Topic: Bloody brakes

[jgary]

I'm bringing back a 1972 CB750 that was given to me.  Front brake was a disaster.  Got the caliper and master cylinder apart, and with Bob Wessner's help, back together again.  No leaks, this time.  The problem I'm having now is bleeding the air out.  I'm thinking that there might be a pump of some sort that I can use to push fluid up from the bleed nipple.  Does anyone know if such a thing exists, and if so where I can get my hands on one?  Thanks.

John.

[dusterdude]

john,its called a mityvac,i used one a couple of weeks ago to do my 72,man that thing is a life saver

[Glenn Stauffer]

Your best bet is to make or buy a pressure bleeder and do it from the top side rather than pump fluid up from the bottom.  I never had much luck with the mityvac though others have.  A pressure bleeder is much cheaper, but only does one thing.

You can make a bleeder using the instructions outlined here:

http://www.bmw-m.net/TechProc/bleeder.htm

I bought one that is essentially the same, but with a pressure guage and a cap for my auto's brake reservoir.  I will be making a cap for my motorcycle brake reservoirs soon.

--Glenn

[jgary]

Holy cannollis!  That's great!  I tried the minivac too, and I didn't have any success with it.  This pump setup looks great though.  Off to eBay to get a spare master cylinder cap.

John.

[dusterdude]

now i will say,with the mityvac you will pump your ass off but i does work

[bransonmowoodman]

I used a  mayonase jar with a lid on it drilled 2 holes in the top, pour some fluid in the jar and attach a plastic tubing into one of the holes down into the fluid, attach the other end on the bleed screw on the brake caliper,fill resevour and squezz the brake handle, when airbubbles stops, tighten the bleeder screw and fill resevoir up to line and you should have it, The brake lever will get tighter as the air is expelled.

[Bob Wessner]

John,

Perhaps I was just lucky, but when I did mine, I found the instructions that have you turn the bleed screw, apply brake, screw in bleed screw, etc. a really pain in the butt. I finally just removed the bleed screw altogether and used my thumb over the hole in the correct sequence of events. When there was fluid in the entire system, then quickly replaced the screw and then just a few times using the instructed method, untightening and then tightening the bleed screw with a tube in the jar, did the trick. Good firm brake lever. Again though, maybe I just got lucky.

[jgary]

This machine is a great learning opportunity.  That's what I keep telling myself, over and over.  I printed out the instructions for making a pressurized bleeding tool.  It occurs to me that, using that pump, I have to compress the air in the master cylinder before I start to compress the air in the caliper.  It then occurs to me that I should be able to put a really long hose on the bleeder nipple, fill the hose with fluid, and compress with the pump, forcing fluid from the nipple back up into the caliper and eventually into the master cylinder.  The reason for this approach is that I have a bunch of tubing just hanging around (literally) and can do it right away, tonight.  To make my own pressure device the other way, I need a spare cap for the master cylinder, plus some fittings.  Not a big deal, but I would have to wait.  The hitch in the giddy-up that I can see in the push-from-the-bottom method is, will the piston in the master cylinder allow the air to pass?  Or should I just forget this crazy idea and stick with what has been proven to work.  Recommedations are welcomed.  Thanks.

John.

[dusterdude]

bob,i had to do mine the same way

[Bob Wessner]

I did notice one thing when doing the final bleed using the tube in the jar. It appeared to me that it would be best to use the smallest inside diameter tubing you can find as long as it will stretch over the bleed screw. It appeared to me that if the diameter were too large, you could lose the syphoning affect.

[dusterdude]

yea that do make sense.

[Dennis]

jgary,
I always bleed my brakes from the bottom up. I hate fighting gravity. The air bubbles want to travel upward. I don't want to fight them.
As far as the master cylinder allowing air to pass through, it better. Beacuse if it doesn't then the fluid can't return when you release the brake lever and your front brake will be on at all times after you push tha piston out as hard as you can.
I usually use a large animal syringe and push fluid into the caliper bleeder until no air bubbles appear in the master cylinder.  It is best to do this with a helper to watch the master cylinder because you may have to remove some brake fluid before you get all of the air out of the caliper and lines. Some of the discount stores, Walmart I've heard carry a gadget to mix two stroke oil in the form of a syringe which I have heard works well. Also you can get a trigger pump style oiler to push fluid into the system, so I have been told that this also works well. Vinyl tubing for all connections. never use the oiler for lubricationg oil, get one brand new and mark it.
Well there you have a bunch of suggestions, most of which have been tried and made to work.
Make your selection and take your best shot.