Dragging Disk Brakes

[MonoRT]

I've got an '85 R80RT with twin disks that are connected to each other by a hard line that arches up under the fork brace and one flex line to the fairing.

The bike has about 265K on it now. A couple weeks back, I got a screw in my front tire, plugged it and set up with my dealer to get a new tire installed. Before the tire change, the front wheel spun pretty freely (I kept checking the plug) and after the new tire was installed, I noticed (not while riding, but just spinning the wheel by hand to get the valve into the right spot to check the air pressure) that there is a fair amount of drag - I'm not all that strong, but a good push on the front wheel only gets about one third of a rotation. The drag is even, no grab and release action.

When I stop after a ride, there is a fair amount of resistance to being spun by hand and both disks are uncomfortably hot (to try and hold on to), but not sizzling hot and, according to my calibrated fingers, there's no discernible difference in heat between the two disks. Even when everything has cooled down, the brake drag remains. The brake lever returns to it's stop when I release it and the reservoir is filled to about 5mm below the MAX line.

There are a bunch of possible culprits here and I wanted to see what the group might suggest.

The flexible brake lines are stainless steel from San Jose BMW and they have at least 240K on them. The front calipers are supposed to be removed when the front wheel is removed, but I can't tell if that was done or not. That might have impacted the lower flex line, but there is no visible crease or crimp. I know that SnowBum was written about invisible failures inside those SS Teflon lined brake lines and that might be a problem...

I last rebuilt the calipers at about the 100K mark, so they might be overdue for a rebuild - I don't see any tears in the dust boots and I don't see any fluid weeping or drips.

I rebuilt the master cylinder a couple of years back and had to 'hone' the bore with a super fine ScotchBrite patch, a shotgun cleaning rod and a drill motor because the new piston hung up on the white oxide that had formed tiny spots in the bore. In that case, the lever would not return to it's stop unless I pushed it back with my fingers. At this point, I'm not seeing any fluid weeping or dripping around the master cylinder.

New BMW brake pads went in a couple of months ago.

What do you all think?

 

[brook.reams]

Hi there,

A couple of thoughts come to mind.
1. Clearly one or both caliper pistons are not retracting fully.
2. HYPOTHESIS #1: In installing the wheel, the technician pushed the pads all the way into the caliper to get the wheel on. That caused a part of the piston that is not typically rubbing on the piston seal to now rub on the seal. That part of the piston is rusty and now binds up on the seal preventing the piston from fully retracting.
3. HYPOTHESIS #2: Crap has been dislodged in the brake line and is now plugging the fluid return passage in the master cylinder or caliper body(s) preventing pressure from being released in the line when you let go of the brake lever. This keeps the caliper piston pushed against the disk.

ONE SOLUTION: Return the bike to the dealer and request they correct the problem they created when installing the wheel.

ANOTHER SOLUTION:
HYPOSTHESIS #1 & HYPOTHESIS #2. Pull both calipers and remove the caliper pistons and inspect them. Also the piston seal. Replace as necessary (or anyway if time in service is 10 - 15 years or more. The pistons are commonly plated steel and will rust in time and the seal gets worn). This will require a drain and fill of the disk brakes which will clean out the old fluid and likely any crap will get purged. You can visually inspect the master cylinder as you pump it to purge air for the return fluid flow into the reservoir. If you don't see that, continue to push brake fluid through the system to clean out whatever is preventing return flow.

I hope some of this is helpful.

Best.
Brook Reams.

 

[ccolwell]

Start by making sure it's the brakes. Unbolt the calipers and wiggle each to push the pads in a bit. Remount and see whether the wheel still drags. It's likely the brakes but make sure you're on the right path.

If the resistance increases after riding, it may simply be air in the caliper(s).

 

[20774]

In line with Brooks' #2, next time when you again spin the tire and there's drag, crack the bleeder(s) and see if any fluid is pushed out. Close the bleeder(s) and see if the wheel spins easier. If so, that would suggest fluid build up in the caliper and no return to the master cylinder.

 

[MonoRT]

So, had a little daylight after I got home yesterday and double-checked. There was still excessive drag. All four caliper pads were firmly in contact with the disk surfaces. Following Kurt's suggestion, I put a 11mm box end over the bleeder followed by some bleeder tubing. Cracked open the bleeder nipple and about 10mm of fluid popped up into the bleeder line. Closed the bleeder and spun the wheel. Much better!

So... the problem appears to affect both calipers and all four caliper pistons. Like as not, the problem is in the brake lines or in the master cylinder. I popped off the reservoir cap and used my vacuum bleeder to suction out the reservoir, refilled it with new fluid and then used the bleeder to suck that one reservoir's worth of fluid through the bleeder nipple. No joy, but at least the fluid coming out of the calipers was light yellow and not murky brown.

Brooke suggested crap (a highly technical term) in the MC return valve. How does one go about making sure those tiny little holes in the MC are clear? Should I pull the MC off, remove the brake line and piston and try to blow it out with compressed air? And, how do the valves work? is it just the rubber bits on the piston assembly blocking and uncovering the tiny holes as it travels back and forth?

I intend to acquire enough parts to replace the lines and rebuild the Master Cylinder and each caliper. In the meantime, I'm just trying to see if I can address the problem in the short term.

 

[20774]

Brooke suggested crap (a highly technical term) in the MC return valve. How does one go about making sure those tiny little holes in the MC are clear? Should I pull the MC off, remove the brake line and piston and try to blow it out with compressed air? And, how do the valves work? is it just the rubber bits on the piston assembly blocking and uncovering the tiny holes as it travels back and forth?

