dust keeps blowing back out of the chuck into my face, even with a vacuum hooked up.

Diagnosis

The issue is caused by a failure in the airtight seal between the SDS-plus bit shank and the internal walls of the chuck. On the Bosch GBH2-28L, the dust extraction system relies on a negative pressure vacuum created around the drill bit. When the rubber dust seal (O-ring) is worn, cracked, or missing, the vacuum "leaks" at the chuck interface. Instead of pulling masonry dust into the extraction shroud, the high-velocity air from the hammer's internal cooling fan pushes the fine silica dust backward through the gap, blowing it directly toward the operator's face. You can confirm this by running the tool with a vacuum attached and placing a small piece of masking tape over the seam where the bit enters the chuck. If the tape is sucked inward, the vacuum is working but the seal is failing; if the tape is blown outward, you have a massive pressure leak or a clog in the extraction path. This often happens after prolonged use with abrasive concrete dust, which acts like sandpaper, eroding the rubber seal over time.

How to Fix It

To resolve this, you must inspect and replace the internal seal and ensure the vacuum path is unobstructed. Follow these exact steps:

1. Disconnect the GBH2-28L from all power sources to prevent accidental activation.
2. Remove the SDS-plus bit from the chuck by pulling the locking sleeve back and pulling the bit out.
3. Use a pressurized air canister or a thin needle-nose pick to clear the groove located approximately 15mm inside the chuck barrel. Packed concrete dust often hides here, preventing the seal from seating.
4. Inspect the rubber O-ring (Bosch part number 2 607 001 041). If the rubber feels brittle, has visible nicks, or is missing entirely, it must be replaced.
5. To install the new seal, lubricate it slightly with a tiny amount of high-temperature silicone grease. Seat it firmly into the internal groove, ensuring it is flush and not twisted.
6. Insert your SDS-plus bit and ensure the locking sleeve clicks fully into place.
7. Connect your vacuum to the extraction shroud, ensuring the hose diameter is at least 1-1/2 inches to maintain the necessary CFM (Cubic Feet per Minute) to overcome the blower fan's pressure.

If That Didn't Work

• Check the extraction shroud's internal bellows. If the rubber boot that surrounds the bit is torn or stretched out, the vacuum will pull air from the room rather than pulling dust from the hole. Replace the shroud assembly if you see any punctures in the rubber.
• Inspect the vacuum hose for "clogging points." Concrete dust often clumps in the corrugated ridges of shop-vac hoses. Remove the hose and run a tennis ball or foam plug through it using a string to clear any solidified debris.
• Verify your vacuum's suction power. If you are using a small portable vacuum, it may not have enough lift to compete with the GBH2-28L's internal cooling fan. Switch to a high-capacity industrial vacuum with at least 150 CFM of airflow.
• Check the bit diameter. If you are using a bit that is significantly smaller than the extraction shroud's design (e.g., using a 1/4" bit in a shroud designed for 1/2"), the gap is too wide for the vacuum to create a seal, and dust will escape. Use a shroud matched to your bit size.

Heads Up

• Avoid using heavy grease on the SDS-plus shanks; excessive grease attracts dust, which turns into a grinding paste that destroys the rubber seals faster.
• Periodically blow out the chuck with compressed air after every 30 minutes of drilling to prevent dust buildup in the seal grooves.
• Always ensure the dust shroud is pushed firmly against the workpiece surface; any gap between the shroud and the wall will cause the vacuum to fail and blow dust back at you.
• Check the integrity of the SDS-plus bit shank. If the bit is worn down or "undersized" due to age, it will not create a tight enough fit with the seal, regardless of the seal's condition.