To Address Clutch Issue, Start With The Primary

Dear Car Talk:

I'm one of the 11 people in America who still prefer a manual transmission. My 2002 Silverado 2500HD has an 8.1-liter gas engine and six-speed ZF manual transmission. The clutch linkage is hydraulic.

Since new, when pulling a trailer in the mountains, it sometimes has issues. It might suddenly develop 3-4 inches of free play at the top. It might go almost to the floor before releasing the clutch. Today, it did what I've feared; it wouldn't release the clutch at all. I had to pull the pedal up and pump it a few times to get it working again.

Any idea what's wrong? -- Norm

I noticed recently that real estate ads started referring to the "master bedroom" as the "primary bedroom." So rather than wait to get hate mail on this answer, I'm going to start referring to the clutch master cylinder as the clutch "primary" cylinder. That OK with everybody? I guess that'll make the slave cylinder the "guest" cylinder.

Anyway, I'd start by replacing your clutch primary cylinder, Norm. The primary cylinder is essentially a hydraulic pump. It multiplies the force of your foot on the clutch pedal by sending hydraulic fluid, under pressure, to the guest cylinder, which moves the clutch release bearing, and disengages the clutch.

So why would I try the primary cylinder first? Because that cylinder only costs about $50. And even though bleeding the air out of it is tricky, at least the repair doesn't require removing the transmission.

If the primary cylinder isn't generating enough pressure -- if there's an internal leak due to a failed seal, for instance -- the guest cylinder won't move the release bearing far enough, and you'll get free play and a low engagement point in the clutch pedal. So that certainly could be the problem.

And it's likely exacerbated by heat. When you're towing a trailer up mountains, the engine is generating a ton of heat, and the clutch is working as hard as it ever works. If replacing the primary cylinder doesn't fix it, then I would guess that your guest cylinder is leaking. That's a much bigger job. The guest cylinder comes as one piece with the release bearing in this truck. And replacing those parts requires removing the transmission. And if you're doing that, you might as well put in a new friction disc and a clutch plate at the same time and have a whole new clutch.

And, in fact, if it comes to that, you should buy the whole clutch kit, which includes a new, pre bled primary cylinder, too. That's going to run you many hundreds of dollars, Norm. If it makes you feel any better, think of it as redoing the guest room.

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