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A couple of things, as I too struggled with this:
1. Rebuilding the calipers helps, including the pin bushings, seals, and boots. And it's cheap and honestly not that hard. But, if you're going to do it, do the pistons as well, as they are likely pretty worn.
2. If you're going to use the brake pedal and the pressure bleeder simultaneously, that's okay, just be *really* careful how quickly you let off the pedal, as to make sure that fluid flows out of the bleeder and into the system, instead of pulling any air in as the pedal comes back up.
3. The thickness of your rear brake pads has a big impact on pedal feel. If you have thin pads in the rear, you can get enough caliper flex + pad knockback that when you first step on the pedal, all you're doing is taking the slack out of the rear pads and not actually applying real braking force. I suppose this is true for the fronts also, but I've found that fresh rear brake pads has solved my pedal feel issues more than once.

Ultimately I abandoned ship on the front calipers and ordered Stoptechs. I'm putting them on now. I imagine I'll get spoiled by them and end up replacing the rear brakes too.
 

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3. The thickness of your rear brake pads has a big impact on pedal feel. If you have thin pads in the rear, you can get enough caliper flex + pad knockback that when you first step on the pedal, all you're doing is taking the slack out of the rear pads and not actually applying real braking force. I suppose this is true for the fronts also, but I've found that fresh rear brake pads has solved my pedal feel issues more than once.

Agree 100% on this point the rears dictate the feel while the fronts do the work.
 

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Discussion Starter #23
Update... and responses to past posts.


I have new pads at all four corners. The car has about 77k miles on it and has been garaged most of it's life (last eight years in Florida).


When I last did a bleed with the Motive Power Bleeder along with a ten pedal pump (simultaneously). I had the tubing from the bleed nipple going into the catch bottle and submerged into a couple of inches of clean fluid.


I don't want to just throw new parts on in the hope of fixing this problem. I want to know what is the problem part first. I searched the web for 'how to test a brake booster', 'how to diagnose a brake booster vacuum leak' among variations of these search questions.


1. Engine off > Pumped brake pedal and hold pedal > Start engine - pedal sinks way down to the floor.


2. Engine started and allowed to idle for a minute or so > Shut engine off > Waited two minutes > Pressed brake pedal - initially the pedal felt firm. I maintained firm foot pressure on the pedal which slowly went way down over a couple minutes of time.


Does this conclusively indicate a bad booster?


I am not sure if I am missing something here. I believe the rod that goes through the booster is basically a solid rod. This is hard pinned to the brake pedal arm. The other end is right up against the piston of the master cylinder. So as the brake pedal is depressed, the piston of the master cylinder must move accordingly wither the booster is good or bad (only the amount of effort to push would change). When I have the engine running, the brake pedal goes way down with out too much effort. I can't think that there is still that much air in the system that does not bleed out.


Suggestions please and Happy Holidays to All!!!!!!!
 

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I have to disagree with the booster diagnosis here. Booster provide brake pedal force assist using engine vacuum, on one side of a diaphragm, I've never heard of nor can I comprehend how it can fail in this mode. Typically when it fails it doesn't provide enough assist, it makes noises or it leads to engine vacuum leaks.
 

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I have to disagree with the booster diagnosis here. Booster provide brake pedal force assist using engine vacuum, on one side of a diaphragm, I've never heard of nor can I comprehend how it can fail in this mode. Typically when it fails it doesn't provide enough assist, it makes noises or it leads to engine vacuum leaks.
The fact that the pedal malfunctions only after starting the engine is the first clue that it may be a booster failure. But I agree that the other circumstances are a bit curious. What I can tell you is that in manual brake setups (booster-less), this sort of phenomenon can occur with well-bled brakes only under one or more of the following circumstances:

1. Bad master cylinder
2. Bad slave cylinder
3. Piston seal failure
4. Brake line failure

But none of the foregoing is triggered by starting the engine. So that leads me to conclude it is the booster.

Just my $.02.

--Peter

PS: I will add that manual or boosted systems with ABS theoretically could have an ABS pump failure.
 

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The fact that the pedal malfunctions only after starting the engine is the first clue that it may be a booster failure. But I agree that the other circumstances are a bit curious. What I can tell you is that in manual brake setups (booster-less), this sort of phenomenon can occur with well-bled brakes only under one or more of the following circumstances:

1. Bad master cylinder
2. Bad slave cylinder
3. Piston seal failure
4. Brake line failure

But none of the foregoing is triggered by starting the engine. So that leads me to conclude it is the booster.

