Front brake: Reverse bleeding

[internix]

I recently had to replace the piston assembly in the brake master cylinder as the old one was leaking. After re-assembly I filled the master cylinder with brake fluid and started to bleed the system in the normal way. However, there was so much air in the system that I was unable to get the brake fluid to pump at all. The fluid level frustratingly refused to drop. So after much head scratching I hunted around my garage and found an old veterinary hypodermic syringe (which I used to use for injecting wallpaper paste into "bubbles" before smoothing). I attached a piece of rubber tube to the end where the needle would usually fit, filled it up with brake fluid then attached the other end of the tube to the bleed nipple on the caliper. Cracked open the nipple half a turn and pumped the contents of the syringe into the system. Hey presto! Big stream of bubbles out of the master cylinder and normal lever pressure was restored. I then bled the system in the usual manner. I would think that you could substitute the hypo syringe for anything similar such as a small (clean!) grease gun or oil syringe or maybe even a windscreen washer pump. I`m sure I`m not the first person to come up against this problem so hopefully my experience will be useful to others.

Nick 

[SteveD CB500K0]

That's a good description Nick.

If you have the bike in bits, another way is to hook the caliper to the garage ceiling so that all the pipework is above the master cylinder and then bleed in the normal way.

[pae]

Gave this a go this morning. Replaced a hose, so I got a syringe, sucked fluid up into it through a short length of plastic tube, pushed the tube onto the bleed nipple, opened it up, and pushed the fluid through the bleed nipple and up into the master cyl - so fast, the job was done in a few minutes. Top tip,

Phil

[Yoshi823]

It also helps to wrap a bit of PTFE tape around the threaded part of the bleed nipple.

A)It stops the nipple from seizing in the caliper.
B)It stops air from entering the system during back bleeding.

[tom400f]

This is a great technique. Filled my front brake from empty this way. In this case if you draw back on the syringe as you go then lots of air comes out of the caliper. Once there is a reasonable amount in the reservoir I worked the lever gently and remaining air in the master cylinder bubbles up. When that's all gone pumping the lever (gently) pushes the syringe back out and you can move the fluid to and fro until there are no more bubbles to be seen. If the system is already full then bleed this way too.

Simple, quick, economical on fluid and very little mess.