I've read about this problem and I seem to behaving it with my AAR. My AAR has the 11" rear drums with front disc brakes. On hard braking, it has a tendency to lock up the rears pretty easily. The back end wants to come around on me when this happens. I've heard that you can put the smaller 15/16 wheel cly on the rear and eliminate this problem. I've yet to try this. I need to check out all the brakes on this car first. With that in mind, I'm working on a 70 RR for a customer. It's a manual drum brake car with 11" in the rear. If she goes with power disc up front, will I have the same trouble with the b body if I leave the 11" on the rear or should I change to the 10" to avoid the problem. Would the 15/16 wheel cly solve the problem easily? Anyone that has firsthand experience have any thoughts?

If everything checks out, smaller wheel cylinders help. Or an adjustable proportioning valve

Check your rear brake hose for internal failure.

Is this a disc conversion? If so, what about the metering valve? If not a conversion, has the valve been checked?

AARs had discs standard.

Ok, but is the valve working properly?

What metering valve are you asking about?

I have a 74 E body with disc in front and 10" drums in the rear and have had a adjustable proportioning valve in the rear line since 85. Makes a huge difference, It saved my car one time for sure when I did a burnout and had to stop fast, the car just grabbed the road all four tire equal and I whoaed it down in time. No way it would have stopped without the proportioning valve restricting pressure to the rear.

Then I have a 71 with disc in the front but 11" drum in the rear. That car is even better than my 74. It has the proportioning valve in the rear line also. Stops very flat gripping the road big time. Very impressive brakes for a stock setup. More people should stick with them or use them.

I don't see the need to change wheel cyl piston sizes, you need a adjustable proportional valve to match the weigh of your car from day to day. How many people do you have in it? that effects the proportioning valve setting.

I was talking about the factory metering/ proportioning valve. I was wondering if it has been tested and verified to be working correctly.

I had trouble with the rear brakes being "grabby" and locking up with my 70 disc brake Challenger after the original master cylinder went bad and I replaced it with the aftermarket master cylinder listed for it. It was a 1-1/8" bore MC, the original is 1".
I later had my original sleeved and rebuilt it, and after reinstalling it, I didn't notice the problem any more.

brad70 had both the smaller wheel cylinders and an adjustable prop valve when he had drums on rear, i believe the smaller cylinders worked fine and he didnt need the prop valve adjust, ive just bought a pair if cylinders from RockAuto delivered for $15 so ill try them first

I think 15/16" is standard, and you are asking about smaller 7/8" wheel cylinders.

Anyhow, on a couple of B-bodies, going from 15/16" to 7/8 did the trick for me.

iirc in that Eberg article on this decades ago he said that they often supercede the 7/8 listings on paper to an actual 15/16 that they sell you so I would check em when you get em or if at the parts counter pull back a boot & measure

Sounds like you are dealing with 2 different situations. An all stock car and a retrofit front disc car.

On the all stock car, original proportioning was an "average best guess". What is the weight in the trunk? Full spare or mini or none? Passengers or stuff in the rear seat or not? Ect. Plus parts changes since original may impact proportioning.

On the retrofit car requirements may change. Excess rear bias seems pretty common. With non stock front discs and 11" drums I had excess rear bias.

In general it seems best that the fronts get full pressure through a functional "T". Some of the factory valves are functionally equivalent to a "T". Also an adjustable valve on the rear.

Reducing rear slave cylinder diameter helps and reduces need for valve adjustment... less adjustment means more linear and predictable braking.

Either the valve, or smaller rear slave or both are better than reducing rear brake size IMO.

I use Diplomat proportioning valves.

If I run 11" drums I run the cop spec one (11" drum Diplomats if you are boneyarding it) otherwise I use the civvy ones (10" drums if you are boneyarding it).

I've done several disc brake updates with them and never had an issue, front rotor size doesn't seem to matter as I have done both common sizes as well with those valves.

What is the application for the 7/8 cly that will fit the 70 up backing plate.

Try Raybestos WC37239 from RockAuto, certify that they are the right size though as I'm just going off memory

Raybestos WC37235 is 15/16", the number you posted shows up at RA, but no specs and no application so I dunno.

Looked up a 78 B100, 318, for my number.

Thanks for the info, brad70 used those in his Challenger to rectify back brake grab, I'll be fitting these shortly to my Cuda

Raybestos WC37236, or Dorman W37236 is the 7/8" ones, Rock auto has them listed for a 73 D100 2WD with 10 1/2" rear drums. I used the Raybestos ones on my Road Runner when I converted to front disks last year.