A tip for bleeding brakes

[Guest]

Here's a new trick to try when bleeding hydraulic brakes:

Very lightly tap the brake lever, and I mean very lightly, just enough to open the little port where the replacement hydraulic fluid is sucked into the piston. This will let tiny air bubbles trapped in the master cylinder escape up through the top instead of pushing it through to the bottom. You really can get a lot of air out of the top of the hydraulic system.

Since air bubbles will collect and rise in the fluid, I sometimes do this after a day of sitting to find more air released.

[jimfj]

I have used the same method and found it very useful as well. Good tip.

[chuck hahn]

mighty vac..no muss no fuss

[jknappsax]

Which Mityvac do you guys use? I see them for various prices, and don't know which to buy, or how to use it to bleed brakes.

[chuck hahn]

I dont know the model number but i got the cheapo plastic one with the plastic collection bottle. Was like 42 bucks at Murrys several years ago.

[willie]

Harbor Freight sells a brass-bodied one with tubes and fittings. I bought one a month or so ago for about $20 when I noticed it on sale.
Willie

[jknappsax]

So, what's the procedure? I've manually bled plenty of systems, and would like to use a better, easier, more efficient method, but I've searched the forums, and can't seem to find a how-to.

[AJ]

The master cylinder on my GS1100 is toast and I replaced it with an old 550e master I had laying around. Just for grins, I wanted to see if I could completely bleed the system from the top.

First, I poured a little brake fluid into the piston where the banjo bolt connects and worked the lever a few times just to lubricate the seals on this master that I know has been sitting in my parts bin since 1989. Then I bolted it onto the handlebars, connected up the brake hose, and filled the reservoir with new brake fluid. At first I got no air out from the top of the master, so I made a couple full strokes on the handle. Soon I had a big bubble locked to the big hole and light tapping on the brake lever got that to release and float to the top of the reservoirs. After awhile, I was getting regular bubbles out the big hole and a smaller stream of bubbles out the little hole, but still no pressure. Every now and then, no bubbles would come out the top, so I would make one or two full strokes on the handle. After a big stroke, I'd wait 5 seconds or so to let the bubbles float back up the hose and up into the master cylinder, then followed with small strokes on the brake lever to be rewarded with even more bubbles coming out the top. Eventually, I was able to coax all the air out of the system working entirely from the top and got good pressure.

Just as a final process check, I bled the entire system the old fashioned way, down at the nipples on the brake calipers. I wanted to flush the fluid through anyway, but was also pleased to see that no air came out of the nipples at the brake calipers, demonstrating that it is possible to get all the air out the master cylinder from the top if the lower part of the system was already free of air. It probably took longer to do this way, maybe 10 minutes total, but it was an interesting experiment.

[Steve]

Which Mityvac do you guys use? I see them for various prices, and don't know which to buy, or how to use it to bleed brakes.

Some of the more-expensive models might have metal bodies or more accessories, but neither one really helps that much, for a weekend mechanic. If you are a professional mechanic and are likely to use it several times a week, go for the better ones, but more most of the rest of us, the cheap one works just fine. Mine was about $30 or so from Auto Zone, it's the one with the plastic body. I have had it for over 10 years and it still does the job just fine.

So, what's the procedure? I've manually bled plenty of systems, and would like to use a better, easier, more efficient method, but I've searched the forums, and can't seem to find a how-to.

I have found that it works better if you remove the bleeder nipples from the caliper (and anti-dive unit, if your bike has them) and wrap them with Teflon tape to seal the threads, then re-install them.

- Assemble the Mity-Vac so that the catch can is between the pump and the caliper, use a clear hose to attach to the bleeder nipple.
- If you are doing a full flush of the system, use the Mity-Vac to suction all the old fluid out of the reservoir. refill with fresh fluid.
- If you are simply trying to remove some air bubbles, make sure the fluid level is almost to the top.
- With the hose attached to the bleeder, pump the handle several strokes to build up some vacuum.
- Open the bleeder, you will see fluid start to move through the hose, into the catch can.
- Add vacuum as necessary (just watch the gauge), also make sure the fluid level stays above the bottom of the reservoir (a helper can be very handy).
- You should soon see fresh fluid and maybe some bubbles come out the bleeder nipple. Continue until you don't see any more bubbles.
...(If you had not sealed the threads with Teflon tape, you would not know if the bubbles are from the brake fluid or the gaps in the threads.)
- Close the bleeder. (I will build up a bit of vacuum so fluid gets sucked into the hose as I pull it off the bleeder.)
- Remove the hose,
- Empty the catch can.
- Move to the next bleeder.
- Repeat as necessary.

Most mchanics agree that this should be done at least every two years, depending on the conditions where you ride. An easy way to tell if it needs to be done is to look at the color of the fluid in the reservoir. If it is CLEAR, great. As it ages and absorbs moisture, it gets darker. When it gets to the color of root beer, it's really past due, so change it NOW.

.

[Normk]

Ajay, I do it that way frequently when servicing brakes. As you noted, it can work very well and reduces the difficulty in moving air downward against gravity action. Vaccum bleeders such as the Mityvac work quite well but lack the flow rate of air powered bleeders which are used in the automotive trade. These guys pull a high volume continuously so don't mind the air leakage around the bleeder threads.

