How to Bleed Hayes Dominion Hydraulic Brakes

If you’ve worked through the diagnostic guide and confirmed a bleed is needed, this procedure covers Hayes Dominion brakes (A4 and A2). Dominion calipers have two bleed ports, which means the bleed has two phases: bleed the caliper first using both ports as a closed loop, then bleed the full system from caliper to lever.

Dominion uses DOT 5.1 fluid only. Not DOT 5 (silicone, incompatible). Not mineral oil.

DOT fluid safety

DOT 5.1 is corrosive: strips paint, irritates skin and eyes, absorbs moisture from air. Wear nitrile gloves and eye protection. Have rags and isopropyl alcohol ready for spills. Cover the floor under your work area.

Used fluid is hazardous waste. Most auto parts stores recycle it.

Never share kits between DOT and mineral oil systems.

Tools and supplies

• A bleed kit. The easiest path is the Hayes Pro Bleed Kit, which includes the right adapters for Dominion. Aftermarket options that work: Park Tool BKD-1.2, Birzman, Epic Bleed Solutions.
• DOT 5.1 fluid
• Hex wrenches (5, 4, 3, and 2.5mm may all be needed depending on the caliper variant)
• T10 Torx for the lever bleed port
• A piston press. The Loam Goat 5 In 1 Disc Brake Tool has one built in, or use a pad spreader or tire lever.
• 11mm bleed blocks (in the kit)
• Isopropyl alcohol and rags
• Gloves, safety glasses
• Repair stand

Step 1: Bike prep

1. Remove the wheel.
2. Elevate the front of the bike so the hose runs upward from caliper to lever. The lever should be at roughly 45 degrees, putting the lever bleed port at the highest point in the system.
3. Turn the bars in line with the bike to keep the hose path clean.
4. Lay something under the bike to catch drips.
5. If using a pad spreader to reset the pistons, do this with the pads still installed so the pads protect the pistons from uneven force.
6. Remove the pad retention pin. 4-piston Dominions (A4) use a 3mm hex. 2-piston Dominions (A2) use 2.5mm.
7. Remove the pads.
8. If you didn’t use a pad spreader, now push the pistons back into the bores using a piston press or tire lever. Push straight to avoid binding.
9. Install the 11mm bleed blocks in place of the pads. You may need to be creative to hold them in (a small zip tie works).
10. Remove the caliper from the frame and let it hang. This straightens the hose path and helps bubbles rise.

Step 2: Bleed kit prep

1. Install the red adapters onto both hoses.
2. Install the hoses onto both syringes. Connections should be snug.
3. First syringe: fill ¾ full with DOT 5.1.
4. Purge air from the first syringe by pushing fluid to the tip, then close its line lock.
5. Second syringe: fill ¼ full.
6. Purge air from the second syringe, close its line lock.

Step 3: Phase 1, bleed the caliper

This is the Hayes-specific step. The Dominion caliper has two bleed ports. We connect both syringes to the caliper directly and shuffle fluid between them to evacuate air from the caliper body before doing the full-system bleed.

1. Remove both caliper bleed port screws.
2. Connect the syringes to the two caliper ports. It doesn’t matter which port gets the ¾ syringe and which gets the ¼ syringe.
3. Hold both syringes vertically.
4. Unclip both line locks.
5. Pull back on the ¼ full syringe to draw fluid (and bubbles) from the ¾ syringe through the caliper into the ¼ syringe.
6. Push back to reverse the flow.
7. Repeat the back-and-forth motion until no more bubbles appear in either syringe.
8. Stop with the original fill levels: one syringe ¾ full, the other ¼ full. Clip both line locks.
9. Install the ¾ syringe into the syringe holder (we’ll keep it for phase 2). Remove the ¼ syringe from either port.
10. Replace the bleed port screw where the ¼ syringe was. Torque to 1.5 Nm.

Step 4: Phase 2, bleed the full system

1. Move to the lever. Remove the lever bleed port screw with the T10 Torx.
2. Take the (now empty) ¼ syringe and install it into the lever bleed port.
3. Unclip both line locks (the ¾ at the caliper, the ¼ at the lever).
4. Push all the fluid from the caliper syringe up through the hose to the lever syringe. Be careful not to introduce any air that might be in the syringe.
5. Once the fluid is at the lever, push it back down to the caliper.
6. Repeat the back-and-forth until no air bubbles appear anywhere in the system.
7. On the final upward push, flick the lever a few times while pushing. This dislodges any bubbles trapped inside the lever body.
8. With no bubbles in the system, push fluid down from the lever to the caliper one last time.
9. Clip both line locks and put both syringes in the holders.

Step 5: Cap off

At the caliper:

1. Unclip the caliper line lock.
2. Add a small amount of positive pressure to the syringe.
3. Re-clip the line lock.
4. Remove the syringe.
5. Install the bleed port screw. Torque to 1.5 Nm.

At the lever:

1. Unclip the lever line lock.
2. Add a small amount of positive pressure.
3. Watch for bubbles in the hose. If you see any, pull back to stop them entering the system before re-pressurising.
4. Re-clip the line lock.
5. Remove the syringe.
6. Install the lever bleed port screw. Torque to 1.5 Nm.

Step 6: Test and reset

Pull the lever firmly. It should feel solid with a fairly short throw.

If firm:

1. Clean lever and caliper with alcohol on a rag.
2. Remove bleed blocks.
3. Reinstall the pads.
4. Install the pad retention pin and torque to 3.5 Nm.
5. Snug the caliper back onto the frame (don’t full-torque until alignment is confirmed).
6. Reinstall the wheel.
7. Pump the lever to advance the pads.
8. Realign the caliper if rubbing. (see: How to Fix a Rubbing Hydraulic Disc Brake)
9. Torque caliper bolts to spec (typically 6-8 Nm).

If new pads were installed too, bed them in. (see: How to Bed In New Brake Pads)

Step 7: Clean the kit

Same as any DOT bleed:

1. Flush syringes and hoses with isopropyl alcohol.
2. Disassemble and wipe with alcohol on a rag.
3. Lay out to dry before reassembling.

Common screwups

• Skipping the caliper-only phase. Dominion has two bleed ports for a reason. Bleeding only through the lever leaves air trapped in one half of the caliper.
• Wrong torques. Bleed ports are 1.5 Nm. Pad pin is 3.5 Nm. Over-tightening any of these will damage threads.
• Wrong size hex on pad pin. A4 uses 3mm, A2 uses 2.5mm. Stripping it makes future pad swaps miserable.

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Based on Park Tool’s video Hayes Dominion Brake Bleeding.