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Brake Master Cylinder Bleeding

Bleeding Air Out Of Master Cyclinders

By Roger Albert

From: Roger Albert <>
Date: Wed, 10 Mar 99 15:17:36 CST

… issues with air in a Master Cylinder and newly rebuilt brake system:

I’ll now pretend to be able to help.

In my experience, it’s tough even with a mighty-vac or similar to extract all the air from some Master Cylinders (MC) and calipers. Any high points (akin to local maxima) tend to trap a bit of air at their peaks. Sometimes, going through the bleed drill with the bike and bars at different and various angles can help. Fiddling and patience are all good.

Without knowing the year or model, we can’t be too specific, but I’ll offer two tricks I’ve found pretty effective, and which I’ve not seen mentioned much in the Beemer/Duc worlds, but seem to be more commonly practiced among the HD and cage folks — not that I’d drawing any link or conclusion there. 🙂

The explanations below are semi-long because they’re semi-detailed. The actual work will likely take less time than the reading. Both are pretty cheap and will save a lot of time/effort in the long run.

1) The first one involves 1 or, ideally, 2 syringes. A lot of Harley shops sell a pair vacuum packed on a sheet of cardboard along with a couple flexible clear vinyl elbows. Supposedly readily available cheaply from any Vet as well – mine have always come from a cousin/HD-mechanic.


The idea, is to use one to draw off most (certainly not all, you want the port in the MC to remain covered in fluid) of the fluid in the MC. The other is then inserted in a fresh bottle of your favorite brake fluid and filled. Using a piece of vinyl hose to attach the syringe to your bleeder screw, crack the bleeder a bit and force fluid into the bleeder, being careful not to overfill the MC. This performs a simultaneous flush and bleed, and has the advantage that it pushes the naturally buoyant air bubbles upwards and out the MC, rather than trying to draw them out the bottom. Why fight nature? Trying to force air down has always struck me as inefficient at best and often is more akin to beating your head against the wall. Anyway, two or three cycles of each syringe and it’s done.

I should explain how to clear the air from the syringe and line to the valve so that air is not pushed into the system from the syringe – it would have become evident to you once you started. With the hose you’ll use attached, push the plunger down to purge the air and insert the hose into the fresh brake fluid. Pull the plunger out as far as possible drawing fluid into the hose and syringe. If you keep it semi-vertical, the air will rise to the top of the plunger naturally, and thus you’ll have only fluid at the bottom. The most air you could introduce would be the volume inside the bleeder screw outlet. Even that can be minimized via filling it via one cycle of the standard old squeeze/crack/tighten method.

About a concern that pushing fluid from the bottom may mess up the valve in the MC, I’ve yet to see an MC design where that would inherently be the case. (I don’t have any experience with linked or ABS so I won’t generalize to cover them – that smacks of danger.)

What might be the grounds for some concern, is that crud and corrosion are much more likely to form/collect in the caliper than the MC (heat and proximity to dirt and moisture from the road and what have you). Pushing fluid up that way could dislodge some of said crud and deliver it to the MC piston. The most likely problem scenario would seem to be blockage of the small return port, wherein each subsequent pull of the lever pushes more fluid into the line/caliper, but the fluid can’t return to the reservoir (low-pressure-side) upon lever release. Pressure builds with each additional lever pull (and thermal expansion – easy to get a vicious circle here) but can never back down. You’ve ‘fabricated’ a hydraulic ratchet if you will. Binding, or even lock-up ensue. So, let me add a caveat, if you’ve got or might have a lot of crud because your brake system hasn’t been well maintained [ no doubt by the previous owner 🙂 ] then don’t do this. For that matter, you probably ought to park it until its flushed/cleaned and/or rebuilt anyway if that’s a major concern or threat.

This works great on my Duc and on handlebar mounted Magura MCs. I’ve had a bit less luck with this on my under-tank ATE setups on /6 /7 era mounts. Haven’t deduced why just yet.

Brings us to number …

2) This requires two fairly readily available widgets which will be somewhere from cheap to free if you’re in good with your bike and/or auto/tire dealer.


This does just the opposite of the above (forces fluid in the top and pulls excess off at the bottom). The disadvantage of trying to force air out the bottom is largely negated by being able to force most or all of the air out at once (as opposed to the short stroke of one pull of the brake lever).

