95 LS weird clutch problem

I have a 1995 Rodeo LS manual transmission with 122k miles on it. Between last Christmas and New Year's eve as I was coming off the interstate on which I have been driving for 40 minutes the clutch pedal went all the way down. I started pumping it and was able to get home. The mechanic said I needed to have the master cylinder changed. We did change it but a few days later I had the same problem, he said it needed bleeding. The same problem kept coming up and he kept bleeding it.
Finally I said the master cylinder must be defectuous and he agreed to order a new one and replace it. It went ok for a while and another time as I was coming off the interstate the same problem occured.
I took it to another garage, there was no leaking and everything else seemed ok. He pressure bled it and it worked fine for a few weeks.
Two days ago the same problem occured, again as I was coming off the interstate.
Now it is obvious that the problem is not a leakage, and that it occurs when I drive fast for a over 20 miles. So most probably something is happening due to heat and is causing air to be sucked into the circuit.
Anyone can help?

have the slave cylinder checked it may be leaking to find out if it is best thing is to remove the bolts holding it in place take it down remove the rubber boot from it if its wet in there its leaking and should be replaced