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740 Intermittent long brake pedal

pshnfry

Member
Joined
Jan 5, 2011
Location
South Australia
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 :omg:.

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?
 
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.
 
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.
 
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.
 
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.

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.


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.
 
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.
 
I drive a 1991 940SE Turbo Wagon that is basically giving me the same problem. Brakes work even when pedal drops, albeit with a lower pedal. No loss of brake fluid. But black. Flushed only the master the other day. Not long before black again. Want to do a whole system flush, but want to get a handle on the issue first.

1. Pedal intermittently drops, but not to the floor. Brakes still apply with a lower pedal.
2. No loss of brake fluid.
3. I can make this happen without driving the car. So I don't think over heating the brakes has anything to do with it.
4. It is very annoying. Confidence in the brakes marginal at best.

I have never heard anyone say that the ABS unit has an accumulator and that could be the issue. When I took a class in brakes that was a possible reason for a low pedal that doesn't reach the floor. Does anyone know if the ABS hydraulic unit on these cars has an accumulator? I can't remember the exact car but I remember a car that had a screw in accumulator, and they would go bad. And if so do they make a repair kit, or can the accumulator be replaced independently?

Any ideas? I would have though Master Cylinder before going further. But if it was the master it would go to the floor. All the way to the floor.
 
I would have though Master Cylinder before going further. But if it was the master it would go to the floor. All the way to the floor.
No. That usually only happens on an ancient single-piston master, usually from the rubber seals deteriorating and allowing fluid to go around the seal backwards, instead of being pushed through the lines.

On modern two-piston masters (dual, tandem, whatever you want to call them), one piston can fail, and you'll still have brakes, just not as much.

On my non-turbo, I have a "split diagonal" system, where one piston runs half of each front caliper, and one rear. The other piston runs the other half of the fronts, and the other rear. If I lose a piston, I still have reasonable brakes, they'll just be less powerful.

Here's a very basic master diagram I stole off the web:

master.png


When you step on the pedal, the primary piston starts to move forward. As it does, it closes off its intake port, and starts to build low fluid pressure in its output line, and behind the secondary piston. That pressure moves the secondary piston, which then builds pressure in the secondary output line. As long as everything is working properly, you have equal pressure everywhere, and full brakes.

If the seals on the primary piston fail, the piston freely moves forward until it physically touches the secondary piston and pushes it forward, creating pressure only in the secondary output line.

If the seals on the secondary piston fail, the low primary fluid pressure pushes the secondary piston until it hits the end of the bore, which then allows the primary piston to create pressure only in the primary output line.

Scratches in the bore, dirt in the fluid, or mostly worn-out seals can let a piston seal sometimes, and leak sometimes, creating your "intermittent" pedal drop.
 
Just dealt with this! Couldn't get them to bleed or so I thought after bleeding for about 3 hours and wasting multiple bottles of fluid. Then the next day it was just...fine. Firm pedal and all. 3 months later, I'm leaving my subdivision and BAM, no brakes, pedal all the way to the floor. Luckily there was a patch of grass to the right to swerve onto and I avoided being t-boned! Drove back home and ordered a MC from IPD. Bench bled, installed and the issue hasn't reared its head since. Kinda wish I had done the Mustang MC for a possible big brake upgrade down the road, but all in all, the process was simple and you can never go wrong with factory parts.
 
Alright. I will throw a master at it. We'll see if that answers the problem. I am curious myself. Thanks for some feedback.

Did some looking into finding the Pump unit for the ABS. Could only come up with a used unit. And the master is a no brainer to change. It would end up being much cheaper, and if I'm wrong I would have probably ended up changing it anyway.
 
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Scratches in the bore, dirt in the fluid, or mostly worn-out seals can let a piston seal sometimes, and leak sometimes, creating your "intermittent" pedal drop.

Thanks for the input, very helpful.

I'm interested in the quote above, how intermittent being the question? Interested for future diagnosis.

Mine is dragging pads overheating contaminated fluid and boiling one circuit and then the other. The pads on one wheel stuck on more than what had previously happened one day as reversing into the driveway - very noticeable.

Haven't tested the fix as when pulled off the stands for a test drive I had a "no start" scenario that I just CBF looking at at the time - 3 months ago.

:-(
 
Slapped in the face. Thank you everyone for setting me straight. I've been a mechanic for far longer than I'd like to admit at the moment. I changed the Master Cylinder and flushed the entire brake system. That was the trick. My brakes have never worked this well. I think this has been a problem for years. Although lately gotten worse. The brakes were never as good as they are now. I am guessing the rear brakes were not really being used at all. So, nothing wrong with the accumulator in the ABS hydraulic unit. The entire problem was the Master. I had honestly never seen a Master that would only have a low pedal, and not a complete drop to the floor. Never to old to learn. Thanks Again.
 
Slapped in the face. Thank you everyone for setting me straight. I've been a mechanic for far longer than I'd like to admit at the moment. I changed the Master Cylinder and flushed the entire brake system. That was the trick. My brakes have never worked this well. I think this has been a problem for years. Although lately gotten worse. The brakes were never as good as they are now. I am guessing the rear brakes were not really being used at all. So, nothing wrong with the accumulator in the ABS hydraulic unit. The entire problem was the Master. I had honestly never seen a Master that would only have a low pedal, and not a complete drop to the floor. Never to old to learn. Thanks Again.

Thanks for the update. I'll make that my next move if I ever get around to dealing with the no start.
 
I’m having the same issue after doing the junction on both my cars yet I have brand new master cylinders
 
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