It’s usually not a fatal flaw but more like a pain in the butt that GM seems to stumble when it comes to making transmission coolers lines. They were bad on the 4L80s and they fail on the Allisons behind the Duramaxs. Two generations ago? Those all metal lines in grandad’s Turbo 400 behind a 6.2 Detroit never leaked a drop because they used metal lines “back then.” Today, though, ease of engineering seems to override logic (and that’s ALL manufacturers, not just GM) and for LBZ and early LMM trucks, one of the most notorious is the transmission cooler line leaks.

It’s a problem that can start small—maybe a few drops on the driveway—but left unchecked, it can snowball into low fluid levels, slipping transmissions, and expensive repairs. The good news? It’s a fix you can tackle yourself with the right parts—and while you're at it, you might even upgrade the cooling capacity of your entire transmission system.

Today, we’ll break down why these lines fail, how to replace them, and whether it’s smart to upgrade your transmission cooler too. (Hint: it probably is.)

Why Do LBZ Transmission Cooler Lines Fail?

The problem isn’t really the lines themselves—it’s the factory crimped hose ends where the hard metal lines meet the flexible rubber sections.

Over time:

• The crimps loosen.
• The rubber degrades, especially in cold climates.
• Expansion and contraction from heat cycles make the joints weep—or worse, blow out.

In a working diesel that tows, off-roads, or just racks up normal wear and tear, those tiny leaks quickly turn into puddles and transmission performance issues, and the very real concern is a blow out at the crimped ends and a series of bad things happening, from simply not being able to drive your truck to losing the tranny and having to rebuild.

Even worse? Replacing the leaking lines with new factory-style units often just resets the clock on the same problem. It’s why so many 2006-2010 Duramax owners eventually move to upgraded replacement lines like these:

Premier Performance Allison Transmission Repair Lines

Option 1: Reliable Replacement Lines

If your cooler is still in good shape and you just need to fix the leak, the Premier Performance Replacement Lines are a smart, straightforward fix. They’re built with better hose material, tighter crimping, and upgraded fittings compared to GM’s originals.

Option 2: Replace the Cooler Too

If you're seeing temps running higher than normal, pulling heavy, or just want peace of mind, upgrading to a larger transmission cooler makes a lot of sense. One of the things we like – especially if you’re swapping out transmissions – is to go ahead and upgrade the cooler, but NOT just for the better cooling. It’s actually because it eliminates all the old clutch materials that tend to get caught in the coolers over time, degrading their ability to cool properly. If you’re dropping big coin on a new tranny, a new cooler and lines is a no-brainer. PPE’s Heavy-Duty Transmission Cooler is a perfect match for LBZ and LMM trucks—and it can drop transmission temps by as much as 30°F under load.

Swapping the cooler while you’re already working on the lines might just be the smartest move you can make.

What You’ll Need for a Driveway Install

Tools:

• Line disconnect tools (for transmission fittings)
• Wrenches (usually 18mm and 22mm depending on your fittings)
• Drain pan (transmission fluid WILL leak)
• Zip ties or clamps (for tidying up loose lines)
• Fresh transmission fluid (you'll lose a quart or two during the swap)

Supplies:

• Premier Performance Cooler Line Kit
• Optional: PPE Heavy-Duty Cooler
• Optional: New clips/fittings if originals are worn

Step-by-Step Overview: Cooler Line Replacement

Step 1: Safety First

Let the truck cool completely. Raise the front end if needed for better access, and chock the rear wheels. Transmission fluid gets hot enough to burn you badly.

Step 2: Drain What You Can

You don't need to drain the entire transmission, but place a pan under the cooler and at the transmission to catch whatever spills when you disconnect the lines.

Step 3: Remove the Old Lines

Using the line disconnect tool, separate the lines at:

• The cooler (front of the truck, behind the grille)
• The transmission (driver’s side of the case) Some connections may need a gentle tap to pop free. Be patient. You’re fighting factory corrosion and time.

Step 4: Inspect the Cooler and Fittings

Before installing new lines, inspect the cooler inlet/outlet ports for damage. If they’re corroded, it’s worth upgrading the cooler now.

Step 5: Install the New Lines

Route the Premier Performance lines using the factory path. Avoid sharp bends and make sure they're secured clear of moving parts like the fan, belts, or steering components.

Reconnect:

• Transmission side first
• Cooler side second

Double-check all clips and fittings for security.

Step 6: Top Off and Test

Add fresh transmission fluid to replace what was lost. Start the truck and let it idle in Park.

Check:

• Transmission fluid level (add as needed)
• Look underneath for leaks
• Watch trans temps after a short drive

How Long Does It Take?

For most driveway mechanics, plan on 2–3 hours to replace just the lines, or 3–4 hours if you’re swapping the cooler too. It’s all straightforward hand tool work—no special programming, no computer resets needed.

And the result?

No more puddles. No more mystery low fluid levels. Better cooling. Longer transmission life.

Final Thoughts: Fix It Right, Fix It Once

Transmission leaks are like termites—small at first, but devastating if ignored. And in the case of the LBZ/LMM Duramax trucks, it’s not a question of if those factory cooler lines will leak—it’s when.

By upgrading to Premier Performance replacement lines (and optionally adding the PPE heavy-duty cooler), you’re not just fixing a problem. You’re upgrading your whole system to handle more miles, more heat, and more hauling without blinking. More importantly, you’re defusing the ticking time bomb sitting under your truck.

• Tougher lines
• Better cooling
• No recurring leaks

Ready to fix your truck the right way?

Shop Premier Performance Cooler Lines at Alligator Performance

Upgrade to PPE Heavy-Duty Transmission Cooler