Losing Brake Pedal When Engine Running

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Brucey
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Re: Losing Brake Pedal When Engine Running

Post by Brucey »

plip1953 wrote: Wed Dec 05, 2018 9:18 am

To satisfy my curiosity, please could someone explain how can we be certain that fluid isn't somehow getting back from the MC into the main brake fluid reservoir? Presumably there is some kind of valving system. And if so, couldn't that be faulty in some way?


once the pedal starts to move the MC isn't much more complicated than a bicycle pump, and can be (and has been) tested by blanking the hoses off.
One other thought - with the engine and and brake pedal pressed, and sinking, does it make any difference if the pedal is pumped? What actually happens in those circumstance?
as soon as the pedal is released, the accumulators in the ABS unit will discharge and the hoses (if they flex) will recover their shape.

I quite like the suggestion to repeat the test without any flexible hoses in the system. This would be easily done by blanking off the outputs from the ABS unit, or temporarily plumbing in a caliper or two via hardlines only.

I also note that on a standard E24, the brake bias valves may alter how the brake behaves when it has and hasn't got the servo driving it; you may only just exceed the threshold pressure for the rear brakes to work at all without the servo. However on M635CSi there are no bias valves so the same logic does not apply.

BTW no leak plus a slow pedal descent that is 'cured' by new flexible lines suggests to me that there was a leak between the liner and the first layer in the old hoses.

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Re: Losing Brake Pedal When Engine Running

Post by plip1953 »

That sounds like a very real possibility.

On all the performance cars that I've had over the years it has been routine to install braided hoses, although I have never been convinced that they made any difference to braking feel/performance. But that's probably because the replaced rubber pipes were relatively new anyway.

My 635 is now 30 years old and I'm almost certain the flexi brake pipes are original, and so even though I'm not conscious of the sinking pedal feeling, replacement is now going to be added high up on my "to do" list.

I really hope this ends up being the OP's problem too.
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Re: Losing Brake Pedal When Engine Running

Post by Boggie »

Thanks for all your ongoing input chaps, very much appreciated. However, I would be pretty surprised if my issue was the hoses; I had them custom built by BF Goodrich out of their top of the range, racing stainless braided pipes and fittings. I have fitted these pipes to quite a few cars over the years and never had an issue.

However, I could build up two steel male to male short brake pipes and connect one between the FR / RL and the other FL / RR on the ABS unit. If I then start the engine up and get the same sinking pedal then I know it is something to do with the ABS system. If it foes not sink I know the problem is either the flexi pipes (or calipers).

One thing to note is that the MC has two outputs, one feeding FR / RL and the other FL / RR. As my pedal sinks right to the floor, surely this means I have issues in both circuits. If only one circuit was affected the pedal travel would increase but stop before the floor due to the 'good' circuit holding pressure. If that all holds true then what are the common elements other than the pipes....?

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Re: Losing Brake Pedal When Engine Running

Post by Brucey »

FWIW I'm increasingly of the view that this may be a fairly normal (or at least inconsequential) feature of the system. I had another early ABS car years ago with a completely different system that worked on the same principles fitted to it.

IIRC that also allowed the brake pedal to descend slowly provided the car was stationary and the engine was running. That car had braided brake lines fitted (which firmed up the pedal feel noticeably but didn't change the pedal drop) and crucially, the pedal didn't behave in the same way when the car was actually moving; the pedal felt fine.

The only rational explanation I have for this is that the ABS pump may be inhibited when the car is stationary but it might be triggered to run briefly when the car is moving, whenever the pressure gets above a certain value at the entry side of the pump, such that fluid is displaced into the parts labelled 'reservoir' on the motorbike ABS diagram above. This is 'safe behaviour ' (because the pedal will only ever drop with a fair positive pressure in the working part of the system such that the car won't roll away because the brakes are on still) and this could allow the pedal to descend slowly when the car is stationary but not when the car is moving.

To test this I wonder if

a) the ABS pump could be manually triggered when the car is stationary; this may stop the pedal from falling or
b) the pump could be inhibited when the car is moving, in which case the pedal may fall in the same way as when it is stationary.

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Re: Losing Brake Pedal When Engine Running

Post by plip1953 »

While at my local garage yesterday I posed the question about the sinking pedal issue and was told straight away that it was "pretty normal". I went on to ask why it does it, but even the mechanic freely admitted that he'd never really thought about that aspect!

I'll be going there again next week and will seek out the garage owner who is very experienced and a previous owner of his own 635.
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Re: Losing Brake Pedal When Engine Running

Post by olympia57 »

plip1953 wrote: Tue Dec 11, 2018 8:17 am While at my local garage yesterday I posed the question about the sinking pedal issue and was told straight away that it was "pretty normal". I went on to ask why it does it, but even the mechanic freely admitted that he'd never really thought about that aspect!

