One more common fault that could sideline your classic

Behold, the newest member to the club of things likely to fail on a road trip and leave you stranded: The clutch hydraulics (this is the master cylinder).
Behold, the newest member to the club of things likely to fail on a road trip and leave you stranded: The clutch hydraulics (this is the master cylinder). Rob Siegel

Last year, I wrote about “The Big Six”—the six things most likely to send your vintage car into the breakdown lane: fuel delivery, ignition (particularly in a car with points and a condenser), cooling system, charging system, belts, and ball joints. (There’s also #0, the one so obvious that we don’t include it: a flat tire.)

The basic idea here is that, while anything can go wrong in a vintage car (like the loose rocker shaft end plug that was about to swan-dive into my timing chain), the Big Six enumerates the high-probably-of-failure items and systems where time and money are best spent sorting out a car before a road trip. Ball joints may seem a forced fit to some, but they are the nexus of the steering and suspension, taking all of the pounding from the road, and if they fail, you lose control of the car, so they deserve special attention.

There are, of course, any number of other potential failure points on an old car that’s on a road trip—weak exhausts can wear through and get blaringly loud; old suspensions can rattle and clunk or seize and jar your molars; brakes can get progressively worse—but, generally speaking, these are not things that abruptly torpedo a road trip.

I mentioned brakes, so let’s hone in on them for a moment. Obviously, your brakes need to be operating correctly for you to safely drive a car, and I’m not for a moment suggesting otherwise. There’s a litany of things that can be wrong with brakes: worn pads; seized calipers that cause the car to pull to one side; rotors with deposits on them (often erroneously referred to as “warped rotors”), which cause the brake pedal to pulsate and the steering wheel to shimmy when you slow down; master-cylinder failure that causes the pedal to go to the floor; and ruptured lines that cause brake fluid loss. But those first three problems are things that slowly get worse, not things that rear up suddenly on a road trip. The last two—master cylinder failure and line rupture—are possible, and certainly would derail a trip, but they’re rare. Plus, the DOT requirement of dual master cylinders and segmented brake reservoirs (implemented on all passenger cars sold in the U.S. since 1968), dramatically reduces the possibility of sudden braking loss. Even if a line ruptures or a seal in the master cylinder fails, you should still, in theory, have half of your braking left. I’ve never had a failed brake master cylinder cause loss of braking while driving, and the only popped brake lines I’ve had were on my 2000 Chevy Suburban, a vehicle so known for it that there was a class action suit and a NTSB investigation (1999–2005 Chevy trucks were notorious for the brake lines rusting through).

So if the brakes aren’t one of the things most likely to strand you on a trip, what problem merits changing the Big Six to the Big Seven? It’s the brake system’s hydraulic cousin, the clutch master cylinder and slave cylinder employed on cars with manual transmissions. When you push the clutch pedal down, it moves a piston in the clutch master cylinder, which pumps fluid down to the clutch slave cylinder, which pushes a fork (lever) that moves the throwout bearing that separates the clutch from the flywheel, allowing you to shift gears. The clutch hydraulics don’t have a tandem circuit like the hydraulic brake system, so when it fails, it fails. There’s no back-up system.

Now, I’ve tangled with clutch hydraulics many times. Both the clutch master and the slave can die the same two deaths (inactivity and contamination) as any hydraulic component. Inactivity is always a bad thing for hydraulics. The fluid is hygroscopic (it absorbs water), and the internal rubber seals, when sitting unused in watery fluid, harden, crack, and eventually begin to dissolve, turning the fluid black. On the contamination end, the rubber boot protecting the piston can split, allowing water in and causing the piston and the bore to rust. Beacuse the clutch master cylinder’s rubber boot is inside the firewall of the car, it is the longer-lived of the two. But the clutch slave lives under the car, typically bolted to the outside of the transmission, and thus experiences a lot of exposure. For this reason, the slave is certainly something that you should expect to replace several times in the lifetime of a car.

The clutch master is sometimes fed from the same brake fluid reservoir, drawing from a tap near the top so, if the clutch hydraulics leak, they can’t drain all the brake fluid. If there’s a slow leak in the clutch hydraulics, the reservoir may drain down below the feed point. When this happens, fluid will stop flowing to the clutch master, enabling air to get into the line. The clutch pedal may feel like it is offering resistance, but the car won’t want to go into gear, or may only do so with audible gnashing because the slave cylinder is no longer separating the clutch and flywheel. Sometimes topping off the reservoir and pumping the clutch pedal (or bleeding the clutch slave) will restore clutch functionality in order to get you home. But other times, the master or slave will simply fail with surprisingly little warning, allowing you perhaps one or two pump-up episodes before completely giving up the ghost.

