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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Kentallen8 on June 20, 2019, 02:43:02 PM
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'89 K100 RS Special with ABS1
Background:
A couple weeks ago, I discovered that my rear master cylinder had literally fallen apart. The piston (or actuator?) was dangling from the rubber boot, and the rest of the internals are either seized inside or were somehow lost.
Regardless, with prices being what they are, I went ahead and installed an aftermarket master cylinder as described in this thread: https://www.motobrick.com/index.php/topic,9364.msg76252.html#msg76252
Mind you, fluid dynamics are not my forte, so that fact probably influences the problems I'm facing as well as the questions I'm asking:
Problem: I can't get the brake pads to actually compress.
Complication: I have gone from point to point through the rear brake line and confirmed fluid is being pumped when I actuate the brake.
Further complication: I cannot get any fluid to pump out of either bleeder port (on the ABS, or the caliper) UNLESS I completely remove the bleeder bolt. When I do that, the brake fluid flows.
Question: What am I looking at here? There seems to be fluid throughout the whole system, but it's too weak to push through the (loosened) bleeder valves, or move the caliper pistons. Has anyone experienced this before? Is this what I can expect to see when/if there is an air bubble hidden in the ABS1 unit, or is it more likely that the new master cylinder is simply not up to the task?
I can see this cutting both ways: With the original master cylinder's demise, I can imagine all the fluid between the back of the cylinder and the ABS unit draining out and leaving an air bubble that is still able to push fluid through the line, but only when completely unobstructed. But I can also see the likelihood that the new master cylinder is just not cutting it.
Hopefully some of you have enough experience with this sort of thing that the answer(s) is obvious...
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I'm putting my money on a corroded rear master cylinder. It seems water collects there and eats the bore allowing the fluid to leak back over the piston.
Shit-can it and get a cheap ChiCom master cylinder on eBay. There are threads here that tell how to do the retrofit in a couple hours. Don't even bother trying to fix the old one.
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I think I was too brief in the introduction/background. I've already installed the aftermarket master cylinder. Now I'm just trying to diagnose why I can see fluid throughout the whole system, but it isn't activating the calipers.
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If, as you stated, your old master cylinder had "fallen apart" then you likely have bits and pieces of crud and crap in your system including your bleeder ports.
A thorough flush with clean fluid is required. you may have to remove hoses and clean with compressed air.
Have you noticed any tiny specs of contaminants suspended in the fluid?
Does fluid flow freely when front brake lever is squeezed?
a crud-free system is a must for brakes to operate effectively.
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Did you bleed the line from the master cylinder to the ABS first? You might have to make sure there is no air in the line from the reservoir to the master cylinder before anything else. Get the reservoir as high as possible above the master cylinder so there are no places to trap bubbles and tap the line.
There might still be a lot of air trapped in the master cylinder. You have to loosen the brake line on the master cylinder and slowly pump the pedal to prime the cylinder. Then move on the bleeding the line to the ABS and then on to the caliper.
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I'd suggest, check, make sure that your bleeders are not plugged with crud. It can, and has happened.
If you say that you get fluid out only when you completely remove the bleeder, that tells me that they are probably plugged.
Like TMG suggested, bleed from MC to ABS 1st, then the caliper.
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Follow up: Have resorted to bleeding the brakes with a Vacula bleeder - First to the ABS, then to the caliper. Took forever, but I managed to get clean fluid to flow through the whole line. Have confirmed the new master cylinder continues to create pressure in the line, but I still can’t get the calipers to compress. Note that I have also replaced the pads, so I assumed I just needed to pump the pedal for a while, but it’s been days and hundreds of pumps to no avail.
Is the sounding familiar to anyone yet?
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. . . but I still can’t get the calipers to compress.
By calipers you mean pistons, right?
It might be that the pistons are hanging up on debris affecting their seals or bores. Did you try to retract the pistons fully before starting all this?
Anyway, try j o's method (http://www.motobrick.com/index.php/topic,6522.msg44087.html#msg44087); it has a meditative quality that might be the solution; otherwise, you might need to get right in there.
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By calipers you mean pistons, right?
It might be that the pistons are hanging up on debris affecting their seals or bores. Did you try to retract the pistons fully before starting all this?
Anyway, try j o's method (http://www.motobrick.com/index.php/topic,6522.msg44087.html#msg44087); it has a meditative quality that might be the solution; otherwise, you might need to get right in there.
Yeah, I meant the pistons. I spent two days off and on trying j o's method. It was only after that went nowhere that I moved on to the vacuum method. If I understand you correctly, yes I did retract the pistons as part of the pad replacement process. (Retract = fully seating piston within the caliper housing, right?)
I've found a descrip on the BMWMOA forum that looks to mirror my issues:
https://forums.bmwmoa.org/showthread.php?10048-k100-rear-brake
I'm going to give this method a go and see where it leads...
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Follow up: Have resorted to bleeding the brakes with a Vacula bleeder - First to the ABS, then to the caliper. Took forever, but I managed to get clean fluid to flow through the whole line. Have confirmed the new master cylinder continues to create pressure in the line, but I still can’t get the calipers to compress. Note that I have also replaced the pads, so I assumed I just needed to pump the pedal for a while, but it’s been days and hundreds of pumps to no avail.
Is the sounding familiar to anyone yet?
Hi
I suggest you rebuild the caliber. Separate the two halves and replace the seals. There is a kit available from motobins in the uk.
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Hi
I suggest you rebuild the caliber. Separate the two halves and replace the seals. There is a kit available from motobins in the uk.
Here's how:
http://www.motobrick.com/index.php/topic,12182.msg107307.html#msg107307
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Here's how:
http://www.motobrick.com/index.php/topic,12182.msg107307.html#msg107307
Thanks. Yeah, I think that’s the next step. I’ve since pulled more air bubbles out and managed to get the pistons to compress for a little while, but they always seem to revert back to an unresponsive state after a few minutes.
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Have you got stainless steel lines, or rubber ones? It’s possible the rubber ones can become porous over 30 odd years, maybe? So there will always be air getting back into the system.
Just a thought...
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Not sure why so many are focusing on clogged bleeders. Even if they were clogged your caliper pistons should move if they are free and clear. As another suggested if enough outside elements got into the master when it fell apart to warrant replacement, you can crud/moisture got down stream to the caliper. In all likelihood your pucks are rusty and the bores are pitted and the seals are toast. Any or all three of these could cause your symptoms. Bad hoses could as well but I'd guess you've already inspected those?
When you rebuild post a pic of what you find.
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Not sure why so many are focusing on clogged bleeders. Even if they were clogged your caliper pistons should move if they are free and clear. As another suggested if enough outside elements got into the master when it fell apart to warrant replacement, you can crud/moisture got down stream to the caliper. In all likelihood your pucks are rusty and the bores are pitted and the seals are toast. Any or all three of these could cause your symptoms. Bad hoses could as well but I'd guess you've already inspected those?
When you rebuild post a pic of what you find.
Will do. Just waiting on the caliper rebuild kit to arrive. (And praying that some bad seals are all I find once I open it up!) I have checked my hoses, and aside from some road grime, they look to be in great shape. (Although I have no way to confirm this, the original owner was reportedly a BMW parts manager. Many of the internals - particularly the splines - are in really excellent shape, so I wouldn't be surprised if the brake lines are newer/replacement OEMs...)