Brake Bleeding

[DerekM]

maybe ima abit dense, but is the brake fluid supposed to flow out from the master cylinder into the brake line when bleeding brakes?

My front brake MC had an ample amount of mud caked in the cylinder, which i've cleaned out and cleared the bigger hole that leads to the plunger unit. the other hole is closed off and i cannot run a copper wire through it.  Putting my finger over the hose fitting location provides a substantial amount of suction so i think the MC is good and went ahead and installed a new SS line and banjo unit. i'm not getting any flow into the line, but i see a small stream of bubbles and darker fluid released upwards in the MC. i'm a bit vexxed at the moment and want to make sure i'm on the right path before i start getting into more project scope creep than i am.

[Bob_Roller]

If you are seeing bubbles and a stream of dark, old brake fluid, the small hole is probably open .

If you have a vacuum bleeder tool, I would use it .

A few members use the method of starting from the bleeder screw on the caliper and push fluid in reverse through the brake system, if you have anything to accomplish it this way,  you may want to try it .

[Luca]

If the lever is mushy then you've probably just got a lot of air in the line, which is hampering the hydraulic action.  Pump like crazy with the bleeders open or try as Bob suggests (don't inject fluid into the bleeders with a full reservoir though!).

If the lever is relatively firm I'd make sure your new banjo fittings are lining up properly.

FYI, a cheap luer-lock syringe will readily thread onto clear plastic tubing... I think 3/16" is the size I usually use.  Very useful not only for brakes, but also fork oil, driveshaft oil, wet cell batteries, etc.

[Barry]

The little hole you say is closed off should open as you pump the lever and that little pulse of fluid upwards in the reservoir you are seeing is normal.

When I've completely emptied the system I always fill from the bottom upwards using a large syringe. Easier to drive air upwards than downwards.  You may need a wrap of PTFE tape on the bleeder threads to stop leaks but be sure to keep it away from the inner tip of the screw.

To give the lever a final firm up tie it back to the bars overnight.

[Bob_W]

I start at the top and slowly work my way down. First use your finger to block the MC output hole to build pressure and then not let vacuum suck air back into the unit. add the line to the front wheel and continue until you have fluid at the caliper.
This may take what seems like a long time. If it seems the MC output is clogged, compressed air into an empty cylinder should help open it up.
Bob

[DerekM]

i've got a syringe that can help for running fluid up from the bottom and can borrow a vacuum pump from a buddy. just seems a bit odd that  i can see bubbles moving up from the MC but no fluid is entering into where the banjo and line should be.

[montmil]

i've got a syringe that can help for running fluid up from the bottom and can borrow a vacuum pump from a buddy. just seems a bit odd that  i can see bubbles moving up from the MC but no fluid is entering into where the banjo and line should be.

You have "cracked" the bleeder nipple... right?

[DerekM]

Actually sans bleeder nipple, found mine this evening. I was testing just the MC sans brake line to test it, but I think the test needs to be on the whole system not just the MC.

[arvo92]

Brake bleeding can be a pain to get it right first time but first you have to get the fluids running.

Like Monte asked, have you got the bleeders open when you pump the MC? As you illustrate, you see bubbles in MC as you should but the fluid needs to go somewhere and not be held back by air pressure in the lines.

The way it is done is to pump the MC so you feel the pressure at the handlebar, open the bleeder and that allows the fluid to flow into lines. You close the bleeder once pressure is gone not to let any air suck back into the lines via bleeder nipple. Lets say that I can get all my lines bled with just pumping MC and using this method. My brakes work as they should.

[DerekM]

I had old fluid blocking the mc. I removed the brake line, plugged the bleeder valve in and pumped it out with a mityvac. I was able to get the brakes bled however I appear to have a leak at the MC, (13mm round version) and stuck pistons in the ate calipers. Not cheap. Any advice on replacing the MC and what caliper rebuild kit to buy? Not seeing much on max's besides a $200 MC. Ouch.

[arvo92]

Cheapest way is always ebay if you can find what you need.
You can always check out UK sites which ship abroad:

www.motobins.co.uk
www.motorworks.co.uk

I can not comment on MC but stuck ATE pistons are a problem. You can use compressor to pump them out or just remove the calipers from the fork and pump the handlebar to pump the pistons out. It can be very tricky it they are stuck and corroded. Mine were so stuck that it was not possible to evenly pump both out at the same time (one tries to put something in between the brake pads to pump both pistons out evenly. In the end you can just pluck them out if they are out as much as they can come before one of them just falls out. Then it is not possible to pump anymore and you have to get the other out some way or the other. I blocked the flow of brake fluid into one of the caliper halves with piece of rubber in between caliper halves. Then pumped to get the other half out. Just be creative!

ATE pistons were chromed but tended to rust horribly. If they are rusted, you need to replace them unless they are modestly polishable. I had a front brake failure due to corroded pistons and that crash taught me not to spare money on brakes. It is not possible to get original pistons anymore, there was a link few months ago on another brake bleeding topic of one USA shop selling stainless steel pistons for ATE calipers. You should go for these as they are cheap. Otherwise it is a custom job to make them and they are very expensive.

If you have pistons as mine, replace them!

[Barry]

or just remove the calipers from the fork and pump the handlebar to pump the pistons out. It can be very tricky it they are stuck and corroded. Mine were so stuck that it was not possible to evenly pump both out at the same time  

That's what usually happens even when they are not badly corroded but you can hold back the the piston that does moves with G cramp and that will make the other one move.

[DerekM]

thanks for the advice guys, i will start with the calipers and see how bad the damage is. I don't want to scrimp on brakes and am tempted to replace the whole set up if the pistons are nasty.

[arvo92]

http://www.etypeparts.com/products/ate15-bmw1

That should be your piston right there. My custom made ones were 50 british pounds a piece which is 85 USD. Consider that price you get from that US site a bargain if you buy 4 of these!

It took me 3 months to sort my brakes out and understand everything. So take it nice and slow and take lot of pictures These pictures will be a source of laughter for many years to come.

[DerekM]

Got the calipers apart and popped the pistons out, via a heavy duty syringe, an o-ring and some luck. Tons of muck and brown crap, that did not get flushed out surprisingly. Pistons are in okay shape with only a very small bit of rust on each but are going to get replaced anyways. New pads while I'm in there too. Once the calipers are rebuilt I'll see about replacing the MC.