Another trick to bleed the master is to push the caliper pistons inward which will often displace enough fluid to eject the air from the master cylinder bore.

You will have noted that one needs position the master cylinder so that the compensating port (the tiny hole between the reservoir and the cylinder bore) is at the highest point so that all air collects against the port and can be ejected. Don't confuse this with the breather port which is closer to the lever end of the master cylinder because air will not be ejected from the breather unless the master was full of air.

It is helpful to bleed the master cylinder before installation as that reduces the amount of air needing to be removed.

The main advantage of the metal bodied Mityvac is that the body is unlikely to be damaged by sucking something like diesel fuel into the unit. I learned this when using mine to bleed fuel injection transfer lines and wasn't paying enough attention.

It was even more of a problem because I was working in Vanuatu at the time and 3rd World countries are short of tool suppliers.

HIH

Norm

[Normk]

Another trick to remove those last stubborn bubbles which are not moving (assuming that you have used an 8 mm or 10 mm wrench to tap the metal parts to knock the bubbles loose) is to pull the lever up hard and wrap with a bungee cord. Leave it sit over night and the increased pressure will have caused some or all of the trapped air to dissolve.

This doesn't work with large volumes of air but can help remove those last stubborn bubbles. The last step is to bleed out the fluid which contains trapped air as it may come back out of solution in a problem area. It usually just finds its way out in tiny amounts or stays dissolved but it is prudent to get it out. After all, the solubility of a gas in a liquid is inversely proportional to temperature which means that air could come out when the fluid is hot and that's likely not the best time to have a spongy brake.

A huge problem area for the amateur wrench is having a reference point to decide whether that lever is spongy or not. The longer one fiddles, the more one becomes convinced that it is spongy. I strongly recommend taking notice of how far the lever and pedal move with a known application force. Any guesses how I figured this out? (VBG)

Norm

[Heintz]

Go to a pet supply or any place to come up whith a large plastic syrenge and plastic tubing to form a good seal on syrenge. with master cylinder cap removed and lever pulled to handelbar and fixed in that position crack bleeder valve open and attach hose to bleeder valve. Use syrenge to pull fluid through system.......make sure fluid level in master cylinder is toped off at the same time fluid is being pulled through system.....a syrenge and tubing should cost no more than a couple buck. This works great for evacuating old fluid or just bleeding brakes.........

[Normk]

It will be easier to move fluid if the lever is in the released postion rather than pulled in. When the lever is released, it moves the master cylinder piston back and clears the compensating port so that there is an opening between the cylinder bore (and also the line) and the reservoir. That's the small hole in about the center of the reservoir which is sometimes covered by a deflector.

If the lever is pulled in any significant amount the piston cup is past the compensating port so you will have to act against the cup in order to move fluid from reservoir. The effort required is not in the order of that required in the other direction but since there is not advantage to having the port blocked, it is better to leave the lever in release position.

Another aspect of master cylinder operation is that the master cylinder piston must be allowed to move past the compensating port in order for there to be proper release of pressure. People sometimes adjust lever position or substitute the wrong lever which prevents the piston from opening the compensating port.

The most common symptom of this condition is dragging brakes when the system heats up. If the master cylinder cup seals as it is supposed to do, the brake will lock up and bring the wheel to a stop, sometimes harder than the rider wishes to deal with.

HIH

Norm

Go to a pet supply or any place to come up whith a large plastic syrenge and plastic tubing to form a good seal on syrenge. with master cylinder cap removed and lever pulled to handelbar and fixed in that position crack bleeder valve open and attach hose to bleeder valve. Use syrenge to pull fluid through system.......make sure fluid level in master cylinder is toped off at the same time fluid is being pulled through system.....a syrenge and tubing should cost no more than a couple buck. This works great for evacuating old fluid or just bleeding brakes.........

[Road_Clam]

I now use a Mityvac. It's the ONLY true way to remove 99.9% of stubborn air from braking systems. I tried hand bleeding my GSXR brakes for 2 dam weeks and never was happy. I got a Mityvac, and I also got a tip to remove the caliper from the fork, bring the caliper under vaccume and pivot the caliper with your hand and bring the bleeder to verticle. This method gave me a sweet firm feel to my GSXR's fron brakes. It's now my ONLY method of brake bleeding !

http://www.mityvac.com/pages/products_bcbe.asp

[bccap]

I now use a Mityvac. It's the ONLY true way to remove 99.9% of stubborn air from braking systems. I tried hand bleeding my GSXR brakes for 2 dam weeks and never was happy. I got a Mityvac, and I also got a tip to remove the caliper from the fork, bring the caliper under vaccume and pivot the caliper with your hand and bring the bleeder to verticle. This method gave me a sweet firm feel to my GSXR's fron brakes. It's now my ONLY method of brake bleeding !

http://www.mityvac.com/pages/products_bcbe.asp

I just purchased mine, but have only used it once. I didn't have any teflon tape at the time, to seal my bleed screws. But never the less managed to successfully bleed my brakes, through constant pumping. Correct me if I am wrong ? The proper way to use it is to pump while the valve is closed, then open the bleeder, and cease pumping ? Also which tip is appropriate ?
Thanx