That clutch loosing pressure... I did a video about replacing the slave cylinder and when I was replacing it I noticed that it did not come with a copper crush washer and they used to.
The copper crush washer is like a seal.
Without it air can slowly leak back in the system causing a soft pedal.
These trucks are old so generally when you're doing a clutch slave cylinder replacement you need to go ahead and do the master and the two rubber lines as well.
If you have not seen the video already here it is.
https://youtu.be/qrMlab-GFAw?si=F_TduyYkJrZwjcmv
