Damn rear brakes - K-Bikes.com - Excellence in Motion
 
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post #1 of 6 (permalink) Old May 17th, 2015, 9:51 am Thread Starter
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Join Date: Jul 2012
Location: Providence, RI, USA
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Damn rear brakes

Not my first issue, but definately the worst. Heres link to previous:
Rear Brake Mystery?!

So first let me describe state of my rear brake. Its shite! But it was worse last year when pads werent retracting upon brake release. Spongy, this season, (after plenty of bleedings) but due to that poor pressure, the pads seemed to be retracting/ releasing. :p improvement from last year, and i dont use it on the road.

Brake pedal falls against pushrod causing activation of rear brake switch when sitting/ riding. So ive been holding pedal up with toe, allowing MC pushrod to fully extend/ brake switch unactivated.

I lengenthend both studs from pedal (shortening play to MC and increasing distance to push/activate brake switch)

Riding this morning, in 75degreeF weather, the pads mustve rubbed, then seized and i pulled over to find pads smoldering, no flame, but probably warped my disc already. (10 MINUTE RIDE NOT USING REAR BRAKE)

So my question, WTF is my issue? Sponginess after having bled and renewed brake fluid. New pushrod, caliper seals, and pads last season. When sitting, weight of pedal depresses pushrod about 3mm. When pedal is raised pushrod extends out slowly, drop the pedal and it depresses again 3-4mm. I checked last season for obstruction in either of the holes in MC, none apparent (air flow possible).

Do i need new MC or pushrod? New caliper? Replace brakeline? Replace caliper seals?

Panhandling on Brooklyn Bridge for a spare clutch plate.
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post #2 of 6 (permalink) Old May 17th, 2015, 11:40 am Thread Starter
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Location: Providence, RI, USA
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rode to work, 2nd check updates.

Backed out brake pedal screws. (Till just out of contact of MC pushrod)
While brake depressed can still spin rear wheel.
Cracked bleeder to very little fluid expelled.
When brake disengaged, pushrod slowly extends back out

Locked the pads outwards to caliper and bled, very little pressure in system. How is this possible when pads seized just a couple hours ago?!

Panhandling on Brooklyn Bridge for a spare clutch plate.
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post #3 of 6 (permalink) Old May 20th, 2015, 1:23 pm
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Location: Honolulu, HI, USA
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rear brakes trouble

There is a small port in the master cylinder that has to be open for the brakes to function properly. Suggest you remove, disassemble, clean the master cylinder, then carefully reassemble.

If this doesn't solve the problem, rebuild your caliper, the piston may be seizing in the bore. Caliper kits aren't expensive and as long as the caliper bore is clean you should be successful.

Also, make certain the flexible hose to the caliper isn't collapsed internally. This has been a problem on a lot of BMW's cars (DAMHIK) where the hose collapses internally. Brake fluid can flow from the master to caliper under pressure of your foot on the pedal, but since there is no return pressure and the hose is constricted, the fluid will not flow back. Result: constant pressure of pads on rotor, heating, etc.

HTH
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post #4 of 6 (permalink) Old May 22nd, 2015, 10:28 am Thread Starter
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Thank you. Yes this issue has been troubling me for a couple seasons now.

Id rebuilt the caliper and master cylinder. Wģout replacing brake line like an idiot
has been replaced and the pads havent seized since. Crossing my fingers.

Panhandling on Brooklyn Bridge for a spare clutch plate.
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post #5 of 6 (permalink) Old May 30th, 2015, 5:48 am
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You will find all the answers in my posts here because I also nearly lost a rear rotor and caliper through a stuck on rear brake:

http://www.i-bmw.com/showthread.php?...ressure+sensor

I have no more KRS brake issues and my rear pads are not wearing out so fast because I can see with my cockpit light lit, when my foot is generating brake pressure, so I use it as little as possible. You only get about 10-15% contribution to overall braking from the rear anyway.
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post #6 of 6 (permalink) Old May 18th, 2017, 1:19 pm
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Hi,
new guy here.
I bought my first BMW, a 1988 K100LT w/ 45k miles. It had sat unused for 12 years so I knew it might have some issues.
I had it into the repair shop where they replaced the steering head bearings, other service and inspection, and the entire rear brake assembly except the master cylinder. The rear brake had been locking up, smoking, etc, and the rotor was trashed.
So every thing from the MC on back is new, lines, pads, everything.
I got it back and made it 16 miles and the rear brake locked up. If I had been at highway speed, I would be in the hospital as all those new parts worked real well aand it locked up solid instead of just smoking.
Took it back to the shop and they rebuilt the MC.
I was a little scared of it as I don't have a death wish, and did a check ride stopping every mile or so to check the brake. 6 miles and no problem. I took it out again and made it less than 5 miles and it locked up. I had to walk home.

Does any body know what the deal is? I really like the thing. I want to use it. But it is a death trap right now.
Dwight
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