The feed for the clutch hydraulics (the blue hose) from the brake reservoir comes from near the top so a leak can’t drain the reservoir and cause loss of braking.
The feed for the clutch hydraulics (the blue hose) from the brake reservoir comes from near the top so a leak can’t drain the reservoir and cause loss of braking. Rob Siegel

Those who’ve read my book Ran When Parked, about the far-from-home resurrection of my 1972 BMW 2002tii “Louie,” know that since the car had been sitting for a decade and had a non-functional clutch, I replaced the slave cylinder. That made the clutch functional, but after driving it about 20 miles to test it before a thousand-mile journey home, the clutch master cylinder suddenly failed as I was about to pull onto the interstate. The lesson there was that, for a car that’s been sitting a long time, you’d be wise to replace both the master and slave, even if replacing the slave restores clutch functionality.

When clutch hydraulics fail, if you have a vintage car without a clutch interlock (one where you can start the engine without depressing the clutch pedal), you can put the transmission in gear with the engine off, start the car, gently feed gas while the car does that awful lurching forward thing that happens when you accidently start it in gear, try to up-shift without using the clutch by matching engine and transmission revs (it takes practice), ease the transmission out of gear when you need to stop, and then shut the engine off, put the transmission back in gear, and start it again to get it going. But doing so is hard on the starter motor, and shifting without the clutch abuses the synchromesh gears as well. Also, the degree of control you have when this happens is limited (forget parallel parking), and so it’s really only advised that you try it when you need to get a few miles back home through light or non-existent traffic, or you need to get yourself and the car somewhere safe. For this reason, when Louie’s clutch master cylinder abruptly died at a gas station next to the entrance ramp to the interstate, I did not pull onto the expressway; I instead returned to the borrowed garage where I’d been working on the car and moved heaven and earth to procure a clutch master cylinder and install it before leaving again.

In a BMW 2002, the clutch master is a bit of a bear to remove, as it through-bolts to the firewall with traditional bolts and nuts, requiring a person on each side to undo the fasteners and remove it. Further, straight-on access to one of the nuts is blocked by the body of the master cylinder, making it so you can’t get a socket on it, and access from the side is blocked by attachments to the pedal bucket, making it very challenging to get a wrench on it. It is one of a handful of times in my life that I’ve used crowfoot wrench ratchet attachments (and rejoiced that a nearby Harbor Freight had them in stock).

Removing Louie’s clutch master cylinder with a crowfoot wrench ratchet attachment. The just-replaced slave cylinder is seen at upper left.
Rob Siegel

The clutch hydraulic issue just nearly bit me again. Every year, I go to an event called “The Vintage,” one of the largest vintage BMW events in the country. It’s in Asheville, North Carolina, the weekend before Memorial Day weekend. I’ve pounded out the nearly 2000-mile round trip from Boston in a variety of 45-year-old cars. This year, I took my 1979 BMW Euro 635CSi (more about that trip in a future story). It’s a car I bought three years ago, generally well-sorted but by no means restored.

I was doing a pre-trip checkout. While I was driving, the brake warning light began coming on intermittently. Previously, this car had an issue with the electrical contact on the emergency brake lever and required adjustment. I fiddled with it, but that didn’t seem to be the problem. I checked the brake fluid reservoir (the brake warning light on most cars is for both the handbrake and the fluid level) and was surprised to find that the fluid level was indeed a bit low. I put the car up on my mid-rise lift and inspected the calipers, the brake lines, and the clutch slave, and all appeared dry. I thought that perhaps the natural wear of the brake pads had caused the fluid level to drop (as the pads wear, the pistons in the calipers sit further outward in their bores, which does cause the fluid level to drop slightly), but the pads looked fine. I topped off the fluid reservoir and continued looking for the culprit.

It was while I was doing something completely unrelated that I found the problem. While I was driving, I felt hot air blowing onto my left leg due to a large grommet around the wiring harness that had become unseated in its hole in the firewall. As I had my head up under the dashboard trying to seat the grommet, I looked at the rubber boot for the clutch master cylinder, and noticed that it was wet and rusty, clearly indicating that the clutch master was leaking.

The rusty fluid oozing from the clutch master cylinder’s boot clearly indicates it’s going bad.
The rusty fluid oozing from the clutch master cylinder’s boot clearly indicates it’s going bad. Rob Siegel

Fortunately I had ample time before the trip to procure another part. While I could’ve thrown it in the trunk and only replaced it if I needed to, what’s the point? The clutch master in the car was clearly failing. I’d sourced the part. Unlike the one in my 2002, this one was easy to remove, with studs on the master and easily-accessible nuts on the inside, making it a one-person job. Why on Earth wouldn’t I change it before leaving? So I did.

Two cars, two long trips, two clutch hydraulic failures, one that stopped me dead in my tracks and another that I caught in time. I’m convinced that it’s time to concede that the Big Six is now the Big Seven—fuel, ignition, cooling system, charging system, belts, ball joints, and clutch hydraulics. I’d still bet on the first five, but congratulations, clutch hydraulics, you’ve made it. Welcome to the club.


Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 30 years. His new book, Just Needs a Recharge: The Hack MechanicGuide to Vintage Air Conditioning, is now available on Amazon. You can order a personally inscribed copy here.

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