I'll be going there again next week and will seek out the garage owner who is very experienced and a previous owner of his own 635.
Not sure what I think of the mechanics synopsis Phil , at over two thousand visits to the thread I'd have expected many similar statements from actual and current E24 owners if that was the case .
In my many years and vehicles owned I've experienced it once and as Ian says his pedal actually sinks to the floor suggests that it was worse than my problem which was unpleasant and definitely not normal.
Look forward to hearing what the Boss says .
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Re: Losing Brake Pedal When Engine Running

Post by Brucey »

in fairness except when you are deliberately testing the brakes you wouldn't necessarily notice this happening; even with an autobox car you instinctively don't press on the brakes (whilst stationary) any more than you need, i.e. to stop the car from running away. This may well happen at a system pressure that is so low you can't push fluid into the ABS 'reservoirs', and therefore the pedal doesn't normally drop, even if it is 'a normal feature' of the system at higher pressures when the car is stationary.

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Re: Losing Brake Pedal When Engine Running

Post by plip1953 »

I share a certain amount of the scepticism expressed by Olympia, and agree that if such a thing is "normal" then why haven't loads others on here commented to that effect. That said, I can pretty easily get the pedal to sink on my Merc ML63 (which I'd not experimented with prior to reading his thread) and there's absolutely nothing wrong with the brakes on that car during normal driving.

Th mechanic I spoke to on Monday has discussed the issue with his boss and told me yesterday that the boss agrees that it's not an at all unusual phenomenon, but his explanation is primarily that as you continue to press hard on the pedal the pressure build up in the system is so great that anything that has scope to be expanded (eg rubber brakes lines, but not exclusively those) will expand and thus the pedal will sink. But he also added that in his opinion there is also a strong possibility that fluid will be finding it's way past the MC seals. I expect to see him in person early next week and will plan on picking up further with him on this topic.

In the meantime, one thing that I really don't get is about these so-called "reservoirs" within the ABS unit (of cars of the 80s) . What possible purpose do they serve and how are they physically configured and controlled. If, under certain circumstances there is scope for them to fill of empty, what triggers it and or controls it, and what's the fluid replaced by when they are empty? The only other thought on this point is around the Mercedes feature of the early to mid 2000s (and maybe many other makes these days) of SBC whereby my understanding is that the entirely braking action (as well as traction control and loads of other stuff) is fully under the control of the electronics.

This Merc marketing video is interesting (even if not especially relevant to the specifics of this thread) and I note that a reservoir is part of the system, albeit that I don't believe it's the same kind of thing as discussed in earlier posts, nor do I believe there is anything like it in the 635.

https://www.eeuroparts.com/blog/2527/me ... s-doesn't/
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Re: Losing Brake Pedal When Engine Running

Post by Brucey »

referring to the diagrams I posted upthread, my understanding is that the reservoirs are simply small chambers which have spring loaded pistons in them. The springs are preloaded so that the pistons can only deflect once the pressure exceeds a certain threshold. They are needed in the ABS system to average out the flow through the pump and because without them you would be likely get something akin to 'hydraulic hammer' in closed parts of the system which would be liable to quickly become destructive in nature.

If the MC is faulty, the pedal would fall all the time, (not just when the car is stationary and the engine is running) and you would probably notice. However if the reservoirs only fill above a threshold pressure (and only don't empty themselves when the car is stationary eg because the pump is inhibited) you may not notice this either.

BTW I suspect that the ABS pump is inhibited when the car is stationary; IIRC the system doesn't work below a threshold speed anyway (15mph from memory?) presumably because the car will normally stop pretty quickly even if all four wheels are locked, from that speed. Maybe the wheel sensors don't deliver a 100% reliable signal below a certain speed, too. Also drivers will feel each stroke of the ABS pump/system as a fairly aggressive 'kick' in the pedal at low speeds; at the least this is unsettling, and at worst it would probably be deemed unacceptable.

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Re: Losing Brake Pedal When Engine Running

Post by ron »

I've been following this thread with some interest as on at least 8 cars I have had the same symptom ie pedal slowly sinks with engine running. I talked to a local BMW specialist garage (I think around 15 years ago) who confirmed that he'd had the same thing with other models and makes and never did work out why.

I think that if all our forum members did this "pushing on pedal with engine running" we would have an awful lot of confirmation!

