I have a 5 speed dodge dakota with 3.9l V6. Recently replaced the master and slave cylinder but the master cylinder won't stay attached to the firewall. I have it connected to the clutch pedal but it won't lock in place with turning it 45 degrees. The old one looks the same as the new part I installed. Please help. How do I prevent the master cylinder from slipping out and dangling in the engine bay.

 

Get some pictures that clearly show how you mounted it and post them here.
And tell us the year.

 

Okay, you're working on your CLUTCH master cylinder. I could place it in my head thinking of the brake master. It's been many years since I worked on a clutch master. I seem to remember that you bolt the master in place on the firewall. Once the actuator cylinder (it's not politically correct to call it the slave cylinder) is installed, you push the rod through the firewall into the master cylinder until it clicks. Then you just rotate the shaft until it cab slide over the mounting rod on the clutch pedal.

If this doesn't help, take some pictures like others have said so we know how things aren't working.