I drive a 79 F150. I'm posting about it here because it has a transmission out of a 90 F250 (a zf5-420), behind a 4bt diesel, and I'm having persistent clutch problems, and need very badly to figure them out. This is my work truck.

I swapped in the diesel and 5 speed about 4 years ago. I used a Cummins-Ford engine side adapter plate for a ford small block bell pattern, Cummins-Ford flywheel, stock ford diaphragm type pressure plate, stock ford clutch disk (10 or 11"), both probably the same age as my truck, or at least that style. Ball bearing type pilot bearing sized properly for the flywheel recess and input shaft tip, concentric (in bell, around input shaft) type slave cylinder PN D951001, selected because it's the one that would've gone in the 90 F250 this tranny came out of. I bought and installed a new one when I did the swap, but it's the one that the tranny came to me with, same PN. A flexible black plastic line with quick connect at the slave end connected that to master cylinder PN D22375, mounted to/through the firewall, with the actuator rod eyelet bolted with a shoulder bolt (no binding) to the stock 70's ford clutch pedal arm in the cab. Am I forgetting anything? Just ask if so..

I drove the truck like this for these past four years, 40-50,000 miles, with zero clutch issues. A few weeks ago, my clutch pedal started getting a little fuzzy/soft/short stroked over a day or two, but not enough for me to realize it wasn't just in my head. Then I pushed it in about a minute into a drive, and got minimal clutch release, then trying it a second later, I got none, total soft pedal, no ability to get into gears, buzzing gears forward and reverse, trouble pulling out, no clutch. I popped under the truck, in the road, for just a second, and got a drip of brake fluid on my face through the view port at the bottom of the bell. Looking up with a flashlight I saw the next drip on the slave, the engine still running, trans in neutral. I limped home and tore down, assuming that the slave had failed, going on the fluid as evidence. I found the master cylinder to be empty, and the fluid left in the system to be very dirty, black. Some people told me to replace the slave and master together, but I didn't, for the money. New slave, same part number, bled until the pedal got stiff, and the clutch worked again! The truck took off for a foot or two in reverse when I first started it, but then it freed right up, clutch pushed in fully and full pushback from it the whole time, and it was tough to get into reverse a minute later, but I figured I must've made the clutch disk a little sticky when I powershifted the truck home after the first failure. I test drove it a couple miles and things were fine.

I started it up the next time this week, and went to do an errand just a few miles from home. The clutch felt good. I got to my destination, went in, came out, restarted the truck, went to drop into reverse, BUZZZZZZ, reverse was spinning fully despite my foot pushing the pedal to the floor. I don't remember if the pedal was still stiff that first moment.. I shut down, dropped into reverse while off, started up in gear and got out of the parking space, powershifted and made it home. The clutch pedal had gone fully soft. I found no leaks on the master or the line, and no fluid on the slave externally. I tried to rebleed the system, since the slave was new just 5-10 miles before. I filled the MC reservoir 3 times, and it just kept going down with pedal pushes. I then checked the slave again, and it was dribbling fluid. I pulled the engine again, and when I unbolted the slave cylinder from the bellhousing, about a cup of fluid spilled out, that had been trapped in the recess that the slave mount flange covers. There was an obvious crack on the rear side of the slave cylinders mounting flange, where it joins up with the tube section of the aluminum cylinder body that goes around the tranny input shaft, the inner wall of the cylinder.

I picked up a brand new slave cylinder, again, and a brand new master and line. I hooked it all up, reinstalled the engine, started it up once just for a second and everything worked great. Forward and reverse were easy, no buzzing, no trouble getting into gears. I shut down, refilled the cooling system, and started up again. I pushed in the clutch, easily entered first gear, held it for a second or two with no issues, and then all of a sudden I heard a moderate clunk from the tranny area, and the truck lurched forward. My pedal was still on the floor, and I still had full normal pushback pressure, there was no fluctuation in the pedal, but the clutch had obviously grabbed. After just a quarter or half second engaged, a couple feet of travel, one momentary lurch, my foot still on the pedal, things disengaged, and were free as they should be. Then things worked normally for a few seconds, a few tries of first and reverse, with normal pedal use and feel. Then, with the pedal to the floor, with good pushback pressure, first and reverse were suddenly full buzz, full speed spinning under lots of engine power. I couldn't shift in, with the stiff feel clutch pedal all the way to the floor. Then it was mostly fine again for a couple minutes of testing after, but still with occasional lurch/catch moments, and some difficult gear entry, but sometimes totally fine. Good pedal feel throughout.

What is going on? Thank you for reading, and please, please help!

 

I'm thinking the throwout bearing is not sliding freely on it's shaft(front part of the transmission).

I used to have this problem with some old Chevy trucks when I didn't put grease inside the throwout bearing upon installation.

 

I'd be very surprised if that was the case. The throwout bearing on these concentric slaves doesn't ever contact the tranny input shaft. It and the rest of the slave ride on a central aluminum tube that sheaths the input shaft, an extension of the body of the slave cyl.

And if it was sticking, how would I get a sudden, momentary engagement, with the clutch pedal to the floor, when that same pedal push had initially resulted in normal free spin. The cylinder would have to extend as normal, then return partially to rest, stick there, then free up to re-engage, all with no feedback feeling through the pedal.

 

Just as a matter of comparison; T-5s on Fox Mustangs have the same type tube arrangement (aluminum) for which a steel aftermarket unit was one of the first upgraded parts made for them to combat throwout bearing sticking due to galling.

Clutch chatter & buzzing was a common complaint when it happened.

Just a thought.

 

Well, the verdict is in. I tore down, all the way this time, hood off and engine fully out, and the clutch disk was seated off center against the pressure plate/flywheel, so obviously something was going on there. I slowly backed off the pressure plate, and when it came off, the pilot bearing fell out on to the ground. I measured the input shaft stickout, and the depth of the bearing recess in the flywheel just to confirm I hadn't assembled this custom setup with lots of play in it, and indeed it would be totally impossible for that pilot bearing to have worked its way out intact when the input shaft was installed through it. I must have popped the pilot bearing out of it's recess with the input shaft tip when I pulled the engine the first time, to replace the first slave cylinder. Since then, running with no pilot bearing, the clutch disk has been swinging around on the free input shaft, jamming itself crooked in between the pressure plate and flywheel, and bashing the input shaft into the inner face of the slave cylinder, resulting in the cracked slaves. It's lucky actually that things failed so fast. The input shaft still has almost no visible sideplay in it, but that wouldn't have been the case for long. The clutch disk was warped from taking torque unevenly, the hub wasn't on the same plane as the fiber, and the rivets were all dinged up where they'd been hitting the loose pilot bearing.

The only question remaining is why the first slave cylinder failed. For that, all I can do is replace the pressure plate with a brand new one of a slightly different style. I'm going to go a little shallower, so the slave isn't taking so much spring pressure before it even engages.

Thank you guys very much for the help! Alex