Bicycle Mechanics - How to bleed Hydraulic Brakes?

I am thinking of buying some hydraulic disc brakes of a friend but the front brake needs a bleed. I know its cheap getting it done by the lbs but i would like to try it myself. So how do i make a 'bleed kit' and what do i do with it? I know I need DOT 4 brake fluid which i can get easily but the rest i have no idea about.

Thanks guys.

Oh and I have tried searching the forums but it hasnt been covered very well.

If you are getting hayes you may need dot 4. But shimano and others use mineral oil.

Most good companies offer a bleed kit to buy and hayes even has descriptions of all the parts to make them. Personally for hayes I simple got a syringe, 2x tubing and an old 1 litre bottle of pop. I forget all the diameters of the tubing but thats all you really need. The syringe + one tube pushes the oil from the caliper out the bleed hole (master cylinder) in the lever body, this is captured by another tube which puts it in the 1 litre bottle. Pretty simple and only take minutes to do (depending on which hayes the tubing changes so definately check the site :))

Ohhh i get it now. So the oil goes in at the caliper and out at the lever? And how much oil do you need to pass through it?

Ohhh i get it now. So the oil goes in at the caliper and out at the lever? And how much oil do you need to pass through it?

It doesn't take very much brake fluid. The trick is to position the lever so that the bleed hole points straight up. That way all of the air bubbles naturally want to go there and they are easy to push out. I think that a 1 liter bottle is kind of big but run a piece of plastic tubeing from the bleed hole into the bottle and submerge it in a little brake fluid. It might be easier to have a helper hold the bottle and tubeing. Now when you push fresh fluid in from the bottom, no air can reenter the system from the top. When you get no more bubbles coming out of the master cylinder, close off the bleed screw and you're done.

Okay, thanks for your help. I completely understand it now. :)

Another question: Seeing as it is just air that causes the problems, can you re-use the same oil? ie but some more oil and do a 'rotation' of oil?

Get somone at your LBS to walk you through it, even if you have to pay. Its alot easier when someone shows you how. The mechanic at my LBS showed me how to do the back brake and then I did the front. And hes really cool i didnt pay a thing!

Another question: Seeing as it is just air that causes the problems, can you re-use the same oil? ie but some more oil and do a 'rotation' of oil?

Probably not a good idea. Another problem is that the hydraulic fluid absorbs water from the air. Not only would I not reuse brake fluid, but be careful to brake fluid containers tightly sealed. DOT brake fluid is cheap. If all that you use it on is a bicycle, a pint, or whatever size container it comes in, will last you the rest of your life.

Okay, thanks everyone.

While on the subject of Brake Fluid it might be good to tell those who may
not know it that brake fluid is quite corrosive and destructive of paint finishes, coatings, etc.
One would not want to get any of it on the fine fancy finish one finds on most bikes these days. Grin. If it does, wipe (dab it ) away and rinse with water and soap immediatly.

Be careful with that stuff.
Oh, it is also a great skin irritant. Does wonders for the eyes too, not.

Thanks for the tip UziBEatle, I didn't know that.

I just recalled something. With modern clear coat coverings over our paints, on cars, and I ASSUME on
bicycles, the corrosive color draining paint finish ruining benifits of brake fluid are mitigated by a fair amount.
So, one has a bit more time to respond before hitting the red panic button, but I'd not tarry getting it off that finish, nonetheless.

I just tried to bleed my rear brake (Hayes). I use a syringe, not Hayes' plastic bottle. My problem is that I need a so tremendous pressure to displace the fluid that I had to give up. What am I doing wrong? In the Hayes instructions they say to tilt the bike 45 degrees. I thought the height difference was what required so high pressure so I tilted the bike almost horizontally (max 30 cm height difference between caliper and master cylinder). But the pressure required is still tremendous. I am able to bleed one drop every 5 seconds with great effort.

Open up the bleed bolt on the caliper. (The port you attach the hose from the syringe to) Just loosening it a tiny bit wont open it enough. You need to open it a full turn.

When you hang the bike up, adjust the brake levers so the bleed hole faces up. Also, wiggle the lines and hold the lines so that the air will travel up to the lever.

Thanks, Dirtbike!
I had the brake levers right but I only opened the caliper bleed bolt 1/4 turn. I'll try full turn next time. I ended up doing it differently though: I connected the syringe to the levers then did the following a number of times:
- open the caliper bleed bolt
- press the lever all the way
- close the caliper bleed bolt
- release the lever (results in pumping in brake fluid from the syringe)
This is how my mechanic bleeds Shimano brakes. On Shimano you have a small reservoir that you can open and fill with fluid every 2-3 lever stroke. Since you don't have it on Hayes, you need to use a syringe.
I'm not sure it's recommended to do it like that though: looks like I didn't get all the air out. Also I have the impression that the new oil got mixed with the old one. I'll have to test it downhill and see if it still works fine when warm.
Btw, I fastened the (rear brake) caliper to the fork so I could operate both bleed bolt and lever alone.
I'm not sure I got

The main reason you start from the bottom is that it aids in pushing the air to the top. You should have a little piece that attaches to the bleed hole on the lever. We have a little plastic cup with an old spoke as a handle that we hand on the handlebar, and then have the hose come out of the lever into the cup to catch the old fluid.