Clutch pedal suddenly firm, clutch slips

Hi all;
I have a very early TR3A, built Oct.'57. I was lapping Mosport race track at very low speed/low revs, when the clutch pedal suddenly went very firm. Testing shows it is slipping too, but is still driveable (just). A good friend suggests this is likely to be the pin that locates the throwout bearing fork on the cross shaft. I don't disbelieve this at all, but I seem to remember having the same problem many years ago, probably with a Mk.1 Spit, and it turned out to be something like internal master or slave seals, whereby the fluid leaked past them (but remained in the cylinder---the fluid is full on my car) and made the pedal very firm. Pressing the pedal therefore felt horribly firm and had a bit of an effect in that the clutch would do its clutching thing, but not to acceptable standards. I must admit it "feels" like it's a hydraulic problem on the pedal, but I could be wrong. Any ideas??? PS, slave cylinder is not very old, nor is the clutch assembly. Thanks! Tim.