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Old 04-21-2021, 12:22 AM   #1
pshnfry
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Default Intermittent long brake pedal

89 build 745 GLE B234F Auto.

Problem with the brakes driving the wagon yesterday, lots of heavy stop start traffic. Looking for suggestions on where to start looking - see the story below.

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Part way through the trip the brake pedal travel went long - probably twice the travel to get braking effect and seemed to be more sensitive to pressure when the brakes did bite. Definitely still getting good brakes, just really long travel.

This progressed towards the end of the trip to becoming more of a "dead" pedal with minimal stopping power .

Pumping the pedal did not correct the long travel.

Noted the pedal when released came back up 80%, paused for a second or two and then went up fully to the expected height.

Left the car parked for 3 hours, started it up and found normal brake travel and feel, drove it back home in light traffic with no issues.

This is the second time in 6 months it has done this to me. Same circumstances and outcome both times.

Last time it happened I pulled the pads in all four wheels and cleaned up the pad locating pins with a drill and emery paper. Pads still had plenty of life and wearing evenly. The front discs had passed minimum thickness and were replaced with careful pad bedding (no replacement of pads).

Master cylinder is at the correct level. Fluid is getting towards needing a flush. Car is original and has low mileage.

Brake booster has made light "groaning" noise on initial pedal travel for several years, but retains assistance for two pedal pumps after the motor stops so I'm not expecting to be looking at that.

Master cylinder may be the original 32 year old part, but the intermittent and infrequent nature of the problem doesn't gel with this failing.

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Any thoughts or ideas on where to look first this weekend?
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Old 04-21-2021, 07:59 AM   #2
cleanflametrap
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Sounds like a caliper is getting hot, usually because the pads are dragging on the rotor. Stuck piston or clogged flex hose. If your car is anything like the 240 with dual diagonal (redundant circuits in the front calipers) one circuit will boil first, and if this warning of a "long pedal" is not enough, the other circuit will boil, the pedal will hit the floor, and you'll rear-end someone. Once you let it cool, or it decides to unstick itself, the brakes will feel normal again and the mechanic will scratch his head.
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Old 04-21-2021, 09:35 AM   #3
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Just drive it around a bit (without heavy braking, just drive gently and normally), and then *carefully* feel to see if one of the wheels is significantly warmer than the others.

Another possibility is a loose wheel bearing letting the rotor knock the pads further back, but the symptoms track better with CFT's theory above.
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Old 04-21-2021, 09:44 AM   #4
283SD
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When the car is cold hold your foot on the brake pedal with steady pressure, it should not sink to the floor [2minuits]. If it does sink bad M/C. Other issues are listed on the above posts. The M/C in my 78 wagon failed after 40 years.
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Old 04-22-2021, 12:12 AM   #5
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Quote:
Originally Posted by cleanflametrap View Post
Sounds like a caliper is getting hot, usually because the pads are dragging on the rotor. Stuck piston or clogged flex hose. If your car is anything like the 240 with dual diagonal (redundant circuits in the front calipers) one circuit will boil first, and if this warning of a "long pedal" is not enough, the other circuit will boil, the pedal will hit the floor, and you'll rear-end someone. Once you let it cool, or it decides to unstick itself, the brakes will feel normal again and the mechanic will scratch his head.
Thanks Art, this is really where I was heading and why I cleaned up the pad pins last time it happened. I'll use the power bleeder to flush out the old brake fluid, it is quite dark and very much overdue. That might help in terms of overheating the fluid.

Hoping it isn't hoses as that is not something I've seen a test for, it would be "throwing parts" at the problem. I'll pull the calipers and check pins again, pads for uneven wear and then just keep an eye on overheating after each trip to see which wheel is causing the issue.

Quote:
Originally Posted by JohnMc View Post
Just drive it around a bit (without heavy braking, just drive gently and normally), and then *carefully* feel to see if one of the wheels is significantly warmer than the others.

Another possibility is a loose wheel bearing letting the rotor knock the pads further back, but the symptoms track better with CFT's theory above.
Thanks John, I did check wheel bearings last time it happened, but will check again. As above, once I've flushed and inspected the hardware again I'll keep a closer eye on checking each corner of the car for obvious heat after each drive - might take a while I don't drive it nearly often enough.


Quote:
Originally Posted by 283SD View Post
When the car is cold hold your foot on the brake pedal with steady pressure, it should not sink to the floor [2minuits]. If it does sink bad M/C. Other issues are listed on the above posts. The M/C in my 78 wagon failed after 40 years.
Thanks for the input, I don't suspect the master cylinder at this point. Pedal doesn't sink, even when having the long travel it still firms up at some point and doesn't sink from there. Same with the booster, still two good assisted pedal pumps after the motor is shut down.

I've had a failed master cylinder twice now on my 240, that was much easier to diagnose.
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Old 04-22-2021, 06:55 PM   #6
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Quote:
Originally Posted by pshnfry View Post
Hoping it isn't hoses as that is not something I've seen a test for, it would be "throwing parts" at the problem.
While yes, it may not resolve your problem to replace your flex hoses, if they look like the original 30+ y/o ones, I'd replace them while messing with the braking system anyway. At under $10 a hose, I'd say it's worth the added safety.
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