Just my $.02.

--Peter

PS: I will add that manual or boosted systems with ABS theoretically could have an ABS pump failure.

I don't disagree with the logic, however , I can't see a failure mode of a booster that allows this to happen, unless the rod/spring ect is deflecting and that's the additional movement and it's not being transmitted to the hydraulic. I'm not saying crazier illogical **** can't happen but I'm still skeptical


That said let me ask the OP this question, when you did the test was the car off and then you started or was it on but the engine not running? If it was off and not not running, I'd suggest turning to on but not start the engine. This way the ABS pump and system is energized but the booster is not under vacuum. I'd also suggest, if you can reset the faults, to disconnect the abs pump and body and try again (or just pull the fuses)
 

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The job of the booster is to make it easier to push the pedal, less force required. Your pedal sinks to the floor when you start the engine. The booster is working! Can it fail in a way that makes it work too well? I don't see how.
Sounds to me like the booster is working with an under-resistant circuit, so air in the circuit, a leak, or bad seals.
 

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Discussion Starter #28
Still a crazy situation...


After I bench bled the master cylinder, I mounted it to the booster out of the car. I am very sure that the booster rod aligned up to the master cylinder piston. Makes sense that the booster is working to allow added force to the MC piston.


I just ordered up all of the parts to rebuild the calipers (o-rings, boots, pistons, guide pins with brass bearings and proper assembly lubricants). I will do the calipers, re-bleed and see if there is any improvement.


I was able to use ISTA+ to work all of the ABS valves and pumps. I don't think there is an issue with it, also since it is a separate closed system when not activated, it should have no effect on normal braking conditions.


When I next test, I will pull the DSC & ABS fuses to remove that from the equation.



This work may take me a couple of weeks to complete as parts were just ordered tonight.


Thanks Guys!!!!



To be continued...
 

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The job of the booster is to make it easier to push the pedal, less force required. Your pedal sinks to the floor when you start the engine. The booster is working! Can it fail in a way that makes it work too well? I don't see how.
Sounds to me like the booster is working with an under-resistant circuit, so air in the circuit, a leak, or bad seals.
I hear you. Just weird that none of that "circuit" seems to fail until the engine is turned on. Generally, such a leak would show itself with the engine turned off as well.

--Peter
 

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I hear you. Just weird that none of that "circuit" seems to fail until the engine is turned on. Generally, such a leak would show itself with the engine turned off as well.

--Peter
My guess is if you compared OPs pedal with your car's pedal with engines off you would feel a difference. Then with engines on your pedal is easier and his goes to the floor.
 

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Still a crazy situation...





I was able to use ISTA+ to work all of the ABS valves and pumps. I don't think there is an issue with it, also since it is a separate closed system when not activated, it should have no effect on normal braking conditions.


When I next test, I will pull the DSC & ABS fuses to remove that from the equation.
I normally avoid brake threads because they are confusing. Sorry I have not read the whole tread just jumped into the most recent replies, apologies if I missed something important.
Have you considered that you have a hydraulic leak in the solenoid pack of your ABS unit? When the car is off only a very small amount of fluid leaks because you can't apply enough pressure to have enough fluid "float" the pintle in the solenoid. When you add the power of the booster there is enough flow of fluid to "float" the pintle up and allow for faster escape. There is no way you can test that, you need to remove the lines from the ABS unit and put either by pass lines on or pressure gauges, usually a combo of both.
The only way without spending hundreds on equipment is blindly replace the hydraulic part of the unit or take it to someone with the equipment. If you actually had a leaking caliper you would see the signs, lift a boot seal and see if there is fluid but if after one or two pumps there would be enough fluid to make your boot look like a water balloon. Trust me you would have noticed.



Unplugging the main cord of the ABS unit or fuses will tell you if you have a fault in the electric side of the ABS unit that opens a solenoid but it will do nothing for a solenoid that has a hair in it or part of the filter lodged in it. They can also just corrode if garage queens, water in the fluid will collect at the noids.
 

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Discussion Starter #33
Thanks Sailor24,



My problem may just well be in the ABS unit. I will check to see if there is a shop in the Orlando metro area who can diagnose & fix ABS unit.


I am still waiting for caliper rebuild parts to arrive and will rebuild them anyway.


Modern cars have become way too complicated...
 

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Discussion Starter #34
My parts are starting to arrive...


