off the differential. Remove the C-clips, slide the axle shafts out. Release the park-brake cables from the brakes and backing plates. Unbolt the backing plates from the ends of the axle tubes. You should be polite and put the axle shafts back into the axle, and reinstall the cover to protect the guts from rain and dirt. If the park brake cables are any good--not damaged or seized--take 'em along with you. Around here at least, they're known for rusting up.
4. If you have 254mm rear brakes, you also should have the weak "10-bolt" 8.5" rear axle assembly. When it was me, I upgraded the
entire axle assembly to the much-stronger 9.5" rear axle assembly, which came with the bigger 11.x brakes already attached. This is a simple swap that requires only gear lube, U-bolts and a conversion U-joint when upgrading a 4WD--you buy a usable 4WD 6-lug axle with the right gear ratio from the Treasure Yard, buy a U-joint, gear lube, and U-bolts at the parts store, and slap it all together. It's tougher on 2WD because you either need custom axle shafts to retain the 5-lug bolt circle for the wheels; or you need to get the 6-lug wheels to go with the 6-lug axle (and then convert the front to 6-lug too.) The 4WD and the 2WD pickup-truck axle assemblies are different-widths. I don't know about the widths on the Tahoe/Blazer/Suburbans. Perhaps they're the same as the pickups, maybe not. Buy a matching axle for your truck.