Clutch slipping
 

More of a general question but I"m having a clutch problem on my 66 E-Type.
The clutch is slipping under heavier loads. If I give it the spurs in third gear it will break loose. It's fine with casual driving.
The clutch disk, pressure plate and TO bearing were replaced about 30 years ago but has maybe 5k miles on it and I am not hard on a clutch. It was a Borg and Beck kit.
I have three working theories. 1) My rear main seal is leaking and I'm wondering if oil could find its way to the clutch side of the flywheel and cause the slip. Seems unlikely but??? 2) The springs in the clutch loose tension over time and don't have the same clamping force. 3) The slave cylinder is not properly adjusted and is applying pressure. This also seems unlikely since the TO bearing would surely be worn away to nothing by now if it was constantly pushing on the pressure plate. The clutch does engage very near the top of the travel. I also think that the slave cylinder is the self adjusting type so I would think that it would find its comfort zone, even with wear.
Any thought appreciated. An engine out refresh is due in the next couple of years but I'm hoping not to have to do it now.
Thanks in advance.

I will be interested as to what you find about the high friction point. I have a ’69 with only about 28k miles and have a very high friction point. It recently has been adjusted on the slave rod but no more room to go. Somewhat better; however, still high. Does mean that a new clutch is in my near future?

Regarding original post: Here's a 4th idea: What is your crankshaft end-float? If its excessive, I think it can lead to clutch slip.

Regarding post#2 - Original clutch? Replacement at 28K miles doesn't seem too bad.

HTH, Dave

My clutch history
 

I drove a 1970 FHC 4.2 as a daily driver, putting about 72,ooo miles on it. In that time the clutch needed replacing once due to wear. This was likely needed at something near to your present mileage.

Thanks for the input, everyone! I have not had time to investigate further but someone suggested that maybe the hydraulic flex line is collapsing internally and not allowing the fluid to return quickly and holding residual pressure. This is easy enough to replace so I think I will try that to see if it helps. Even if it doesn't, the line is 30 years old.