I some times reverse fill the brake system using a 60cc syringe filled with brake fluid. Start with little or no fluid in the mc resiver and push the 60cc of fluid through the slave bleed port.
Good point ckahleer, a three Dollar 100 ccm syringe i use to bleeding the break system can vaccumise and pressurise the bleeding process of the brake system. No need to buy expecive special tools. You only have to to remove/clean the break fluid from the piston or the rubber part will swell up and sized in the syringe cylinder!
A common problem when bleeding is getting the caliper pistons in the proper position, close to the disc. Unless the caliper piston moves close, into its proper position, the hand lever will feel spongy. Also, you need to prime the master before it will build pressure. And, as mentioned by others, if the small return port is plugged, you will never have properly working brakes.
It's critical, with an old brake system, to de-crud the calipers on the inside by taking out the piston and descaling everything. Also, replace the brake lines with new. Preferably, teflon/stainless lines.
If all this is done and the system still won't build pressure like it should, then pump the lever and tie it back overnight. This will force the piston to creep forward, toward the disc, and then the lever will firm up.
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I've never had much luck using a vacuum system by itself. The best method I've found for motorbikes is to:
- Push the caliper pistons all the way in
- Open the caliper bleed screws, attach hoses/vaccum bleeder.
- Vacuum bleed the system as best as possible. If you don't have a vacuum bleeder you can just pump it as normal, but that's hard if the system is dry
- Take off the bleeder, and pump up the calipers until they are where they need to be
- Bleed the calipers once more, which should take out any remaining air in the calipers
- Check that the brake lever is firm. If it's not, there's air in the lines. Bleeding themagainst gravity is difficult so:
- Push the caliper pistons back in all the way (make sure you don't overflow the reservoir). Any remaining air in the lines should be pumped back up to the reservoir with the fluid, and because it's going up, gravity will assist this time. As you're pushing the caliper in, tilt the brake splitter junction (or tilt the bike if still attached) and tap it with something hard to dislodge any bubbles that may be sitting in there.
- Pump the calipers back up with the lever again. The lever should now be firm. If it's not, you may need to bleed the master cylinder (take off the lever and pump with the full travel of the piston with a pushrod of some sort), or there is a issue with seals or incompatible parts (eg using a 14mm master cylinder on a dual caliper KZ).