Especially When They Get Hot

Hydraulic disc brakes are one of the biggest performance upgrades in modern cycling, offering power, consistency, and control in all conditions. But like any hydraulic system, they only work perfectly when free of air, contamination, or old fluid. One of the most common issues riders experience with SRAM hydraulic disc brakes is hidden air in the brake lines—an issue that often gets dramatically worse when the brakes heat up.

In this post, we’ll look at what happens when there’s air in a SRAM brake system, what happens when the lever goes hard, why old fluid can make everything worse, and how to fix it properly.

Why Heat + Air Causes Brake Problems

Hydraulic brakes rely on incompressible DOT 5.1 fluid to transmit pressure from the lever to the caliper. Air, on the other hand, is compressible, which introduces problems the moment it enters the system.

1. Air Expands Under Heat

As you brake—especially during long descents—the system heats up. DOT fluid remains stable, but any air inside the line expands dramatically.

This leads to:

• A spongy or soft lever feel
• Increased lever travel
• Reduced braking power
• A lever that may pull all the way to the bar

This is known as heat fade, and it’s a classic sign of air in a hydraulic brake system.

What Happens When the Lever Goes Hard

A hard or rock-solid brake lever points to a very different problem than air. Instead of compressible bubbles, the system is creating too much pressure, usually because something is preventing fluid from moving normally. Common causes include:

• Pads fully contacting the rotor because pistons are over-extended or sticky — a disc brake pad spreader helps reset them.
• An overfilled brake system with no room for fluid expansion
• A blocked compensation port inside the lever
• Heat expansion in a system with no relief space
• Stuck pistons unable to retract smoothly — a hydraulic brake piston press can help free them.

Old Fluid: Why It Makes Everything Worse

SRAM brakes use DOT 5.1, which is hygroscopic—meaning it absorbs water over time. Old, moisture-contaminated fluid causes multiple symptoms:

• Lower boiling point → increased fade when hot
• More expansion when heated → hard lever or brake drag
• Thicker, darker fluid that struggles to flow
• Corrosion and debris inside the lever and caliper
• Clogged compensation ports leading to hard lever feel
• Sticky pistons caused by deposits and seal wear

Old fluid can cause BOTH effects:

• Spongy feel (from micro-bubbles)
• Hard lever (from fluid expansion or blocked ports)

Regular fluid replacement is essential to keep SRAM brakes working properly. High-quality fresh DOT 5.1 like Liqui Moly DOT 5.1 ensures consistent performance.

Why This Happens Often in SRAM Hydraulic Systems

SRAM’s hydraulic road and MTB brakes use a sealed bladder reservoir inside the lever. While reliable when bled correctly, this design can make it easier for small air bubbles and contamination to get trapped in the system if the bleed wasn’t done meticulously.

Because of this:

• Tiny air pockets can survive an imperfect bleed
• Micro-bubbles tend to migrate upward into the lever
• Old or dirty fluid can clog internal ports
• Heat exaggerates both air expansion and fluid expansion

The result is a brake that feels fine at the start of a ride, but gets soft, spongy, or hard and draggy once things warm up.

Brakes Feel Better After Cooling? That’s Air.

A very common sign of air in SRAM brakes is this pattern:

• On a descent → lever gets soft and braking power drops
• At the bottom → brakes cool
• Lever firms up and feels almost normal again

This is simply the air expanding when hot and contracting once cool.

The Fix: A Proper SRAM Bleed

To eliminate the issue, the solution is straightforward: perform a full SRAM-style two-syringe bleed using a proper SRAM brake bleed kit.

A proper bleed will:

• Remove all air from the system
• Restore firm, consistent lever feel
• Improve braking power
• Reduce lever travel and fade
• Replace old, moisture-contaminated fluid

Tips for Getting a Perfect Bleed on SRAM Brakes

• Use fresh DOT 5.1 fluid (old fluid absorbs moisture)
• Use two syringes as required—one at the lever, one at the caliper
• Tap the hose and lever repeatedly to dislodge trapped bubbles
• Use the lever flick method to chase out micro-bubbles
• Clean and reset pistons before bleeding (a piston press tool helps)
• Keep the lever positioned exactly as shown in SRAM’s bleed instructions
• Move fluid slowly—fast pushes create new bubbles

Final Thoughts

If your SRAM brakes feel great when cool but fade when hot, go soft under heat, or go hard during heavy braking, you’re almost certainly dealing with air expansion, old fluid, overfill, or restricted fluid flow.

Luckily, the solution is simple: refresh the fluid, clean the pistons, and do a thorough, methodical bleed using SRAM’s proper technique. Once the system is healthy again, your brakes will feel stronger, safer, and far more consistent on every ride.

For cleaning or maintenance, products like disc brake cleaner and a rotor truing fork help keep your braking system performing at its best.