the clutch does not have as high a pressure as the brakes. you could run braided all the way. the clutch slippage is caused by to small a line. with a small line there is a lag time from when you let the clutch out untill the pressure bleeds of on the slave cylinder. on a hard launch the clutch does not grab quik enough and slips. it is the same as letting out the clutch slowly and the clutch will slip until all the way out. Fran uses 1/4" line
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I thought that the default, and correct, size for the Ricardo clutch line was 3/16", as Cam, I, John Salmon, and all the other Ricardo-based SLCs I've seen have. I know for a while, there was a discussion that engagement issues when hot were line-size related, but I think in the end the final diagnosis was that the fluid was boiling because the then uninsulated line near the slave was picking up too much heat from the exhaust (which is why the original FGT had that foil-like insulation on that part of the line). The original issue, IIRC, wasn't too-slow release, but an inability to disengage the clutch when hot.
When I drove an SLC with the 3/16" hardline, a Ricardo, etc, it didn't have an engagement issue- in fact, the clutch felt pretty good. That's only one data point, but it reassured me about the issue, anyway.
I also think that running flex hose for that length might actually worsen the problem of too-slow response. One characteristic of flex hose is that it swells under pressure. Rubber-core lines are worse than the teflon-lined hoses in this respect, which is why the use of teflon-lines hoses in braking systems is a typical upgrade for performance cars-- the effect is to firm up the brake pedal slightly. As is pointed out elsewhere, the clutch hydraulic circuit doesn't have as much peak pressure as a braking system, so perhaps that might mitigate the tendency to swell.
The swelling issue, and the concomitant change in the response curve is why I think it is normally better to run hard line as far as possible, and to use flex line when needed to account for movement of vibration.
Which, incidentally, is just how RCR designed the car.
My $.02, anyway.