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Ok, so installed my new 105 R7020 shifters and brakes today but am getting a result I cant quite understand. on the rear there is a long lever throw, almost hitting the bar before the brakes apply, yet simultaneously the brakes are rubbing with no pressure to the brake lever. The front is set up perfectly, with light travel and a hard stop about 1.5cm away from the bar.
I did a full bleed and back bled the caliper, and have then tried the '5 min bleed' a few times using just the funnel and pumping the brakes, with to no effect. So, question 1, each time I do the 5 min bleed I get quite a few air bubbles, does this mean there is an air leak in the system somewhere, or just that when you open up the system you inherently get some air inside at it's this air that is then coming out?
Question 2, how can I fix this issue? The pads are a bit worn already as it's a second-hand set, if that matters.
It sounds like there's air in the system. What I find helps is to connect your syringe to the bleeder on the caliper, open the bleeder, and get a friend to gently squeeze the lever while you draw fluid out using the syringe. Close the bleeder, release the lever, and top up the reservoir, then repeat until air bubbles stop coming out the caliper. This make sure there aren't any bubbles trapped in the caliper and I find it's much more effective than just trying to use a syringe. Then use the syringe to pump fluid from the caliper to the lever until air bubbles stop coming out of the reservoir. Close the bleeder and it should have a firm lever feel.
The other problem may be that caliper isn't centered properly or one piston is extended and sticking. With the pads in, gently wedge the pads apart with a clean flat screwdriver to make sure the pistons are fully retracted. Do this with the reservoir port open to allow excess fluid to escape. Then refit the wheel and check that the caliper is centered over the disk and that the wheel spins freely. When you apply the brakes, check that both pistons are moving and that they both release when the brake lever is released.
I did try gently pumping the brakes with the bleed port open, though I didn't close it before releasing the brakes. I also tried squeezing the brakes and quickly opening and closing the bleed port and then letting the brake lever return to normal. This sounds similar to your suggestion?
I did look at the pistons, and it looks like they both move and retract somewhat evenly. I have heard the centering method of applying the brakes before tightening up the bolts doesn't always work, can anyone explain why it doesn't? In my head, it should always straighten and centre the brake.
I did try gently pumping the brakes with the bleed port open, though I didn’t close it before releasing the brakes.
This will suck air into the caliper.
I also tried squeezing the brakes and quickly opening and closing the bleed port and then letting the brake lever return to normal. This sounds similar to your suggestion?
Yes, but doing it with a syringe connected, then pumping fluid back through the system ensures you are pumping bubble-free fluid back through the system and flushing all the air out. Pump a full syringe full of fluid back up to the reservoir to make sure the entire system is flushed.
I have heard the centering method of applying the brakes before tightening up the bolts doesn’t always work, can anyone explain why it doesn’t?
Sometimes the caliper moves when you tighten the bolts. It often helps to put a feeler gauge or strip of coke can between the pads and rotor while you tight the bolts. This can prevent the pistons from extending too much and causing the brakes to drag.
"This will suck air into the caliper."
I'm not questioning but how, where does the air come from? I have a syringe of fluid on the bleed port and a cup full of fluid at the lever at the time.
I just saw this video too, would this be doing the same thing?
If the lever comes to the bars before there's any contact, this doesn't sound like air in the system to me. Even with air in there, the pads should still move - it would just be spongy when you start to apply more pressure.
I have heard the centering method of applying the brakes before tightening up the bolts doesn’t always work, can anyone explain why it doesn’t?
Poorly faced disc mounts could do this. Or as above the caliper is moving when you tighten the bolts. But it can also be caused by one side being stuck. Which, given your other symptoms, could be the issue.
If you put something light behind the brake and shine a light on it (or do it in bright light outside) then you should be able to see the gap between pad and disc. For referene also, the disc should be in the middle of the caliper. If you do the squeezing the lever to tighten it up thing, and the disc's not centred in the caliper, then this is a dead giveaway that one of the pistons isn't moving at all, or is only moving a bit.
Big lever throw is indicative of air in system. Being a rear brake - probably with internal routing also points towards air in system.
Have you tried adjusting the reach on the lever ?