Vacuum Bleeding a Shock

WARNING: ALWAYS WEAR EYE PROTECTION. I’ve had suspension fluid sprayed in my eyes and it sucks. It can happen even if you do everything right.

Degas the Suspension Fluid 

Suspension fluid has a lot of dissolved air in it, and before bleeding this air must be removed for two reasons:

• Dissolved air makes the oil more compressible.
• It allows the oil to absorb any small air bubbles left after bleeding.

Fill the vacuum bleeder reservoir no more than halfway with suspension fluid. Close the compressed air valve, open the vacuum valve, start the vacuum pump and run it to -30 in of Hg, then close the vacuum valve and turn off the vacuum pump. The the fluid sit in vacuum for 15-30 minutes. The vacuum can then be relieved by slowly opening the compressed air valve.

Bleed the Damper

NOTE: These bleed instructions are for bleeding a reservoir shock with a bleed port and an IFP. Although the basics of the bleed procedure are the same, the details for other damper architectures are different, so be sure to familiarize yourself with those before bleeding. 

Preparation

• Service/rebuild the damper and assemble it without suspension fluid.
• Connect the appropriate bleed adapter to the bleed port on the shock, and position it so it’s the highest point of the damper.
• Install the IFP and the reservoir cap retaining clip. It is very important the IFP be retained inside the shock during the bleed process.
• Fill the vacuum bleeder reservoir no more than halfway with the desired suspension fluid.
• Set your compressed air regulator to 45 PSI.
• Connect the compressed air source to the vacuum bleeder.

Bleed

1. Close the compressed air valve, start the vacuum pump, and slowly open the vacuum valve. Once there’s a good vacuum, close the vacuum valve and turn off the vacuum. Be sure to close the vacuum valve before turning off the pump.

NOTE: What look like bubbles under vacuum are actually pockets of vacuum and will collapse during the pressure phase of the bleed.

2. Verify the vacuum valve is closed, and slowly open the compressed air valve. Oil should start flowing from the reservoir into the damper. Once the pressure is stable, cycle the damper several times by hand. It can also help to tap the damper to dislodge any bubbles.

4. Repeat steps 1-2 several more times (I usually do 5 cycles) to help insure all the air is evacuated from the shock. Any small bubbles which remain will be absorbed by the degassed suspension fluid.

5. Close the compressed air valve, and use the pressure relief valve to release the pressure.

6.  Disconnect the air compressor, open the compressed air valve, and set the IFP depth. The excess oil will be pushed back to the vacuum bleeder reservoir.

7. Disconnect the vacuum bleeder from the bleed adapter, then remove the bleed adapter and close the shock's bleed port.

8. Install the reservoir cap and pressurize the reservoir to spec.

Building a Vacuum Bleeder

A suspension vacuum bleeder uses alternating vacuum and pressure to bleed a damper. It's much easier than hand bleeding, does a better job, and is a less mess.

Bill of Material

A bleed adapter specific to the damper will be required to connect the vacuum bleeder to the damper. ANSO (https://anso-suspension.com) makes good bleed adapters at a reasonable price, and the list below reflects using ANSO adapters. If different bleed adapters are used, the "3/8" FNPT V-Style coupler" would need to changed to work with those adapters.

• 1/4" NPT 50 PSI pressure relief valve - $9 -  https://a.co/d/7lOzOpp
• 3/8" OD 1/4" ID polyurethane (PU) tubing - $9 - https://a.co/d/7cUTqff
• 1/2" NPT to 1/4" barb 90* fittings - $10 - https://a.co/d/fgpZFRI
• 3/8" spring hose clamp - $6 - https://a.co/d/gdm5f48
• 3/8" FNPT V-Style coupler - $9 - https://a.co/d/8DQBxJ5
• 1/4" NPT pressure gauge - $13 - https://a.co/d/f1TbRqJ
• 1/4HP 3.5 CFM vacuum pump - $55 - https://a.co/d/2L3zF0F
• Hydronix HF2-10CLWH12 filter housing - $23 - https://a.co/d/dYMTLSv
• 3/8" NPT 3/8" tubing push to connect fittings - $9 - https://a.co/d/auJHnwp
• 1/2" NPT mini ball valves - $13 - https://a.co/d/72oPZEk
• 1/4" FNPT air hose quick connect adapter - $4 - https://a.co/d/aGIQmXG
• 1/4'' NPT 1/4" barb fittings - $5 - https://a.co/d/cOngmrj

TOTAL - $165

Assembly

NOTE: Whenever installing a fitting the threads should be wrapped with at least three layers of Teflon tape. Be sure to orient the wrap so that tightening it doesn’t try to un-wrap it. 

