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post #1 of 11 (permalink) Old Sep 26th, 2013, 4:01 pm Thread Starter
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Rear Brake Mystery?!

Hey all, owned my '93 75 brick for second season now, and noticed it was time to replace rear brake (pad non-existant) I was also leaking from pedal side of master cylinder (boot), so that replacement came shortly after pads. But here was my process

Ordered pads, and decided to renew o-rings and seals in rear caliper. Noticed a loss in pressure to brakes, so I renewed master cylinder piston. Bled brakes from caliper and reservoir side. rode for a 100 miles and repeated bleed.

My issue is i've noticed the pads weren't retracting from rotor completely so as I spin wheel i heard almost metalic squealing caused by outboard pad rubbing on rotor.

I can fix this by pushing the pad away from rotor with finger or screw driver- but then I lose pressure to pedal until a few pumps where pressure returns and the pad sticks again slightly rubbing rotor, causing rotor to get super super hot from any riding let alone at highway speeds.

I've disassembled, and cleaned piston cups, and re assembled, bled and re-bled, and I can't figure out a solution to this. All insight is appreciated, especially as I will probably be renewing front caliper components similarly within the next 1000miles.
Thanks

Panhandling on Brooklyn Bridge for a spare clutch plate.
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post #2 of 11 (permalink) Old Sep 26th, 2013, 6:23 pm
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Whern they stick try opening the bleeder. If fluid under some pressure comes out and the pads release you may have a bad hose. Internally degraded then acts as a one way valve, keeps the pressure on.

You said you replaced the MC piston but....How was the bore? Shouldn't have any wear or corrosion marks.

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post #3 of 11 (permalink) Old Sep 27th, 2013, 10:09 am Thread Starter
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outside of the MC had corrosion from leak, but I didn't notice any issues on the inside of the cylinder.

Other issue was that I didn't soak parts in brake fluid before installing. realizing this issue I disassmbled components and soaked them (except for orings in rear caliper) (everything was pliable enough to install without rupture) could this be a cause for concern, or will they be exposed to fluid and soak naturally.

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post #4 of 11 (permalink) Old Sep 27th, 2013, 12:43 pm
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Oh...I am very surprised the caliper pistons went in without lube if that's what you mean. Doesn't take much to make those stick and not fully release. I have done quite a few of them over the years, always lubed them and even at that they'd need a good push to sit in there. Dry that's asking for trouble.

Did you really clean the groove inside the caliper, doesn't take much corrosion or crud in there to bind things????

I don't like that method of lubing parts with brake fluid. It does work but you have to wipe any residual on the outside. If left there will attract moisture and cause corrosion.

There are special "brake mounting lubes". I use the "red rubber grease" but that's hard to find in North America.

As for the MC...there is a tiny hole in the cylinder, brings the fluid back up to the reservoir when you release the pedal. If plugged well brakes won't release. Crack that bleeder open with the brakes stuck that will give you the clues.

Adjustment rod??? It does need some play.Even a tiny pressure on the piston is enough to keep the brakes on and then heat builds up, things expand and binding gets worse.

Benelli 50cc at 14
Yamaha RD 200 at 16
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Honda CB 750 F at 18
Honda V45 Sabre at 24
BMW K100RS at 27
BMW R100GS at 34
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post #5 of 11 (permalink) Old Nov 3rd, 2013, 1:42 pm Thread Starter
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I cracked the bleeder and didn't notice the fluid to be under pressure. I'm going to check the master cylinder again now,
any ideas on whether to look on mc or caliper for problem, Im guessing fluid isn't a problem, wouldn't seem as hose was issue with the bleeder test.

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post #6 of 11 (permalink) Old Nov 3rd, 2013, 7:31 pm
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If the pad on one side is releasing and the other one isn't, I would rule out the MC for now.

My money would be on sticking pistons. Like H I have spent happy hours with old Brembo calipers. In some cases you just have to put in a new piston/seal.

I test mine with an airline, calipers off the bike and the pads removed. I set for a max pressure of no more than 10 psi, then see what air pressure is needed on each piston to push it forwards. The lower the better, but around 4 psi on every piston tells me they are o.k.

