The thing is we are talking volume, not area. Volume is radius squared times height, thus the master cylinder must travel exponentially to make up for the increased volume not just for one piston cylinder but for four pistons in the larger front calipers. Say the NA is 2 and the SRT is 2.2 in diameter. 2 x 2 =4 but the SRT is 2.2 x 2.2= 4.84. Now .84 x the four pistons would mean the same master cylinder must move 21% more fluid to fill the cylinders. 19.36 vice 16. Everything else being equal the travel of the brake peddle travels 21% farther. That's a bit simplified, but you get the idea. I am no mathematician, so I maybe all wet.
Wish you had more than one other SRT 6 to compare with your car. Plus each car is a little different and even more so with the passing of time.

 

I am sorry but you failed the maths test, area is = pi x r x r.
I said the pistons only move a few thou, they are practically rubbing lightly all the time.
The seals pull back the pistons by returning to their original shape also the seals are pushed back by any wobble in the disk, it is hard to have a disk rotate without a few thou runout. Too much runout and you feel it in the brake pedal.
Pushing the brake pedal initially takes up the slack in the mechanism, the amount the MC piston moves is about 1/8” to 3/16” from what I remember, if the area of the pistons was twice that of the NA then the movement would be at the most 3/8” and that would not account for the massive pedal drop that happens. As I said with the ignition OFF the pedal works fine and does stop the car from rolling albeit with greater pressure on the pedal.
I have tried another SRT and the owner of it says my pedal drops too much.
I will update my figures later, as I have to go out.
Edit
I did the calculations but lost them, the extra volume of oil moved is about 50% more as I recall.

 

I have decided to change the ABS module as my current thought is that one or more of the valves in it are jammed. I have just to check some more of the power to the original unit, confirm that it is at fault and assure myself that I can bleed the new ABS unit when it installed.
I will start a new thread on that attempt at restoring the correct pedal travel.
I will be relying on the DRBIII clone to complete the task.

 

I finally got up the gumption to run the Micropod through some steps, I had the pump run on the ABS module and it told me the steering needed centring. I only tried the ABS menu and it has not given me the confidence to replace the ABS module. I have read as many articles as I could on the subject but nothing covered my problem except one article did say that a stuck valve could cause a low pedal.
I do not think that there is air in the system as the pedal is rock hard at the bottom of the pedal stroke.
If I did change the ABS module I would have to bleed the system, the Crossfire manual says this involves bleeding the brakes, bleeding the ABS module and bleeding the brakes again. This means bleeding each calliper three times. I assume that the pump is activated to bleed each line when bleeding the ABS module.
The ABS works fine in the rain and on gravel, do I really need to worry about the low pedal? It is still some distance from the floor.
At my age I want the problem solved, going through the whole process above and not solve the problem would be a real downer. When I was younger I would have done it in a flash.
Any tips out there?

I saw these pages which were shown in the attached files in a previous John and Terry post. CLICK
I think the bleeding is done by activating the pump in the menu on a page not shown which is under the Anti-lock Brakes menu. Does anyone know for sure please.

The ABS on the Crossfire is a Teves ESP Mark 20 system.

 

After reading your post and the detailed explanation you provided I was trying to analyze the generic power braking system and determine where in the system a problem as you described might be the cause. The thought that I have is related to the rubber hose brake lines connecting the rigid metal brake with the wheel backing back plates. Aged rubber hose deteriorates internally and can produce the symptoms you have described and experienced. Rather than me explain the various symptoms and resultant effects refer to the attached online link for a complete explanation. If your rubber hoses have deteriorated replacement is not very costly. I hope this helps solve your problem. Jack
https://www.yourmechanic.com/article...ing-brake-hose

 

Thanks for the reply but I have replaced the hoses within the last 15,000 miles with Goodrich braided cables. The car has only 40,300 miles on it.

 

Okay if the brake hoses have been replaced I would completely empty and refill the lines with new fluid. You probably have a considerable amount of air in the system. I recommend you not attempt to do this as it should be done by a quality auto repair shop and more than likely involve 2 people. You indicate the pedal goes almost to the floor then becomes solid. This to me indicates the presence of entrapped air in the line. You are compressing the air sufficiently to equal the hydraulic pressure needed to actuate the brakes.

 

.
Their is no need to be 2 people if an mityvac brake bleeder kit, or something similar, is used and it is a way better method than using the brake pedal to do so . . .

First,install car on lift or on 4 jack stand and remove weels. Open the master cylinder reservoir and note the oil level because at the end of the procedure you need to finish with that same level. Then, suck the old brake fluid almost to the bottom of the master cylinder reservoir, refill to top with new brake fluid, put cap on but do not completely screw the cap.

