Situation: Replaced rear wheel cylinders both sides. Did not disassemble ANYTHING else - brake shoes (80 to 90% thickness), springs, anything. Brakes worked perfectly going in except for leakage on LR cyl and starting to seep on RR cyl. Was able to R&R cyls without undoing anything else. Bled the lines in the normal fashion (in the distant past I was ASE certified in several categories so I am familiar with most aspects of car service). Drive test was definitely unsat. Inspection revealed no rear brake application. Could turn the drums by hand with brake pedal applied. Then I went to my (factory) shop manual and I finally found where it describes a specific procedure to bleed the rear brakes using the accumulator pump. So I went through that drill and a test drive was better, but not much. When the brake and ABS lights are off, applying the brake gets immediate slowing (and both lights come on), but strong braking comes only after 1 second or so and the pedal rises at the same time.
Question is - what am I overlooking or leaving out? I'm going to bleed them again, paying EXACT attention to the procedure and run a full quart of fluid (this is getting expensive) through it. I'll report results.

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'89 Bonneville SSE daily driver (prior 3800 was '90 Buick LeSabre - solid car)
'06 Suburban 5.3 280,000 miles (now 308,000 and running fine - May,'22)
'67 Corvette Coupe 427/435, 1 owner (motor in storage, now running with 4 bolt 327)
'38 Chev Business Coupe, all original (wife won't let me "improve" it).
'80 BMW R100 Frankenscooter from a basket case.

Follow up and enlightenment. After spending lots of time reading other car forums all relating to Teves 2 ABS systems (many other brands used them during late '80s and early '90s), I found one particular post describing exactly my symptoms which pointed to the accumulator. The post was from the North American Turbo Coupe Organization > Tech Garage > Tech Articles > Troubleshooting the Teves II electro-hydraulic braking system.

Searching for the part results in MANY choices which "might" fit ranging in price from about $75 to $600. They are listed for Jaguar, Range Rover, Mercedes, Buick, Mitsubishi and many others. They all perform the same function but differ in size a little. Based on what I read regarding "seems to fit", I chose one made by Bosch for a Mercedes. It's a little smaller, but the thread fits and seals and now it works normally. The lights cycle as they should and the pedal feels right. Cost was (new, not used) $82 from a seller on ebay. The alternative was to go to a shop where the whole assy which includes master cyl, reservoir, valve block, pump and accumulator is all that is available through NAPA or ORielys for $1500 plus labor. I talked to several mechanics and none of them had even heard of the Teves system. Hardest part to R&R was to get a wrench on it. Oil filter wrench fit, but no room to turn it. BIG channel locks worked. Hope this helps someone.

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'89 Bonneville SSE daily driver (prior 3800 was '90 Buick LeSabre - solid car)
'06 Suburban 5.3 280,000 miles (now 308,000 and running fine - May,'22)
'67 Corvette Coupe 427/435, 1 owner (motor in storage, now running with 4 bolt 327)
'38 Chev Business Coupe, all original (wife won't let me "improve" it).
'80 BMW R100 Frankenscooter from a basket case.