I have the condition where the brake pedal is too low when used, others have reported the same condition here before. The brakes stop the car very well but when I got the car, an SRT, I noticed at once than the pedal movement was not the same as the NA that I had before.
I have bled the brakes many times and there has been no improvement.
With the engine stationary the pedal moves the amount or almost the amount I would expect with the engine running. When I start the car with my foot on the pedal still, the pedal sinks towards the floor so that the gap under the pedal just lets me slip my toes under the pedal. As I said the brakes will work fine and the ABS works.
What I do not understand is that there is a series of rods and pistons that make a solid contact with each other when the pedal is fully depressed and each piston will move brake fluid to each calliper piston.
My question is why is it that when there is a vacuum in the system the pedal moves more when it should be the almost same amount as when there is no vacuum in the system?
The pedal is not spongy at all so l have discounted air in the lines.
There is no leakage of brake fluid to be seen anywhere.
The pedal does not drop when a constant pressure is applied to it with the engine running or not.
With the engine running or not the same amount of fluid is displaced to move the pads against the rotors.
With the engine running there is a negligible effort to move the pedal to the point that the braking effect can be felt.
The car has the wire wrapped brake lines, even though I think this has a minimal effect on braking feel, I think the wrapping is more for hose protection and looks than anything.
I see only three things that could be wrong, the ABS unit with air in it or with sticky valves, a faulty brake booster or a faulty master cylinder which I replaced it with no improvement so I discount this as a problem.
From what I see the brake fluid bypasses a lot of the valves when the engine is not running and goes straight to the calipers pistons. I surmised that with the engine running the ABS would is active and the brake fluid enters areas where there are reservoirs that have sprung loaded pistons in them and the brake pressure compresses the springs and the reservoirs are filled and this caused the extra fluid to be required before the caliper pistons see enough pressure to move them. From this I reasoned that the ABS unit had some valves stuck and allowed the brake fluid into areas where it would not normally go unless the ABS feature was activated.
I am hesitant to replace the ABS unit as then it will definitely require bleeding.
I have the DRBII setup but I have not found the ABS bleeding program on it, I have had the pump on the ABS unit run for a second or so but apart from some readings that have no bearing on my issue that is all I found.
At the moment I believe the ABS unit valves are my problem.
Does anyone have any other ideas. The ABS unit is made by Teves and is model number MK20 ABS ESP, (Electronic Stability Program). The Crossfire one has the # 193 545 05 32, the ABS motor is #A 004 431 74 12 Q02.
I will add a schematic of half of the hydraulic circuit of this unit a little later, the other half is identical and is for the other side of the car.
So, anyone with other ideas or think my ABS unit is the cause?



Electronic Stability Control
https://www.aa1car.com/library/stability_control.htm

Teves Mk20 ABS
https://www.aa1car.com/library/abs_teves_mk20.htm

ESP explained in French.
https://www.vag-technique.fr/ext_fil...20_ABS-ESP.pdf

 

Dave, just for an idea, remove and plug the vacuum line to the booster, and drive it. See where the pedal stops the car. What would this prove, I have no idea!

 

Thanks pmI will try this, my street has no kids on it😀, also I will bleed the brakes and find a gravel road and do some emergency stops, I read this can get air out of the ABS module when you bleed it again.
I am loathe to do anything that makes it worse.

 

Sounds like the brake booster is pulling the pedal down. Think there is a control valve inside it that is supposed to sense when you are pressing the pedal and when you are off. It may be leaking. I have seen this in certain older cars. Start the engine and watch the brake pedal drop.

 

With my foot on the pedal and lightly pushing on it. The pedal will fall away as soon as the engine is started. Is that what you mean?
Thanks for the replies to this and the PM.

 

Yes, that is what I mean. Typically occurs in a defective brake booster, or an over-vacuumed brake booster. The latter is probably not an issue for you, but the former could be. Is it worth fixing? Only if it affects brake operation.