This can be done on most any MC, but is particularly easy on the old under-tank ATEs. The idea is to take an old master cylinder cap (even a marginal or cosmetically challenged will do just fine) and install a standard automotive Schrader valve in it. Pretty readily available in most any motorcycle dealer’s used parts room, or perhaps from your or a friend’s parts-bike. My Schrader valve was given to me for free by my regular auto tire dealer who gets lots of my business and who didn’t want to be bothered to ring up such a small amount. (OK, not a lot, I need new car tires like once every 5 years at most, but 100% of my minimal business, anyway.) Get the type of valve that has a threaded stem and ideally a nut and rubber seal, as opposed to one of the rubber molded press-in deals. With the round plastic ATE cap, pop out the electric switch and any cheap drill will bore a hole to mount the Schrader valve. Something similar will work for most rectangular metal caps, but often the text molded into the cap must be ground off for a good seal. A thick pliable seal should eliminate the need for grinding if you’re not so inclined.

At any rate, install the Schrader valve in your spare MC-cap to make the pressure bleeder. Top off your reservoir and substitute your pressure-bleeder for the normal cap. Run some vinyl hose to a small catch jar/can, and ideally put enough fluid in it so the outlet of the hose remains covered. Now, using any low pressure air source (<=10psi seems good and safe, yet effective in my experience — a regulated compressor, small syringe type pump, or even a bicycle tire pump) apply a bit of pressure to the MC-cap/pressure-bleeder just as you crack the bleeder screw (just a bit 1/8-1/4 turn usually adequate). Fluid and air come streaming out. Just keep an eye on the fluid level in the MC reservoir. Naturally, tighten the bleeder before backing off much on the pressure or before the fluid becomes low. A big advantage here is, the long stream of fluid (and trapped air) that is pumped out at once. Using the standard crack/squeeze/tighten cycle, air that doesn’t immediately make it out the bleeder often rises back to its original location before the next cycle can take place and finish forcing the air out. Here, with the pressure bleeder, you are limited in how much you can force out at once only by the capacity of your MC. I usually use two fill-MC/force-air-out cycles and my /6s are immediately done. This is made even easier with a set of $13 speed bleeders with their built in checkvalves.

You can very likely make the pressure bleeder for under $10, and surely both construct it and pick up a speedbleeder or two for a one-time expense of <$25. I’m a tool/gadget geek, so I’ve rationalized (I believe quite correctly) that this is very worth it. The first time you perform a good bleed on a normally difficult system in about 10 minutes, including clean-up, you’ll probably feel it was time and money well spent. I built mine in well under 10 minutes as well. Maybe 20 minutes for the text-laden, metal Ducati version.

Hope some of that helps. Its really much easier than it sounds. If anyone thinks its worth the trouble but has questions, I’d be glad to write or call with help. Same thing works on cars too and is even cheaper, as junkyards will almost always cough up an old MC cover for $0-2.

My next version (always gotta tinker) will be to build one with its own little primeable reservoir so I’m not limited by the capacity of the built-in reservoir.


Roger Albert, Ex-Illini, Motorola Wireless/DSP, Austin

From: Cuno Walters <>
Date:Thu, 24 Jun 1999 02:35:50 -0500

I have a trick for the tech article :

If you think there is air in the braking circuit or the brakes feel soft put the brake under pressure for a night by binding a rope around the brake lever. The next morning the brake will feel much harder. The air trapped inside the circuit will “bubble up” due to the pressure.

All fore now

Cuno Walters

Hints for bleeding handlebar mounted reservoir hydraulic systems

From: Brian Curry <>
Date: Fri, 11 Dec 1998 00:59:19

When you are bleeding your front brakes put the bike on the side stand and swing the handlebars fully to the left. This makes the master cylinder to the brake fluid reservoir port, and the reservoir the high point of the system. So, any air bubbles in the hoses to the calipers or the ABS modulators will migrate to that high point (gravity and density difference is a good thing.) and out of the system. :):) (You may have to rotate the grip a bit to keep the reservoir horizontal in one plane.)

Joe Katz (retired wrench) brought this up at the K Tech Session at RA in MA, but it is applicable to any bike with a handlebar mounted brake fluid reservoir. So, pay attention R and R11 people. While it is obvious once you hear it, I had never done it. But I will in the future. :):) Anything that makes working on brakes easier is a good thing.

In the case of a hydraulic clutch, (K12RS, and R11S) I would put the bike on the center stand, and turn the handlebars to the right to elevate the reservoir. (Same principle.)

Thank you Joe Katz!!!

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