My Brake Pedal Feel is Kicking My Butt... - Page 3 - BMW M5 Forum and M6 Forums
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post #21 of 158 Old 12th December 2018, 03:11 PM
uhad2point
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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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post #22 of 158 Old 12th December 2018, 03:27 PM
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Originally Posted by uhad2point View Post
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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post #23 of 158 Old 13th December 2018, 11:14 PM Thread Starter
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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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post #24 of 158 Old 14th December 2018, 12:43 AM
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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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post #25 of 158 Old 14th December 2018, 04:29 PM
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Originally Posted by Ferris View Post
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.

Last edited by herrubermensch; 14th December 2018 at 04:30 PM.
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post #26 of 158 Old 14th December 2018, 04:42 PM
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Originally Posted by herrubermensch View Post
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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post #27 of 158 Old 14th December 2018, 05:19 PM
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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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post #28 of 158 Old 15th December 2018, 03:08 AM Thread Starter
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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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post #29 of 158 Old 15th December 2018, 04:41 PM
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Quote:
Originally Posted by xrviz View Post
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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post #30 of 158 Old 15th December 2018, 05:14 PM
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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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