Topic: Brakes grab and don't let go

[fmctm1sw]

I have a caliper with a new seal and a rebuilt master cylinder.  The tiny hole in the master cylinder is clear.  The line is clear and I'm able to push brake fluid down the line.  When I'm bleeding them though, the caliper tightens up on the disk and never lets go.  And I'm not talking just some drag, I can't even push the bike.  What force is responsible for the caliper backing off?

[robvangulik]

Did you clean the groove the new seal sits in? Meticulously?

[Kickstart]

Without more information... my best guess at possible things to look at are:

• Possibility that the master cylinder rebuild was somehow done incorrectly causing the piston not to retract back past the small hole (and not allowing the pressure to be released)
• A kinked brakelines (that could be acting as a one-way valve)
• Caliper piston catching/not aligned properly somehow and not releasing

After it tightens up, if you open the bleeder valve is there pressure behind it?

[Kickstart]

Another question... when you rebuilt the master cylinder, did you install the check-valve in the correct orientation?

[Stev-o]

I recently had a similar issue on my K8.  A disassembly and good cleaning of the caliper took care of it.

[martin99]

Original rubber brake line? They will deteriorate inside over time. The rubber becomes too soft and will bulge under pressure rather than retain a uniform inner diameter. This can cause the caliper piston to stay put rather than retract, as the fluid sits in the line in the larger space created rather than get sucked back up the line by the action of releasing the lever. If this is the case, only cure is a new line.

[fmctm1sw]

Thanks for the replies.  I'll check over those things tomorrow.

[Bodi]

With the brake lever released, the m/c piston pressure cup should sit behind the relief hole (tiny hole in the reservoir through to the cylinder). That will release any pressure in the system, and allow the caliper piston to retract.
Your problem could be
- some mechanical problem with the caliper piston making it seize in the cylinder - the hydraulic pressure is able to move it to lock the brake but it's so stiff it doesn't move back.
- a blocked relief hole.
- misassembled or defective m/c rebuild kit causing the piston to be past the relief hole even when fully retracted.

[78whiteorbs]

as cheap as brand new masters are I don't see why anyone rebuilds them ... other than for "historically correct" bikes. Most I know though aren't white glovers and just want a bike that stops well without pulling teeth.

my $.02

[rm56]

I had the same trouble with my 74. What was happening was because the brake piston bore was pitted and destroyed by corrosion and crud when you pull the front brake lever to apply the brake and then release it. The caliper piston will not back off as the pressure is released. It only moves a small amount and is nearly impossible to see with the eye. The same thing happens with car disc brakes, you apply them and instead of releasing they lock up on you. The other thing you have to make sure is you adjust the front caliper screw just right. Too much either way and it grabs or locks up. My bike was a bugger to get it right, but I finally got it. Have your bike on its center stand with a jack stand holding the front wheel off the ground. That way you can apply the brake, and check that the wheel stops and then releases as it should. You have to keep in mind our bike are almost as old as we are and the parts can go bad just sitting in a garage. My favorite line when reading adds here in California when some dummy is selling an old bike is "It ran when parked" Yeah 40 years ago. Depending on how your bike was stored you may get lucky and just need to clean up that caliper bore with some gentle scuffing and make sure to clean the seal groove perfectly. I also had to buy a new piston $105. US ouch.
I completely rebuilt the master cylinder, I also replaced the outboard part of my front caliper the piston and seal. The bore and the small piston can wear and also if water and dirt gets in there that bore gets pitted corroded and ruined. That is what happened to mine. Brake fluid attracts water like a sponge. Never leave clean brake fluid bottles open to the air or your just going to ruin the brake system with water contamination. I also replace all the front brake hoses, its not a good idea to ride around with 39 year old hydraulic brake hoses on your bike. CMSNL or David Silver has all you need in NOS.
Good luck!

[fmctm1sw]

I had the master cylinder back apart and it looked fine.  The inside wasn't chewed up and I'm certain it's assembled right.  I've done a dozen or so master cylinder rebuilds.  I had the caliper apart again too.  It's very clean.  One thing that has me wondering is the brake pad itself though.  It came with a screw that I screwed into the top center of it.  I would guess it keeps the pad from rotating in the caliper.  Once I removed the caliper, I couldn't get the pad out.  I thought it should pop out with my fingers, no?  I'm wondering if the pad gets snagged in the caliper somehow.  I got it all back together and tried it again and it still doesn't let go.  I'm out of time this weekend. 

[fmctm1sw]

Here's what I was trying to communicate. The caliper has a channel at the top which I assume is to match up with the screw that came with the brake pad.  I think the pad may be binding.  I guess this weekend I'll try removing that screw or maybe try another pad if I can find one.

[tomkimberly]

Try dumping the screw.

Tom

[Henning]

The pad must be loose enough to just fall out of the caliper. Mine kind of rattles around in there. I've read that some aftermarket pads are too big a diameter so you need to sand off some material (paint if I remember correctly). Same applies to the B-pad. You need the screw at the top or else the pad will rotate under braking and wear the inside the caliper.

[Duanob]

Try dumping the screw.

Tom

Ditto, I've never bought pads with a screw before. try Honda OEM next time.

[flybox1]

Dont remove the screw.  it keeps the pad from spinning in place.
Its the black coating on the pad backing. 
It makes the pad OD too big, and binds in the ID of the caliper.
take a wire wheel, or some emory paper and remove it all.
that pad needs to be able to move in and out freely. without binding.
and rotate ever so slightly (with the screw in its channel)

[MCRider]

Dont remove the screw.  it keeps the pad from spinning in place.
Its the black coating on the pad backing. 
It makes the pad OD too big, and binds in the ID of the caliper.
take a wire wheel, or some emory paper and remove it all.
that pad needs to be able to move in and out freely. without binding.
and rotate ever so slightly (with the screw in its channel)

I agree completely, OCICBW.

I can't remember one that doesn't have a locating screw. The screw itself doesn't do anything as a screw. But it's head locates and guides the pad in the groove.  What did the OEMs have?  A peg of some sort.

My most recent set had the painted circumference problem. Had to remove the paint completely to a shiny surface to get the pad to work properly. Should be metal to metal, IMO.

[fmctm1sw]

Thanks guys, that was it.  I grinded that black paint off the pad.  The pad was getting wedged inside the caliper when I pulled the brake lever.

[MCRider]

Thanks guys, that was it.  I grinded that black paint off the pad.  The pad was getting wedged inside the caliper when I pulled the brake lever.

Painting them keeps them looking good in the blister pak, free of rust. But no es bueno in application. Use as little to none lubrication as possible. TwoTired has coached us about a certain high temp grease to use, that is not the typical auto disc brake grease.

Be sure to use the proper stuff (Search disk brake grease) or none. Don't want anything on braking surface.