[RichardJ]

saxart
>>at one point earlier this system worked fine. If that is true, then you can rule out any issues with booster push rod length.<<

Aussieinnc
>>1/ Suggest you unbolt the master cylinder from the booster and place one 1/16" flat washer on each stud,<<


I think there is some possibility that it is the booster rod length causing this problem, but there may be a secondary issue.
About 3 months back, I had a problem with my C4 Vette that may be the same problem you have described.
At lights, I always take manual transmissions out of gear with my foot off the clutch and if possible my foot off the brake. The Vette rolls very easily and is sensitive to any uneven ground, more so than other vehicles.

Out of the blue, it stopped rolling at traffic lights and I could sense a difference in brake feel when coming to a stop in stop-n-go traffic.

The only thing that had been touched in years was a new booster about 3 years ago.
This issue would only start after being driven in traffic for awhile and the brakes heated, but not necessarily with any hard braking.

My Vette manual covered the usual brake dragging problems, but also mentioned a problem with the element of heat added.

Old, contaminated brake fluid heats up more and expands more than clean new fluid.

I had decided to check booster rod length and in the process I discovered the Rubber reservoir cover on the MC had a large crack and the fluid was near black.

The booster rod clearance was under by about .002" when it should be over by .010 - .020"

I always check pin length and I attribute it having changed to the cheap aftermarket steel booster. Original C4 Plastic boosters don't hold up well in the high underhood temperatures of the C4 Vette. I have replaced the Plastic three times in 20 years and decided there wouldn't be a fourth. They're just a couple inches from the headers and fail on a regular basis. Probably the same reason the reservoir cap failed.

I adjusted the booster pin, flushed the brake system, replaced the cap and the brakes are perfect.