I found a posting on how to check the wheel speed sensors (diodes) and will do that shortly. The post also talked about re-soldering the ten terminals that bridge the DSC/ABS module to the hydraulic module. I will look at my terminals and see if I should do this too. Once done, I will use ISTA+ to cycle the pumps and valves to make sure that is working.


I will also do another 'in car' master cylinder bleed. I am thinking of plugging two of the MC ports and only run one hose to the reservoir and pump, then move the hose to another port, plugging the latter and pump, and again for the third port. Not sure if this makes any difference as opposed to running a hose from all three ports at once. I would like to know for sure the the MC is bled completely!!!


Hope to do all this before the Holidays get busy!!!


Happy Holidays to all!!!!!
 

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Glad I found this thread, as I'm experiencing VERY similar issues with the 2000 M5 that I just bought this week with a long, soft pedal, no improvement with bleeding, but also no warning lights. A question for you though, perhaps the strangest symptom that I'm experiencing is that the pedal is actually very firm and difficult to push while I'm trying to bleed the brakes, even with the caliper nipple open! Happily though, I actually have an extra ABS unit, albeit with a bad controller, but I'm just going to pull the hydraulic unit off the back of it and try swapping that out tomorrow to see if it improves matters.
 

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I found a posting on how to check the wheel speed sensors (diodes) and will do that shortly. The post also talked about re-soldering the ten terminals that bridge the DSC/ABS module to the hydraulic module.
What ten terminals? Inside the electrical half? I don't recommend that. You are referring to the Bimmerfest thread? Only one guy of all who attempted that had success and even he had to redo it. Some of the wires inside are like 30 AWG. Aka angel hairs. Been there done that, got the headache. If you don't have ABS/DSC codes you should not open the module.

My .02.

Good luck sorting this out, seems like a common issue.
 

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What ten terminals? Inside the electrical half? I don't recommend that. You are referring to the Bimmerfest thread? Only one guy of all who attempted that had success and even he had to redo it. Some of the wires inside are like 30 AWG. Aka angel hairs. Been there done that, got the headache. If you don't have ABS/DSC codes you should not open the module.

My .02.

Good luck sorting this out, seems like a common issue.
There is an old thread here where one of our members did a repair. See I this is helpful.

https://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/167399-diy-dsc-abs-module-re-solder-assy-terminals.html

Regards,
Jerry
 

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Discussion Starter #38
gsfent, xrviz,


Maybe I won't open the ABS/DSC unit to avoid adding headaches to this situation that is driving me nuts!!!


I wish I still had my 1964-1/2 first year Mustang!!!! Keep it simple, easy to work on and repair!!!!


Earlier today I tried to test the wheel speed sensors (the diode in each), but my multi-meter did not show any readout even when I reversed the meter probes. I tried this at the ABS/DSC connector and the sensor connector (did FR and RR sensors). I would like to know how to confirm that the meter is working and if I have to do something at the sensor. There are pictures in the Old Terminal post that just shows the probes touching the two sensor terminals. And I did have good batteries and meter set to diode setting, but I only see a 0 for readout. I can't imagine two sensor are bad. I only checked the two for now.



I would like to confirm that things are good working order so I am not chasing unknowns.


The rest of my caliper parts arrived today so I can start rebuilding them.



Thanks Guys!!!!
 

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Discussion Starter #39
I Googled 'How to test a diode with a multi-meter'. It seems to be so simple to do.... I put brand spanking new batteries in my meter and went after the wheel speed sensors (AKA Pulse generator, ABS sensor). I disconnected the sensors at the wheel wells and tested the sensors (all four). All the meter showed at all locations and reversed terminals was .0L. All I can conclude is that all four diodes are toast? I did a parts search and see that there is a very wide range of prices by the different manufacturers. I will shop middle of the road prices from RockAuto.com. First thing I will do when I receive the new sensors is test them with the multi-meter. I better get a reading or I will be major PO'ed.


I better get some Aspirin now....
 

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Explain how you tested might be easier than me typing out how to test a diode. Include what ohms scale you used on your meter.
I will also give you a save from another forum to read most the links point to the wrong place. I did not really know how to save things like this when I did it. I hope this works, I am sending it as a zip because I don't know another way. You will have to unzip and you will get one folder with pics and an html file, they must be in the same folder.
View attachment ABS Help.zip

Three tries later and it worked.

<form action="showpost.php"><table class="tborder" style="border-top-width:0px" width="100%" cellspacing="1" cellpadding="6" border="0"><tbody><tr><td class="tfoot" align="center">
</td> </tr> </tbody></table> </form>
 
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