That is the way it works as far as I know. Do you have the original rubber lines? These can break down and balloon which doesn't let the fluid work back to the master cylinder. You have the handlebar MC. I seem to recall that there should be a small "puff" or squirt of fluid back into the MC when you release the brake lever. They have systems to push fluid from the caliper to the MC. As a first shot, could you do that and see if fluid can be pushed backwards...maybe push some debris out of the small hole. Beyond that, you'll probably need to move to overhaul as you were thinking.

 

[MonoRT]

I ditched the rubber lines early on in the life of the bike. They did swell, and paired with a pea-shooter 13mm MC, I could pull the lever to the bar - and the feel was abysmal.

I switched to the (increasingly hard to find) 16mm MC and the aforementioned SS braided lines from San Jose BMW which are by now way past their sell-by date. The feel at the bar is great with this setup - you can feel the pads touch the disks (or, at least I could when the pads were not touching the disks already).

I've noticed the little 'puff' of fluid in the handle-bar mounted MC reservoir in the past, but I'm not seeing it now. Perhaps that's another clue...

I have a pneumatic vacuum bleeder that I am less than impressed with. The "bottom up" syringe bleeders make more sense to me, but I've baulked at the price. Still, they seem to be selling for more reasonable prices now and I should just get one - sure makes more sense than trying to pull bubbles downhill.

 

[brook.reams]

Clues You Can Use

I ditched the rubber lines early on in the life of the bike. They did swell, and paired with a pea-shooter 13mm MC, I could pull the lever to the bar - and the feel was abysmal.

I switched to the (increasingly hard to find) 16mm MC and the aforementioned SS braided lines from San Jose BMW which are by now way past their sell-by date. The feel at the bar is great with this setup - you can feel the pads touch the disks (or, at least I could when the pads were not touching the disks already).

I've noticed the little 'puff' of fluid in the handle-bar mounted MC reservoir in the past, but I'm not seeing it now. Perhaps that's another clue...

I have a pneumatic vacuum bleeder that I am less than impressed with. The "bottom up" syringe bleeders make more sense to me, but I've baulked at the price. Still, they seem to be selling for more reasonable prices now and I should just get one - sure makes more sense than trying to pull bubbles downhill.

So, you should see that little squirt of brake fluid in the MC when you release the lever. You aren't seeing it, so that confirms the return path is not working correctly. I would drain and remove the MC. I'd take a piece of stranded copper electrical wire, strip one small strand from it and try to poke that through the small holes in the MC. I'd also try squirting some brake cleaner through them to confirm it can move through the the small passages.

Since you found both sets of pistons pushing on the rotors, it seems likely the problem is at the MC return passage or in the single rubber line between the MC and the steel lines going to the calipers. If the rubber line is pretty old (10 years +) I'd likely replace that since I'm already there and eliminate it as a contributing factor to the problem.

My current hypothesis is this is rubber bits that have plugged the return passage in the MC. The source of the rubber bits could be the rubber brake lines or the rubber seals on the MC piston, or ???

Hope this helps.

Best.
Brook Reams.

 

[MonoRT]

Having established that there was some back pressure north of the calipers, I decided to loosen up the two long 4mm cap screws that hold the MC to the lever assembly. This would allow the MC piston to slide back a bit and I should then be able to tell if I still had pressure in the lines or not. Well... that was harder to do than I thought because the 'nose' of the MC that inserts into the lever assembly was a big blob of corroded aluminum - in fact, I go the MC off - everything inside was a mess of corrosion. However, that did relieve the back pressure. I pulled the MC piston out and looked at the bore - again way too much corrosion for my tastes. This is a 28 year old MC and has been rebuilt many times.

My dealer had a new MC in stock and gives me a discount to make a better deal than the online vendors, so I bagged a new MC, a few banjo bolt crush washers and a new bleeder nipple (old one was corroded and bunged up). I already had new flex lines on order, but that will have to wait a bit to install them.

My aftermarket manuals did not specify the torque to use for the banjo bolt. I found a few references online that seemed to indicate about 16Nm, so I went with that and got a drip-free connection.

Now, I have less drag than when I started, but still more than has been normal for this bike. As mentioned, I have new flex lines on order and having looked at the somewhat incomplete caliper rebuild kits offered by BMW, I think I'll order one from Bevel Heaven as their kits appear to include the o-ring to seal the fluid passage between caliper halves and new pistons.

I'll pull the pads and check for damage. I don't think that the forks could get so out-of-line as to case brake drag - could they?

 

[MonoRT]

Well... it looks as if I was dealing with a double-whammy. Perhaps both problems were caused by the front tire change.

I had back-pressure in the brake system that appeared to be caused by a master cylinder with at least one clogged port. The tech may well have attempted to push the pads back to re-install the front wheel without having to loosen or un-bolt the calipers and that could have pushed some junk back up into the MC. As it turns out, the MC was a mess and needed replacing anyway. That solved the back pressure issue, but the brakes still dragged (less).

The first time I peered into the calipers, it was evening and I did not get as good a look as I should have. The dust boots for both outboard caliper pistons at the 6:00 mark had been pushed up from below and pulled out of the caliper bore and then folded back into it. That meant that there was a fold of rubber that was a bit proud of the piston surface, causing the pads to drag some even when the hydraulic pressure was off. I'd have to guess that during the wheel re-install, the edges of the disks got caught between the caliper body and the pad's backing plate.

I may want to have a quiet word with the tech next time I visit the dealership.

Given that I last split and overhauled the calipers about 165K ago, I've ordered a caliper rebuild kit from Bevel Heaven. Those guys also sell the hard-to-find O-ring that seals the caliper halves. And, I got the new Teflon coated aluminum pistons to go with the rebuild.