Front brake bleeding nightmare

[warmachine81]

  I've recently replaced the front brake line and fitted the original master cylinder and fitted new pads. Have bleed it to absolute buggery and can't get pressure. The cylinder worked fine prior to this. I did use a clamp to push the Pistons back in the caliper when putting in the new brakes. Have a screwed my seals and got my pistons stuck or did my master cylinder die while it was off the bike for a couple of months?

[barry905]

One thought that springs to mind is that after you have bled the brakes and removed all the air you need to advance the pistons to get the pad in contact with the disk. This can sometimes seem to take forever and needs a lot of pumping the brakes.

Good luck.

[warmachine81]

I spent hours bleeding the system over and over and the Pistons haven't moved one bit. I can't even pull them out.

[Janx101]

Sounds like a remove the line, fit an air spigot and apply steady pressure with compressor job? Perhaps? ....

Also... with line removed make sure fluid pumps out the end with a press of lever?!

[fetor56]

Shouldn't have been necessary to use a clamp to push the pistons back in.Pistons externally lubricated with brake fluid (along with their seals) and worked in by fingers should have been enough.
Use air pressure (GOOD air foot pump/12v air compressor/etc) to see if the pistons have movement.If your going to remove the caliper place something resembling the disk rotor in the caliper so the pistons don't over-reach their travel and pop out.

[Fatasianboy]

Did you use a vacuum bleeder? When I didn't have a vacuum bleeder it was a pain in the ass to get the air out of the lines. Once I got the bleeder it was done in 5 mins.

[barry905]

Thinking about how the brake operates, the brake lever moves a piston in the master cylinder and this moves a column of fluid down the brake line. When this fluid movement hits the caliper it puts pressure on the piston which clamps it on the disk. So if you could bleed the brakes a column of fluid must move down the pipe to the bleed nipple and then out of the system. If this works then the master cylinder is OK. Which implies you have a problem in your caliper. If neither piston moves, then either they are both stuck or there is no pressure pushing them. But you have fluid movement and thus pressure (when you bled the brakes), and if the pistons ere stuck then you would have pressure on the lever but no braking!

Sorry to ask the simplistic question, but when you bled the brakes you did make sure that the master cylinder was topped up at all times. I have known cases where is was not, and more air was introduced into the brake system. You have a system where you can bleed it, which means that the fluid moves, but the pistons don't move and you have no pressure, which implies air in the system. Try pumping the lever to see if you can get pressure.

[Big Rich]

One thing I want to mention: if you use air pressure to move the pistons, DO NOT leave your fingers in the path of the pistons!! They can come out with a lot of force, and possibly amputate a finger. A thin piece of wood to catch the pistons works well.

[lucas]

Fingers crossed they read that warning before they start.  This could turn into a real brake bleeding nightmare 

[boggy]

Hi, had same nightmare when I replaced the seals in the caliper, could not get any pressure when bleeding it, I solved my problem by taking the pipe and bleed nipple out filled the caliper up with fluid using a shringe, then reconnected the pipe and nipple and bleed them. It worked for me

[warmachine81]

I've left it for a couple of days now. Am about to take the caliper into work and hook it up to the compressor. Fingers crossed. I haven't paid anyone to do anything on this rebuild yet. I'll be dammed if something as simple as brakes stuff that up. At no point did my master cylinder suck in air. Fingers crossed

[warmachine81]

Plus I'm just using a simple one man bleeder. I didn't like the aussie price for a vacuum bleeder

[warmachine81]

So the air compressor was able to move the Pistons out enough for me. I gave everything a good clean with brakes cleaner. Had to push the Pistons back to get it to fit on the disc. I'm a fairly strong guy and still needed to use a clamp to push them back. Did the rebleed and still no pressure or pad biting the disc. With the line off the caliper it still pumps fluid out. Prior to changing to the braided lines my brakes worked fine. I'm completely stumped on this. I have another ebay master cylinder. I might try that again. It worked prior to changing the lines and pads.

[Big Rich]

If the pistons are moving freely, and the master is pushing fluid, there has to be air in the system somewhere. There's no weird loops in the brake line, is there? It only takes one little pocket of air to mess up the entire system. I've gone as far as removing the entire brake system and mounting the caliper to a rotor OFF the bike, to ensure the brake line is straight up and down.

Keep at it man, you'll get it.

[warmachine81]

Hahaha no weird loops. I can't understand how there can still be air in the system if I'm only bleeding fluid.

[fetor56]

It concerns me that u STILL had to use a clamp to push the pistons back in.Having said that u can try(with the pistons fully in/home,so there's absolutely minimal air in the caliper) injecting brake fluid FROM the caliper TO the master cylinder.This is done via a syringe and plastic hose....medical syringe's are the cheapest and u already have the hose.
Be careful not to overfill the MC.

[warmachine81]

Reverse bleed! I'll give it a whirl

[fetor56]

Reverse bleed! I'll give it a whirl

Sometimes u need to do a combination of both, but that's pretty rare.

[warmachine81]

Sound I hold the lever down while pushing on the plunger. I tried it with it out and made a massive mess when the tube came off the syringe 

[fetor56]

Sound I hold the lever down while pushing on the plunger. I tried it with it out and made a massive mess when the tube came off the syringe 

Don't hold the lever down...just let it flow up the line and into the MC with only moderate pressure.Be patient,it MUST flow unless something else is wrong.
PS...not meaning to be a smart ass but i trust your leaving the cap off the MC as your doing this and leaving the bleed nipple open.