 

The pedal only falls away when a light pressure is applied to it. This because a slight movement of the pedal takes up the slack between the pedal pushrod and the MC piston rod. A small movement of the MC piston opens up the inlet behind the piston and closes the inlet for any required make up fluid in front of the piston, I’ll ignore the secondary piston other than add that it starts to move after the primary piston.
By this time the diaphragm of the booster has started to move as the vacuum valve has closed and atmospheric pressure has started to push the piston. Even if the booster is defective how come the pedal falls away so much? That is the explanation I am seeking.

In the meantime I am out tomorrow to find replacements for my rear Hankooks, they are down to the wear bars on the inside and 1/32” away on the outside. The fronts are evenly worn with a lot of tread left. The TPMS has been disconnected and went dead last year after 16 years and 40,000 miles. The fronts were aligned 10 years ago but they could not do the rears, evidently they did not need it anyway.

 

Id like to add my thoughts, here is a pressure valve assy for brakes that may be of interest. At least its worth investigating the concept. Its been a while since I had to wotk on my C. For what its worth, enjoy, WOODY

https://www.ebay.com/itm/13415369647...id=14033642653

 

If you try these, would you please get back to us and let us know the results.

Thank you,

Jim

 

Thanks for the idea Woody, but I think the OEM system should work without any additions to it.
Thanks for the thought though.

 

Dave
Perhaps it’s time to start installing the brake system parts you’ve already purchased. If you don’t remove possible causes off the list, this issue will always be one of speculation for you.

if you’ve already acquired the parts it would be a shame not to get them into the car.

Besides it’s summer in ON and the humidity will make your time under the hood 10x more enjoyable

 

I would but I am not sure how to bleed the ‘new’ABS module. It seems the DRBIII tool I have cannot do the bleeding and I do not want to be worse of than I am now.
My thoughts at the moment is that a solenoid has stuck. If only I could fully understand the way it works I might be better for it.
I have been at it for ten years another ten and I will be long past caring.

 

After much reading, I came across a thread about the Teves Mk20 ABS-ESP which mentioned that the valves leaked prematurely and this resulted in a low pedal to varying degrees. The valves should not leak and provide high pressure fluid in front of the MC piston to stop it going to far forward. This leakage could explain why some people have less of a pedal drop.
I would replace the existing unit if I could assured of being able to bleed it.

 

I have an idea on why the pedal drops.
If we look at the picture below we can see that after the brake pedal has moved a little the piston has blocked of all the inlets from the master cylinder reservoir, but a port from the reservoir has been re-opened behind the piston to prevent a vacuum. At this point the brake cylinder pistons should start to move forward pushing the brake pads against the rotors, but if the solenoid valves that operate the ABS system are leaking or one or more of them is jammed in position then the master cylinder piston will be able to keep moving forward. As the fluid is now in a closed loop condition the pedal will drop even more, because in the fluid circuit is an accumulator which is normally kept closed by a spring. The MC piston will be free to move forward until the accumulator
is filled at which point the brakes will start to work. The fluid is only under pressure when it comes up against an opposing force. This force will occur when the accumulator is full and it’s spring compressed to its maximum. See the translated img015.pdf below.
Normally the MC piston only moves a few thousandth of an inch, because the seal only retracts the caliper piston that much. CLICK
The caliper piston only moves a few thousandth of an inch, therefore the master cylinder piston only has to move the amount of fluid required to move all the caliper pistons a few thousandths to hit the rotors. This assumes that there is no air in the brake lines, master cylinder or the ABS unit. The brake fluid is not compressible otherwise.







Here is how the brake cylinder piston returns to its original position, it is nothing fancy, the seal returns to its original shape and takes the piston with it, that is why the pins on which the callipers ride should be kept clean and greased allowing the callipers to move smoothly. Rapid rotor and shoe wear can be expected if the calliper cannot move or can only move one way, my rotors practically look as if they were just machined. CLICK