A month or two ago I replaced the transmission in my ‘63 Scout. The previous transmission was, well… it had a number of deficiencies. The cause of those problems is still undetermined. Clutch drag seems to have resulted in over stressed synchros, and when they failed, lots of gear clash, chipped teeth, doom, gloom, more doom. True, there was no gear oil in it when I got it out, but that might be a separate failure point. What caused the clutch drag? While the old transmission was out, an alarming amount of oil and grease was noted on the flywheel. This will cause clutch drag, and is usually a sign of a bad rear main seal. I wasn’t in the mood, and didn’t have the garage for long enough to replace the rear main seal, so I just stuffed the replacement transmission in.
The previous owner, who was helping me, had installed a very dry clutch and a happy, oil filled used T-13, which immediately started popping out of 2nd gear. A new Girling slave cylinder had gone in at the same time. So less than 20,000 miles ago it had a nice new clutch and a transmission with at worst a bad 2nd gear synchro, and maybe just a busted spring in the tower shifter.
Happily, the used T-13 that I installed (at least the third transmission it’s had) seemed to be in good shape. Shifted okay, didn’t make any noise (surely the presence of gear oil made it quieter.) It still felt like the clutch wasn’t disengaging, and I didn’t want to ruin another T-13. (T-13 is the IH designation for the ubiquitous Borg Warner T-90; T-13 for 2WD Scouts, T-14 for 4WD Scouts, and an IH input shaft in both.) Before I went pulling everything out and replacing the rear main seal, I wanted to be sure the hydraulics were working.
There’s three main elements to the clutch hydraulic system, as with the brakes. Master cylinder, lines, slave cylinder. One of these things wasn’t developing or holding enough pressure. The lines were cleaned and observed for leakage and none was found, though the truck had been losing clutch fluid from somewhere for years, another mystery. Since the master cylinder was the oldest thing in the system, I decided to start there.
My master cylinder was an Australian aftermarket job, and I wasn’t sure I could source a rebuild kit. Super Scout Specialists had a Wilwood “high performance” replacement on sale, so I got that and installed it. The pushrod on the Wilwood was threaded 5/16ths fine thread; after some searching I found a coupler nut in that thread and a grade 8 bolt about 3-1/2″ long and that became my linkage, along with the homemade clevis from a previous owner.
After installing and bleeding the new master, clutch performance was significantly worse. once pumped up to pressure, the clutch barely disengaged, sometimes not even enough to get out of gear if the revs dropped. Also, the whole system would lose pressure if the pedal was allowed to return to full-up. This was exactly how my ‘61 behaved after I rebuilt the master cylinder, and I was determined to figure it out this time.
What did I do? What didn’t I do? What I didn’t do was bench bleed the master cylinder. In bench bleeding you run a bleeder nipple directly into the output of the master and a short bit of tubing straight back into the reservoir. Short circuit it, basically. Open the bleeder and run the piston ALL the way to the bottom of the stroke, close, return, repeat. This is done in a bench mount, as the throw of your pedal may not drive the piston the whole length of its travel. After a few repititions, all the bubbles should be gone. I managed to bleed the master cyclinder with it installed in the truck by disconnecting the pedal and shoving the rod by hand, while keeping the other hand under the hood to open and close the valve. That’s just another convenient thing about Scouts! I probably would have needed a helper in another vehicle.
When I had the bleeder valve closed, there was no give in the piston, no amount of force would move it. This was a good thing, it meant the master cylinder was not faulty. In my checklist, this meant the slave cylinder must be the source of my trouble. So why did a new-ish slave cylinder fail? The short answer is, wrong hydraulic fluid.
The slave cylinder, as mentioned above, is a Girling unit. I remembered from my Land Rover days that Girling systems used Girling fluid (I even remember inheriting a little red can marked ‘Girling Fluid’), in much the same way that old Ford automatic transmissions used Type-F fluid. Ask anyone why, and they’ll mutter something about keeping the seals bloated, or keeping them from bloating, lubricating the rubber, or using “shark oil” to achieve the goal. In any case, the bottom line is use the right fluid.
I had originally put DOT 3 fluid in, but on the second bleeding I switched up to DOT 4 because it matched the service level of the old SAE-70 R3 spec’d in the owner’s manual. I finally concluded that I needed to rebuild my slave cylinder. Before I went ahead with that, I went to my local Jaguar repair garage and picked their brains about Girling fluid. The mechanic there said he used to use Castrol GT LMA, but had switched to DOT 4, even in the older cars. Not convinced, I went and ordered two bottles of GT LMA along with my rebuild kit. Logic? I don’t think the original slave cylinders were made by Girling. I have an OEM rebuild kit and it’s for a 3/4″ piston (the part number checks out in the IH MT-113 parts catalog) and the Girling unit is a 5/8″ piston. So’s the master cylinder, how ’bout that! Also, GT LMA will not harm non-Girling systems, but DOT 4 will harm Girling parts.
The crucial part is the rubber seal, and in a Girling unit (even one you bought a new-in-2010 rebuild kit for) they’re made of natural rubber (as opposed to what? Nitrile? I don’t know!) and natural rubber needs something that Girling fluid or GT LMA have.
The Jag mechanic did give me a good idea, though. He recommended disconnecting the slave pushrod from the clutch, shoving it all the way back into the cylinder, wedging a stick in to keep it there, and bleeding the clutch that way. The logic being, less volume for bubbles to hide out in. Bleeding one clutch line, he said, can be as much of a pain in the ass as four brake lines.
So now have a reasonably well functioning clutch. All the fluid I put in is still there, months later, I’ve downshifted from third to second for the first time in years, though I’ve had to get used to putting it into first at a dead stop, rather than at walking speed. Down the road she goes!
Bench bleed your master cylinder.
Shorten up your slave cylinder when you bleed it.
Use Castrol GT LMA on your 1960s Girling parts.
UPDATE: I found out through an IH forum that the clutch master and slave cylinders are a match for certain forklift parts. Certain inexpensive forklift parts!
Slave: Crown Forklift #074815, Hyster #3014115, Intrupa Parts BP-1024
Master: Intrupa parts BP-1016, Wilwood 260-2636