Clutch problems

I took a 91 sonoma s15 to a shop to have a small clutch fluid leak fixed. They replaced the slave cylinder. After 30 miles of driving the clutch pedal started acting like it did when low on fluid. There was no loss of fluid so I took it back to the shop where I was told that the replacement slave cylinder must have been bad. They installed another one. 40 miles later, I am losing pedal stroke again as if the fluid is low. What can they be doing wrong?

They’re not replacing the clutch master cylinder.

Tester

Sometimes the new cylinder is bad, I had three in a row like that once.
are they bleading the system? If the clutch peddle is depressed several times any bubbles in the system tend to break up and the clutch will operate, but after sitting for a while the bubbles re-form and then the clutch won’t operate. The only way to overcome this problem is to blead the system. When they did the repair, did they flush the old fluid and add all new? Did they use the correct new fluid? not all fluids are compatible.