Why do disc brakes rub?

[rudy]

Hey all-

So after observing that my rear disc was rubbing a bit I did some research to find out that this is "normal" and possibly inevitable with disc brakes. Can someone please explain why?

My theories based on about a 3% understanding of how hydraulic brakes work are:
1. either the fluid doesn't completely travel back into the line or
2. the "piston" needs to be lubricated better

I'm much better with bicycles... you know, the ones you pedal. This hydraulic business has got me a little confused.

Muchas gracias amigos

-rudy

[markk53]

When the piston is pushed out the piston seal flexes with it, then pulls it back. This is like hundredths of an inch at best. If it's just scuffing lightly, that's somewhat normal. If it is actually dragging enough to slow the bike, that means the caliper needs cleaned and rebuilt. The O ring can not pull the piston back far enough.

It's not hard, really not much worse than doing a hub rebuild on a bicycle. The trick is to get the pistons to carefully pop out if there is more than one. From there, no real problems. I carefully scraped the crud in the O ring groove and in the caliper using a metal clay sculpting tool. I also sanded the caliper and piston with 1500 grit wet/dry and polished them up with Flitz. Then reassembled it all "lubed up" with DOT3 or 4.

If you do the brakes, get a vacuum bleeder from Harbor Freight - $30. Use low vacuum and you'll find it the best $30 you spent on tools for the bike.

[primalmu]

I bought a Mityvac and honestly felt it was a waste of money. I couldn't get it to seal well enough with either the tubing or the little adapter to actually pull brake fluid out. I found that the old fashioned way worked best. I also read that its not necessary to close the bleeder between each pump, a fact that I confirmed while bleeding my brakes when I changed to stainless steel lines.

Of course, I guess there is a small possibility that one of the previous owners had installed speed bleeders...

[Dodsfall]

I also read that its not necessary to close the bleeder between each pump, a fact that I confirmed while bleeding my brakes when I changed to stainless steel lines.

Of course, I guess there is a small possibility that one of the previous owners had installed speed bleeders...

I'm guessing that's the situation. If the bleeder is left open as fluid is drawn back into the master, air certainly will fill the vacuum left at the bleeder end. The air sucked in may eventually make it's way to the top of the fluid, but it's not a very efficient way to bleed brakes.

[z9nine]

I love my speed bleeders on my truck, one man brake bleeding.

[rudy]

Thanks mark. Sounds like my brakes are still alright. No harder than a hub rebuild? Well I can do that! haha.

[Eye_m_no_angel]

When the piston is pushed out the piston seal flexes with it, then pulls it back. This is like hundredths of an inch at best. If it's just scuffing lightly, that's somewhat normal. If it is actually dragging enough to slow the bike, that means the caliper needs cleaned and rebuilt. The O ring can not pull the piston back far enough.

It's not hard, really not much worse than doing a hub rebuild on a bicycle. The trick is to get the pistons to carefully pop out if there is more than one. From there, no real problems. I carefully scraped the crud in the O ring groove and in the caliper using a metal clay sculpting tool. I also sanded the caliper and piston with 1500 grit wet/dry and polished them up with Flitz. Then reassembled it all "lubed up" with DOT3 or 4.

If you do the brakes, get a vacuum bleeder from Harbor Freight - $30. Use low vacuum and you'll find it the best $30 you spent on tools for the bike.

Nailed it.

[WintrSol]

When I get a new-to-me bike, I put fuel-system rated thread sealing tape on the bleeders. That way, there is less air leakage around the threads when using a vacuum device like the MityVac, and less air drawn in if I don't get the bleeder closed before the MC is released. Usually, there is no air leakage with the sealing tape, depending on the condition of the bleeder threads.

[markk53]

All I can say is once I learned to draw a low vacuum with the pump, then barely crack the bleeder, I got results. Quick easy results. I cracked the bleeder relatively briefly, then closed it before the vacuum was gone. No air in the lines. Less than five minutes from putting the first fluid in the rear brake master cylinder and finishing bleeding the system.

Front brakes took a bit longer because I was new at using the pump and pulled too high a vacuum, drawing the master cylinder low. After figuring it out, it was a few minutes.

But that's just my experience.

[Eye_m_no_angel]

You're right Mark. With a Mitey Vac you have to keep a low vacuum and just crack the bleeder and it works fine. I did the same thing the first time I used one: I was impatient and I opened the bleeder way beyond what I could close in about a 1/4 turn, and I pumped like crazy thinking a higher vacuum would help. Didn't work.

I also have a Vacular that works in about the same way. I use full shop air pressure but it's designed to pull a low vacuum so I attach it, lock the trigger open, and crack the bleeder about 1/4 turn. I can then walk around to the master cylinder and pour in new fluid as it draws the old through the system. It takes maybe....10 minutes? To run an entire can of fluid through the system, which is about as good a flush as I've ever needed to do.

I've never had to use tape or thread dope on bleeder screws.

[WintrSol]

+1 on Mark's method. As long as you don't get air into the banjo fitting, I find this to be the best method. Taping the threads is like adding suspenders to your belt when doing it this way, but on 40-yr old bikes, is almost necessary.

One other thing about the rubbing disc; many MC's have a small relief hole to let any excess pressure in the lines bleed off. If that hole gets blocked, pressure builds up in the slave from heat build up.

[markk53]

+1 on Mark's method. As long as you don't get air into the banjo fitting, I find this to be the best method. Taping the threads is like adding suspenders to your belt when doing it this way, but on 40-yr old bikes, is almost necessary.

One other thing about the rubbing disc; many MC's have a small relief hole to let any excess pressure in the lines bleed off. If that hole gets blocked, pressure builds up in the slave from heat build up.

Good point. I usually will "bleed" the fittings by squeezing the lever/pedal and cracking the banjo bolt slightly. Obviously I have a rag around the area to sop up fluid.