I started this thread because some others, as well as myself, have the impression that the SRT has too much brake pedal travel before braking begins. Anyone with any ideas on the subject is invited to add their thoughts. I think that should a seal fail suddenly there would be not enough travel to pump the brakes, having had a master cylinder fail suddenly when braking for a stopped bus I had to pump furiously and only just stopped in time. That was nearly sixty years ago and I remember it as if it was yesterday, a passenger who was about to get off using the rear exit of the London Transport bus saw the panic on my face and showed a similar expression on his own face. After getting off the bus he walked by me wiping his brow and laughing.
I had a similar experience in a rented car in Mexico in the middle of nowhere except for the resort we where going to, why did you drive by they said, I replied that the brakes did not work. It was a slow 50 mile return trip back to the car rental. The manager just shrugged when I told him there was no fluid in the reservoir.

I have had my SRT for 11 years and I have thought that the brake pedal travel was too much and had more travel than the NA I had before. I have replaced the Master Cylinder hoping it was the problem although I doubted it They are not spongy, they have no leaks anywhere and work well, I have changed the brake fluid three times in the nine years I have had the car and bled them at least twice between changes. The fluid was changed the third time last week. In all, I have run two litres of fluid through the brake system. The first time I changed the fluid I had my buddy help me and then twice I used the Goodridge self bleed nipples.
Last week I bled them with the engine stationary and then with the engine running, however, there was no change in the brake pedal travel.
The power brake systems are all basically the same, a diagram showing the basics is shown below. I used this diagram by Encyclopaedia Britannica because of the colours and component description, the one in the Crossfire Service Manual is poor.



You can see that with the engine not running the force felt at the pedal is provided by the spring in front of the brake booster piston. The spring is stronger than the atmospheric pressure on the back face of the piston, the difference between the spring and atmospheric pressure is provided by your foot pressure. The vacuum in front of the piston reduces the pressure needed. The different pressure needed from your foot on different cars would be due to the spring strength, the atmospheric pressure, piston diameters and the effective pedal length.
With the larger piston areas of the SRT callipers compared to the NA and because they use the same MC the travel required by the pedal will be more, how much more I cannot say. The calliper pistons move away from the rotor by only a few thousandths of an inch due to the seal lip returning to its no-pressure shape.

I shall continue after a short intermission.

 

Wow 180,sound like your down to your last nerve.I haven't seen this mentioned but you said you changed the MC.Did you make sure the pushrod is just touching the mc bore.Different brands of mc's could have a different bore length. If you checked it then that's all I got.These xf guys are keeping brake fluid companies in business.My new brakes are getting better,firmer the more I drive it.Do the pads wear in and get harder? Weird.

 

I used the identical Mercedes MC.
There is or was at one time a gap between the pedal push rod and the MC push rod, this was to allow for the fluid to expand when warm. If there was no gap the brakes would be activated by the expanded fluid. I cannot see how that would happen in today’s cars. If the gap was too large I can see that more pedal stroke would be required, some systems have an adjustment to get the gap correct to allow make up fluid to enter the system from the reservoir and set the pedal travel.
I really know little about our brake systems inner workings, I hope I can learn more from those with more know how.

 

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Having seen all the posts (compared to all the posts mentioned here et-al), I can only add my personal experience with my N/A. When I experienced a longer pedal travel, all I did was 'adjust' to its different feel. Finally, I purchased a bleeder kit and performed a DOT-4 fluid change using this vacuum kit at each caliper based on the recommended procedure.. Immediately the pedal feel was factory fresh (high pedal and brake activation). The numerous other posts about this exact issue is only akin to the various different attempts to right this issue. There are many different reasons why this is happening, only hope the issue is solved to the satisfaction to all who are experiencing it.


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180 and Pop Tro - Very good diagram and excellent comments from both of you. I looked at that vacuum line and was wondering if having a oil recovery can which is hooked up to the engine vacuum would have any effect on the brake system. I have the Needswings oil recovery system but I think most all of them use a vacuum assist. Just a thought!

I too, am perplexed by the long pedal travel in these Crossfire's. Mine is a NA but it just seems like the pedal has to go a long way in before the brakes stop the car. Like others, I have bled the system three times, had new rotors and pads installed, and put on the stainless steel wrapped brake lines. New fluid of course, on all three bleedings. My regular car is a Jeep Charokee that stops firmly with just barely touching the brake pedal. It's tough going from the Jeep to the XF. It's scary sometimes. You wonder if the XF is going to stop in time. Anyway, hope we can find a solution somewhere.

Thanks for the diagram and the comments.

