1996 F-350 diesel, manual. I have an unusual clutch issue. A while back I blew two master cylinders right off my firewall trying to push the clutch in. I now have finessed 5000 miles out of my last master / slave cylinder replacement and this is what I have experienced:

When shifting up through the gears with the 7.3 revs 2500 to 3000, the clutch works absolutely normally. Shifts easily and the clutch depresses easily to the floor with little resistance.

When coming down through the gears however, trying to shift from 5th down to 4th, or 4th down to 3rd with the revs at about 1200 to 1500, sometimes the clutch will not even depress. I can actually hear the firewall tweaking from the pressure. I can maybe get an inch on the clutch, pull it out of gear, but if I push it to the floor, the master cylinder winds up in the engine compartment. If I rev the engine up at this point, the clutch is fine when depressed, but the revs need to be about 2000 or so.

The only thing making sense to me is that perhaps the throw out bearing is bad. I have not experienced this before and the only thing that I can think of is that the bearing is worn enough that it is binding at slower speeds. With the engine spinning up at higher revs, I am thinking the bearing would be spinning at a higher speed when engaged and perhaps not bind as much. I am grasping at straws here and just wondering if anyone has experienced anything like this.

 

There are multiple problems I can list for you here for you to check.

The main one is the rod end on the master, where it connects to the clutch.
This rod and its piece wears.
there is a Heim Joint fix for this.

The next is the actual bushings the clutch pedal pivot on. This bracket can break, or the bushings wear, or both. This will cause really strange issues and with you saying that you feel the firewall moving, then thus could be your issue, but the clutch should work better as well.

With the Higher RPM's, the clutch might just end up moving a bit better with the throw out perhaps.
But I would venture to guess, if your blowing masters, that your throw out is bad.
They are of really poor design, and only last so long.
Also, the clutch fork and pivot **** that the clutch fork pivots on can become worn and cause strange issues.
The clutch fork could be worn causing some of these issues as well.

When buying a master/slave, be sure to purchase it from the dealer. The ones at discount stores are of poor shoddy design and do not last long.

The other issue on top of these is that you have a bur on the spline of the input shaft, and its causing the disk to bind while the clutch is depresses, thus not fully pulling away from the flywheel.

 

check to see if the pedal box is cracked.

 

I said that.
You were supposed to back me up with those pictures. :P

 

hold on in school.... let me find em

 

 

Heim joint is good to do whether or not you think you need it. I just got one for $32 shipped from ebay. Type in like Ford Clutch Fix or something to that nature. I think you can see the throw out a little if you pull the inspection window off. Maybe even a little through the fork hole if you pull the rubber boot.

The pedalbox bushings are pretty cheap from Ford and easy to swap. If yours has never been done then most likely they are a little(or a lot) worn. I bet the issues you're describing could be a cominbation of multiple of these issues. During the master cyl changes did you swap out the llittle crappy bushing that goes into the pushrod hole?

As I finished the last sentence my trans guy called me. I took my truck to him for potential warranty work. My trans he built in June was pissin ATF out like crazy from the front. He said that the fork was worn in a weird way that when the clutch was pushed in it was making the throwout move at an angle binding really hard causing a leak(only when the clutch was pressed). He's done a LOT of ZFs and said, like William, that the fork is notorious for weird wear but had never seen one cause a leak. A little OT I know but I thought that was interesting especially as I was typing this reply.

 

The OP most likely has a Throwout binding problem, or a problem with a bur on the imput shaft.
While these other things we have described are possible, pushing the master off the firewall just tells me that something is binding and the force he's pushing the clutch with is not enough.

 

Thanks for all the input guys. I think I will have to pull the transmission at this point and check for burrs, throw out bearing and fork. When replacing the last master, I replaced the rod and the plastic bushing, so don't think that's an issue. The bushings were all good at the time, looked as if they had been replaced fairly recently, no play or other concerns. Couldn't see any cracks or problems with the pedal box, but did not actually remove it for inspection, so worth taking a closer look. Hadn't thought about a burr on the input, well worth polishing it up once I have the transmission out. The fork is an interesting thought. I can see where it could be a problem, but not sure why it would work better when spinning up at higher speed. Probably worth replacing the fork and pivot when I get the transmission out just for the future. Anyone replace the syncros in a ZF out there? I have nothing left in 2nd and not sure if I should replace the syncros or try the wreckers for a low milage unit.