Caution . . . do not suck more brake fluid than the master cylinder reservoir contain at once or air will enter the system. Check level and reajust often.
.
Unscrew the Rear Right caliper's bleeder and put teflon tape on the bleeder's thread to prevent air bubble in the clear tube while you bleed.Reinstall and thighten bleeder,leave a closed end wrench on the bleeder and connect clear tube,Build vacuum and check the guage to see if their is no leak, The tip of the bleeder need to be free of dirt or rust in order to have a good seal with the tube. , If their is no vaccum lost you are ready to proceed . . . then, you open the bleeder slowly and keep your vacuum steady. Has soon you see the change in color of the brake fluid comming out in the clear tube you have the confirmation that this line is properly done .If their was any air at all in this line you should have seen it passing by in the clear tube. Repeat procedure with the Rear Left caliper followed by Front Right and finally the Front Left.

Adjust brake fluild level in master cylinder reservoir to the same level previously noted and thightly screw reservoir cap. Test brake pedal travel and you are done.

Daniel

 

I have replaced the oil twice and bled them at least three times, with help and on my own with and without speed bleeder nipples. I have replaced the Master Cylinder hoping it was at fault, I primed this with oil before installing it.
I have bled brakes for years so this is not my first time at the rodeo as they say. I did think there was air in the ABS module but tend to think there is none, maybe a sticking solenoid valve in the ABS module.
With the ignition OFF the pedal stops where I would expect it to stop and it does apply the brakes, you have to use more pressure as expected. Turn the ignition to the ON position and the pedal falls, the brakes and ABS work fine the pedal is hard and no feeling of sponginess. I think this is because the ABS valves are bypassed with no power going to them. If you see the hydraulic circuit schematic there are more lines than you would expect. I will add it if I can find it.
My Micropod 2 setup says the ABS pump is OK, the power brake system is OK and only the Steering Angle sensor and pollution readings need resetting this is because the battery was disconnected.
Basically I learnt nothing from the Micropod 2 that I did not already know. I only checked the ABS system, my BlueDriver told me the pollution settings problem which is easy to guess.
I have a spare ABS module and I would instal it if I was sure I could bleed the ABS properly using the ABS pump.
I think sticking solenoids valves could allow oil to get past them because they are open thus building up no pressure in the system until it comes to a closed circuit and allows pressure to build and the brakes to work. Pressure should start to rise once the Master Cylinder piston has been pushed past the make up holes in the cylinder walls, instead it travels farther causing the longer pedal stroke.

 

You said you replaced the master cylinder, but it might be defective internally. The bore diameter could be slightly out of tolerance causing fluid to leak by the seal or there is surface corrosion. Remember standard brake fluid absorbs water over time which can create internal corrosion related issues throughout the hydraulic system.
I can see you are trying to do an in-depth engineering analysis as a way of solving your problem. On a personal level after reading about what you have tried to diagnose on your own, I would seek the services of an experienced MB mechanic. Many times, these people are not working for MB dealerships but instead work for small independent garages that only work on MB, BMW, etc. type vehicles. My experiences using independent types of garages has been very positive. They analyze the problem, call before doing any work, explain what the problem is and provide a price. I like to work on my vehicles but there are times when the services of the experienced is the better way to go.
Jack

 

Thanks for the reply, but there is no leak past the seal as the pedal does not sink with pressure on it for a good while. I have had a MC fail on another car so I know what it is like. I am trying to understand how the ABS works, I have a good idea and suspect stick solenoid valve(s). I am against giving it to a garage, they change parts until they solve the problem or give up leaving you with a hefty bill. The car stops fine, the ABS works and I want to find why my pedal is so low. I can put my foot behind the pedal when it is down, it is not to the floor.

 

I repeated the ABS tests with the Micropod 2 this morning and got the same results as I had before.
I found no menu that bled the brakes or the ABS module, I assume you activate the pump to do these operations.

 

I know You have been strugling on that problem for a long time,but, did You had ocasion to compare this depressed pedal getting lower as soon the ignition is only turned to the " ON " position, with any other XF ?

If Yes and only on your XF this occur . . . test i would perform if this problem would be on my XF and here is why: If the depressed brake pedal goes down as soon the ignition is turned only to the " ON " position, this would mean that some brake fluid is going somewhere as soon the ignition is turned to " ON " since the engine is not running and their is NO Vacuum at all comming in the brake booster at this point.
Then, this mean that the brake fluid have to go somewhere by some sort of electrical mean only. This would also mean that " IF " the brake pedal is depressed to a fix point, and then, the key is turn " ON " , pressure would drop down in the brake system .

Having the car on four jack stand, place proper lenght of wood between seat rail and brake pedal to the point you can feel a slight presure on all 4 calipers by turning each wheel by hand, then turn ignition to " ON " and compare the force needed once again, to turn each wheels to see if pressure in the brake system as really fallen.

Daniel