In yesterday's fiasco after a few miles of driving the brakes went to full lock with no play whatsoever in the pedal, an d I had to pull off the road and wait about 30 minutes until the pressure let off enough so that I could make it back to the shop. The consensus opinion, both on this forum and on the Healey autox email list, is that there was not enough free play in the master cylinder pushrod, thus preventing the MC piston to go past the return hole and relieve pressure.
Earlier this afternoon, with everything cool, I put the car up on jack stands and while under the car I actuated the brake while spinning the left front wheel. Whatever free play I had--or thought I had--last put into the pedal linkage had disappeared and with no free play the slightest bit of pedal movement brought about brake activation. So after loosening the lock nut I rotat the shaft about 1/4"-3/8" until there seemed to be the appropriate amount of free movement and, again using moving the pedal from under the car and spiing the left front wheel, there seemed to be some gradual brake actuation. So I locked the shaft down and went out for the proverbial test ride. At first all seemed good but as the engine warmed up and after using the brakes several times with different degrees of pressure, etc. I could again feel the brakes begin to drag and by the time I got back to the garage the pedal height had changed with all free play having disappeared.
I can't help but feel this phenom is--at least partially--related to heat transfer into the MC. It is very warm here in MD and, with the same pedal shaft adjustment as yesterday, prior to temperatures going up there did not seem to be any issue. Then again maybe there has been some deterioration within the MC and/or debris within the cylinder itself.
Anyway, I will make another attempt at adjustment tomorrow morning and report back. In the meantime thanks for the many helpful suggestions. Craig C's in particular was very presceient and as it turns out this was the procedure I followed and reading his post was most encouraging. However it appears that more work needs to be done.