Just my 2 cents worth.
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Re: Losing Brake Pedal When Engine Running

Post by plip1953 »

Brucey wrote: Wed Dec 12, 2018 11:05 am referring to the diagrams I posted upthread, my understanding is that the reservoirs are simply small chambers which have spring loaded pistons in them. The springs are preloaded so that the pistons can only deflect once the pressure exceeds a certain threshold. They are needed in the ABS system to average out the flow through the pump and because without them you would be likely get something akin to 'hydraulic hammer' in closed parts of the system which would be liable to quickly become destructive in nature.
Thanks for that explanation. In the way you describe it that does seem like a plausible explanation.
Brucey wrote: Wed Dec 12, 2018 11:05 am
If the MC is faulty, the pedal would fall all the time, (not just when the car is stationary and the engine is running) and you would probably notice. However if the reservoirs only fill above a threshold pressure (and only don't empty themselves when the car is stationary eg because the pump is inhibited) you may not notice this either.
Intuitively I tend to agree on the MC point, although conscious that pressure in the MC is hugely greater once the engine is running.
Brucey wrote: Wed Dec 12, 2018 11:05 am
BTW I suspect that the ABS pump is inhibited when the car is stationary; IIRC the system doesn't work below a threshold speed anyway (15mph from memory?) presumably because the car will normally stop pretty quickly even if all four wheels are locked, from that speed. Maybe the wheel sensors don't deliver a 100% reliable signal below a certain speed, too. Also drivers will feel each stroke of the ABS pump/system as a fairly aggressive 'kick' in the pedal at low speeds; at the least this is unsettling, and at worst it would probably be deemed unacceptable.
My understanding is that the speed threshold is more like 5mph and the prime rationale is that in snowy conditions in particular at very low speed the car will actually stop more effectively if the wheels are locked/skidding - because snow is accumulated and pushed along in front of the tyre forming a kind of buffer.
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Re: Losing Brake Pedal When Engine Running

Post by Brucey »

Note that the geometry of MC seals is a lot like that of seals in a bicycle pump, i.e. the seals are cupped. This means that they usually seal better and better at high pressures. Also the leak rate through a hole goes as the square root of the pressure in most cases. Both things work in the same direction so to a first approximation

time (for any given pedal drop through leakage) x pressure (at low pressure) ~< time x pressure (at high pressure).

This means it is less likely that a pedal that doesn't fall when the servo isn't working and does fall when it is, is caused by a fault in the MC; given the nature of the MC seals, and the way leak rate varies with pressure, one would expect it to be the other way around.

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Re: Losing Brake Pedal When Engine Running

Post by plip1953 »

Another thought/question.

Do braking systems incorporate any kind of over-pressure relief valve mechanism? My thinking here is that by pressing really hard on the brake pedal (engine running) the combination of foot pressure and boost pressure (or maybe just he boost pressure) will be extremely high and potentially lead to component damage if allowed to remain in that state for an extended period.
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Re: Losing Brake Pedal When Engine Running

Post by Brucey »

no overpressure valves in hydraulic braking systems....

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Re: Losing Brake Pedal When Engine Running

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Was this problem resolved ?
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Re: Losing Brake Pedal When Engine Running

Post by Boggie »

Yes!

Last week I spoke to the chap I take all my classics for MOT testing about this issue as he is a classic car/bike nut too. I explained that I had replaced all the braking components, including the ABS unit, bled the system a number of times (in conjunction with cycling the Bosch ABS valves/pump) but whilst I had a firm pedal with the engine off, once it was running the pedal sank to the floor under heavy pedal pressure. He said that this was not uncommon with early (and some newer) ABS-equipped cars. He asked me if the brakes worked and if I had driven the car, I said yes and no. He told me to take it for a drive around the block with normal braking and try again. I did so and when I returned to the workshop, the pedal stayed firm even with the engine running. I will keep an eye (foot) on it over the coming weeks and let you know if this situation changes.

So, I drove the car down to him on Saturday and after 15 years, laid up in a barn and almost exactly one year in my workshop; the Shark passed its MOT (much rejoicing). :D

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Re: Losing Brake Pedal When Engine Running

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All's well that ends well!!
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Re: Losing Brake Pedal When Engine Running

Post by randall977 »

My cars going in for it's MOT tomorrow after many years off the road. I have replaced every brake component apart from the MC. When I looked at the last MOT in detail last week I noticed that it had failed on 'excessive brake pedal travel' - mysteriously it passed the next day...

The brakes seem to work fine but the pedal does travel a long way down both with the engine off or on. So in a panic on Saturday I ordered a new and very expensive ATE MC. I fitted it, bled the brakes and they felt absolutely solid...

...that is until I started the car and then they felt just as before! My conclusion - that's how the brakes are! I suspect the bomb maintains some pressure when the engine is off.

So I think I removed an almost unused MC - it does look new...doh!
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