Vacuum Pump

• Fill the vacuum pump’s sump with suspension fluid, roughly the same viscosity as you’ll be using.
• Remove the inlet fitting and replace it with a 1/4” NPT barbed fitting.

V-Style Coupling

• Install a 3/8” NPT push-to-connect fitting.

Quick Connect Coupling

• Install a 1/4” NPT barbed fitting.

1/2” Ball Valves

• Install the 1/2” NPT barb 90* fittings so they point away from the valve handle.

3/8” Push-to-Connect Fitting

• In one of the fitting, drill out the bottom of the fitting so a tube can extend all the way through the fitting.
• Meticulously clean any metal swarf out of the fitting.

Filter Housing

• Drill a 19/32” hole in the top of the lid and and thread in the drilled out 3/8” push-to-connect fitting. Teflon tape is not needed on the thread. The fitting will tap itself into the plastic of the lid. Keep threading it in until the bottom is just proud of the inside of the lid.
• Drill a 15/32” hole in the top of the lid, then thread in the pressure gauge. Teflon tape is not needed on the thread. The gauge will tap itself into the plastic of the lid. Keep threading it in until the bottom is just proud of the inside of the lid.
• Drill a 15/32” hole in the side of the lid and thread in the 1/4” NPT pressure relief valve.  Teflon tape is not needed on the thread. The valve will tap itself into the plastic of the lid. Keep threading it in until the bottom is just proud of the inside of the lid.
• Clean out any plastic shavings from the lid.
• Install the two two ball valves into the two threaded holes in the lid. The valve handle should face upward, and the barb fitting should face downward.

Putting it all Together

• Insert the polyurethane (PU) tubing through the push-to-connect fitting in the lid and feed it down until it sits about 1/4"-1/2” above the bottom of the reservoir. Install a zip-tie or clip to prevent the push-to-connect release collet from being depressed.
• Cut the PU tube to length. It needs to be long enough to go from the vacuum bleeder to the damper with some slack in the tube. However, the tube should be kept as short as practical to aid in bleeding.
• Connect the V-style coupling to the end of the PU tube. Install a zip-tie or clip to prevent the push-to-connect release collet from being depressed.
• Cut a length of PU tube to connect the reservoir to the vacuum pump. Put two spring clamps on the tube, warm up the two ends with a hair dryer or heat gun, connect them to the barb on the vacuum pump the barb on one of the ball valves on the reservoir; this is now the vacuum valve. Put the spring clamps in place.
• Cut a length of PU tube to connect the reservoir to the air compressor. Put two spring clamps on the tube, warm up the two ends with a hair dryer or heat gun, connect them to the barb on the quick connect coupling and the barb on the other ball valve; this is now the compressed air valve. Put the spring clamps in place.

Testing

• Test the reservoir to make sure it holds vacuum. Close the compressed air valve, open the vacuum valve, and run the vacuum pump until the pressure gauge registers 30 in. of Hg vacuum, then close the vacuum valve and turn off the pump (air can bleed through the vacuum pump when it’s not running so close the vacuum valve before turning off the pump). The reservoir should easily hold the vacuum for several minutes, if not hours. If it doesn’t, there’s a leak which needs to be found.
• Test the reservoir to make sure it holds pressure. Pressurize the reservoir to 45-50 PSI and close the compressed air valve. Set the air compressor’s regulator to 45-50 PSI, DO NOT pressurize the reservoir with higher pressure air and try to stop it at 45-50 PSI using the pressure gauge. Again observe the pressure gauge for any changes in pressure. If the pressure drops over the next several minutes, there’s a leak which needs to be found.

If the system passed both the vacuum and pressure tests, install it in its final position or install it in its stand/frame.

Andreani Thread

Many vacuum bleed adapters use "Andreani thread" named after the Andreani vacuum bleed machine. It's actually an 1/8" BSPP (British Standard Pipe Parallel) thread. 

Adapters from BSPP to NPT are available:

https://www.amazon.com/dp/B0CLHK1CP2

https://www.amazon.com/dp/B01LQU8FV8

NOTE: There is a British Standard Pipe Parallel (BSPP) and also a British Standard Pipe Taper (BSPT). Make sure you purchase the Parallel.