Jim

 

GREAT POINT! I never associated any loss of vacuum (reduced vacuum) with this issue. Interested to knowing if everyone who continues to have an issue (after fluid change) with long pedal travel, if they have an oil recovery system installed. Interesting.....

Just consider, you are not maintain high RPM's when braking, so the engine vacuum is much lower. Maybe simple? Now if many who do not have a oil recovery system who did replace fluid and has a tight brake system, has this issue, who knows if it (oil recovery system) doesn't effect braking?


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My troubles preceded the installation of the oil catch can. The power brake has a reservoir that holds the vacuum from dropping as the engine revs fall, the reservoir has a one-way check valve on it.
It takes a couple of brake applications after the engine has stopped to deplete the so-called vacuum, it really is not a vacuum just lower air pressure. The car can pull nowhere near a vacuum, but we call it a vacuum anyway. Even with the car idling the vacuum is enough to assist the braking. If the catch can restricted the vacuum it would not affect the other vacuum lines.

 

Guys I do not get what you are saying on vacuum. Vacuum in the line going to the brakes comes off the intake manifold and that vacuum varies with the position of the throttle plate. When the car is at idle the vacuum increases because the throttle plate closes. When you are cruising down the road vacuum reading is considerable lower because throttle plate is open. When you apply the brakes your foot should be off the throttle, throttle plate closes and vacuum increases to the brakes through the line connected to the master cylinder. You have greater vacuum measured in inches of vacuum (-Hg) at idle than at open throttle cruising down the road.

 

My next thought would be, how far are the caliber pistons backing off when not applying the brakes? The SRT does have different calibers than the NA.
Are the seals pulling the pistons back further than needed causing long pedal throws?

 

Your point is taken, and is true. Thanks.

 

That would be a cause of longer pedal throws but I would think that the seal design would be similar and the push back the same. Larger pistons need more volume of oil to move the same distance than smaller pistons. The volume being the surface area of the piston times the distance.
The two pistons inside the MC must move a greater distance to move the extra oil required in the SRT brake system.
It is the pedal throw, I will use that word going forward, that this thread is about. We have learnt about what causes the vacuum and is the complete reverse of previous assumptions. Wiki has an informative article on the subject for those who want to expand on zip439’s post. Physics was not my strong point ever so I was lost pretty quickly.☹️

I will add to and modify post #1 as pertinent information is gathered from these later posts.

It is never to late to learn, you just have to keep the wheels in your head spinning or you are done and dusted. I would like to keep up with my grandfather 😉 Lorne on this forum, it is tough I can tell you as he is as sharp as a tack.

 

I think the point about the caliper pad travel is very interesting. This could well be the answer we're looking for. Is there an adjustment on the caliper? I'm not aware of any but perhaps someone knows of a way to shorten the caliper pad travel. If anyone has installed new calipers it would be informative if they experienced any shorter pedal travel.

Jim

 

I don’t think caliper travel is an issue, it’s not like they have a return spring. I believe the pedal travel difference is a result of the fluid volume difference between the two systems.

 

It definitely makes some difference but I have had mine looked at by another SRT owner and he feels mine is too low.
If it was due to the cylinders requiring more volume then the MC should have been larger.
I could calculate the extra cylinder surface area and compare them, using the published cylinder diameters in the specification sheet, but the cylinders only move a few thou so the pedal throw should be not too much more.
I wish that I had measured the free play between the MC push rod and the pedal push rod when I had the MC off for replacement. Hindsight is a great thing.

 

If your foresight was as good as you hindsight you would be a damn sight. Per my dad.

 

I must remember that one,thanks.

 

You guys could confuse a genius.The pushrod I'm talking is the one entering the mc.Natually it cant push the internals but should just touch.If the internals back off to the retainer clip that hold them in then expansion doesn't even come into play.I don't care if the mc is merc somebody makes the parts and that could be a hair different.Did the mc get bench blead before install?Has the fluid been pushed backwards a lot while changing pads,thats a no-no.Everything is designed buy smart people,second guessing will drive us nuts,in my case its a short drive.Pops

 

In the post where I mention expansion, I use the words "is or was". In the post #14 I mention the fact that I should have measured the gap but failed to do so. Some systems have an adjustment to set the gap, our cars do not have it. Your comments on various lengths are understood but having no adjustment in the system would call for tighter tolerances on the parts used.
The brake pedal was noticed to have a large throw before I changed the MC, the new MC was bled using dummy tubing as normally practised, no difference in the throw was found when it was all put together and bled. The pedal is not spongy but solid and the car brakes well, it is the pedal throw that is too much.

 

Well shut my mouth,no adjustment,who'd a thunk it.All my blabbing just went out the window,sorry.

 

Mea culpa! Got it bassackwards again! Thanks for the refresher!


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