i have an 83 F250 with the 4.9l6, with a NP435 and the clutch hydraulic system from an 86 F150 i used to own (includes bell housing, external slave). when i set up the system, the mechanical linkage couldn't handle the 11" clutch i installed (breaking pedal bracket, etc), so i installed the hydro system from the 86, which i had already modified by lengthenging the drop-down arm to the MC to increase teh stroke into it and thus get the clutch to release. the firewall has been reinforced with 1/4" plate steel and no longer has movement around the MC. the problem i'm having is that the clutch pedal suddenly developed about 2 inches of slack, and now releases right on the floor. after this, i opened the MC, found the rubber cap insert stretched fully downward, and holding all the fluid it could, with the rest of the MC dry. i poured what was in the rubber back into the MC, and topped it off with clean fluid. the system still responds as it did before, indicating there is still air in the system. i figure vacuum bleeding should take that out pretty quick, but i'm more interested in getting to the root cause. does it sound like i'm needing a master or slave cylinder?

this truck isn't my daily, so its not killing me how it is, but i want it right. also, has anyone else upgraded to the 11" and experienced the need to modify clutch linkages? or is this a sign that my hydraulics were worn out from the start?

 

How did the fluid get on top of the rubber piece? I am thinking you didn't realize the rubber piece was in the reservoir, and where filling the top of the cup instead of the reservoir when topping it off. I have made this mistake before myself. Now that you have fluid in it, drive it around the block a couple of times, and the air will work out of it by itself. The more you drive it, especially on hills and bumpy roads, the more the air will work it's way up to the reservoir by itself.

 

The first time I filled my new clutch slave I filled up the black rubber cup lol. My buddy laughed his butt off at me when he caught that.

 

the weird thing about it is that every time i take off the cap, i take out the rubber piece, fold it back up on itself like its supposed to be, etc. because of this, i'm sure the MC was full half a year ago. i can see how this is an easy mistake to make, but pretty sure i didn't make it... but how else could i have it full of fluid.... whatever it is, i guess the system is starting to lose fluid, as this truck used to be my DD without ever a problem... but i guess adding fluid every 2 months to my firewood truck is no worse than blowing a tire every time i drive it (junkyard tires + overloading)