and found, that after disconnecting it from the power booster, the problem still remained. What I did notice however, was that the brake lights were staying on long after the pedal was released, but if I helped the pedal to return by pulling it up with my toe it worked OK. Lifting the pedal up each time after use is not conducive to competitive lap times 
so it was time to check the function of the pedal box and return spring. If you like working in cramped, confined spaces this would be such a fun job 
, but for someone whose youthful flexibility is but a past memory, it's a literal pain in neck, and the back, and the shoulders and ..... 
. Anyway the pedals were removed, and the pivots cleaned and lightly greased. The clutch pedal was found to be cracked near where the cable fits on, so that was welded up as well. A new clevis pin with a split pin instead of the spring clip arrangement was adapted to take the brake pedal return spring after an inspection of the old pin showed indications of its 40-year history. The pedals were then put back in place (that's a hell of lot easier to write than to perform
) but the problem still remained. 
. That left only the power booster - so the pedal and master cylinder were disconnected again and the booster extricated from between the firewall, the master cylinder, the extractors, the clutch cable and all the wiring that springs forth from that corner of the engine bay.
On the bench it was obvious that the pushrod (and the large plastic piston that it connects to) was very hard to push in, and once released it didn't return immediately to its resting position. Applications of all manner of chemical concoctions failed to improve the situation, but luckily a spare of unknown heritage but obvious age was found, and to my delight it seemed to be functioning correctly. After another not-inconsiderable battle to reassemble with the replacement unit, I am pleased to report that the problem is solved
.A biopsy of the old unit showed that the rubber seal that fits around the large black plastic piston connected to the pushrod, had gone dry and hard, and was binding on the piston slowing down its return. This was effectively keeping the brakes on until the vacuum port was uncovered again by the piston moving back to the rest position.
I now have a perfectly functioning power booster, but due to it's age I am concerned that it too may have deteriorated with age and will eventually fail - and most probably on a 6-day rally in the wilds of Tasmania. So in the next week or 2, I guess I'll be repeating the power booster swap, but this time with a nice new one.
Oh, the joys of classic car ownership.
