Hydraulic clutches in today's vehicles operate via a master cylinder. The cylinder is easily visible in the engine compartment, bolted to the firewall on the left side just above the steering column. When the driver depress the clutch, hydraulic fluid is pushed down a copper tube to the slave cylinder. The slave cylinder shifts the transmission. When the slave cylinder goes bad, it leaks, or fluid bleeds back into the line resulting in a loss of pressure.
Open the bonnet. Locate the slave cylinder. It is located on top of the transmission, is approximately four to six inches long.Trace the copper fluid line coming from the master cylinder to the slave. If you can't see it from the hood, try locating it from under the vehicle.
Look around the cylinder for wet spots that indicate it is leaking. If there is fluid coming from the cylinder, it needs to be replaced.
Watch the cylinder as an assistant depresses the clutch pedal. On the front of the cylinder is a hydraulic rod. When the clutch is depressed, the rod extends out, pushing a small fork that shifts the transmission. If the rod does not move when the clutch is depressed, the slave needs to be replaced.
Get in the car and start it. Depress the clutch. If it is hard to shift, the cylinder might be bad. If it does shift, hold the clutch down for five minutes. Try to shift again. If the car will not shift or is very hard to shift or grinds, the slave is bleeding back and needs to be replaced.
Some slave cylinders have a bleeder valve. You can attempt to bleed air out of the slave using this small valve located on the front of the slave. Normally, if the system is sucking air, it needs to be replaced anyway. Also, If you cannot build pressure when you press on the clutch, or the clutch feels spongy or soft, the master cylinder could be bad as well.