Clutch Master Cylinders Keep Breaking????

[rrack]

Ok. I'm helping my non-mechanical minded dad out with his work truck. He recently had a rebuilt transmission put in then the trouble just kept going...I think that it's a 1994 Chevy Cheyene 2500. It's a beast...4x4 with the granny gear & goose neck hitch in the bed...

Anyway, he had a hydraulic leak in the clutch line somewhere. I put a new slave cylinder and master cylinder. It worked for one week! The black plastic clutch master cylinder sprung a small hairline crack under the tank and just sprayed fluid all over the place until it bled down. I returned it and got another thinking it was a defect. It did the same exact thing a week after that. I returned that one to Autozone and asked for a cast metal one instead of the cheap/weak plastic. They didn't have one. UGH! I then went to another parts house and got another with a better warranty. However, it looked exactly the same...black plastic and made in the USA! This time it lasted one day and he broke down 50 miles from home without a clutch pedal! Ugh again! He managed to put a fourth one on and is scard to drive it now! They keep breaking in the exact place! right under the fluid tank on the bottom of the cylinder next to where the line connects.

I've cross referenced and there doesn't seem to be a heavy duty master cylinder for this truck or is there? Why do they keep cracking? Is it just our bad luck or are we being sold the wrong part? or could there be something wrong with his rebuilt transmission that he got? Or just a bunch of bad Master Cylinders floating around? I have no idea????

Any help or tips would be great!

Thanks,
Robert