These Brembo calipers unlike car calipers have no method of retracting pistons when the MC releases pressure which is why you sometimes do get the pads just touching the rotor. But there should be no force.



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post #7 of 11 (permalink) Old Nov 3rd, 2013, 9:04 pm
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replace the o-rings that seal the pistons in the caliper. That O-ring is what retracts the piston

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post #8 of 11 (permalink) Old Nov 4th, 2013, 10:07 am
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I never used the air pressure method to push the pistons out. But interesting...I push them out with air then push them back in and thumb pressure is my gauge.4 PSI, could be just about right. If they stick no way my thumbs will push them in.

Whatever you do...DON'T split them calipers. Altough some have been successful and did that without causing a leak at the o-ring, too many out there haven't had such success and had to buy a new($$$$) or good used caliper.

It takes very little to make a caliper stick....a little forgotten corrosion under the seals or same unpolished on the pistons and that is if it can be polished, wear marks etc....and maybe even a little spray of brake cleaner can swell the seals.

What I'd like to know is if they are even sticking. Is the rotor getting hot even without using the brake??? If not may have to do with the pads, some (too many) have chunks of brassy metal imbedded in the pad. Depending on the pad quality/brand, them chunks can be substantial in size and may curl out a little and cause the metallic noises.I won't get into the brands here but the pads with very tiny specks of metal or none at all seem to do much better against the BMW rotors. And that's a lesson I learned expensively.

Benelli 50cc at 14
Yamaha RD 200 at 16
Yamaha RD 350 at 17
Honda CB 750 F at 18
Honda V45 Sabre at 24
BMW K100RS at 27
BMW R100GS at 34
BMW K1200RS at 53
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post #9 of 11 (permalink) Old Aug 28th, 2014, 12:24 pm Thread Starter
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Hey guys. I'm at wits end. Rear brake wont retract aftrr releasing. Ive been pushing pads out after stops, lose pressure for couple of depressions. Did 1600mi. Trip and found that inboard pad worn to shoe and outboard pad glazed.
Swapped pads, cracked caliper cleaned o-ring joint surface, wiped out puston cups lubed piston seals and reassembled.
Diconnected brake line and cleared. Checked MC for return after depression and it returns.

Checked to see if pushed fluid with bleeder cracl ed with no brake and no fluid expeled. Fully bled. And pads still rubbing,
I backed out foot pedal actuator stud and foot pedal sags until contact with MC. Am I missing a spring to hold up brake lever? Is this pressure enough to cause pads to rub?


To add to this, I checked front,swapped fluid and bled and now front left caliper outboard (left pad) is rubbbing in front with new fluid in system?
I know the error is mine, but what am I f#&!ing up?!
Ive got a rear brake line on order but I suspect it wont resolve issue (solely based on my luck)

Look above and you'll see I had replaced caliper components and MC cup seal within last 5k.

Panhandling on Brooklyn Bridge for a spare clutch plate.
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post #10 of 11 (permalink) Old Aug 28th, 2014, 12:31 pm Thread Starter
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Hey guys. I'm at wits end. Rear brake wont retract aftrr releasing. Ive been pushing pads out after stops, lose pressure for couple of depressions. Did 1600mi. Trip and found that inboard pad worn to shoe and outboard pad glazed.
Swapped pads, cracked caliper cleaned o-ring joint surface, wiped out puston cups lubed piston seals and reassembled.
Diconnected brake line and cleared. Checked MC for return after depression and it returns.

Checked to see if pushed fluid out of bleeder whike cracked with no brake and no fluid expeled. Fully bled. And pads still rubbing,
I backed out foot pedal actuator stud and foot pedal sags until contact with MC. Am I missing a spring to hold up brake lever? Is this pressure enough to cause pads to rub?


To add to this, I checked front,swapped fluid and bled and now front left caliper outboard (left pad) is rubbbing in front with new fluid in system?
I know the error is mine, but what am I f#&!ing up?!
Ive got a rear brake line on order but I suspect it wont resolve issue (solely based on my luck)

Look above and you'll see I had replaced caliper components and MC cup seal within last 5k.

Panhandling on Brooklyn Bridge for a spare clutch plate.

Last edited by gullymonster; Aug 28th, 2014 at 4:38 pm.
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