Ever since I have had the SRT I felt that the brake pedal feel was worse than my old NA, if both cars use the same master cylinder I know why.
I had thought that a larger master cylinder bore would improve the brake pedal feel and possibly improve braking, this is patently not true I find and would lead to worse braking due to the extra force required at the brake pedal. See the explanation below.
The pedal length from the pivot and the master cylinder push rod distance from the pivot are designed to give the best overall results.
Soft pedal can be caused by air in the brake system or water in the brake fluid turning to steam when the brakes get hot.
Excessive pedal travel can be caused by too much play in a wheel bearing, sticky calipers and too much free play in the pushrod that actuates the master cylinder pistons. Our free play is not adjustable like many other cars brake pedal. Some free play is necessary to allow the expansion of the brake fluid as it warms up otherwise the fluid expansion would start to move the caliper pistons.



In this diagram if the master cylinder had a bore that was one square inch then there would be 6lb/sq.in on the caliper pistons.
But if the master cylinder was increased in bore size to two square inches then there would be only 3lb/sq.in on the caliper pistons and require twice the pressure on the brake pedal.
So increasing the MC size has an adverse effect on braking.
During braking the seals in the caliper piston grooves distort toward the rotor, when the pressure is released the distorted seal returns to its normal shape and pulls the pad away from the rotor by a small amount. The area of the seal that is touching the bore has a resistance to moving, so this area stays where it is and the seal in the groove moves distorting the seals shape, when the fluid pressure is removed the seal returns to its original shape and forces the piston away from the rotor.
If the piston is sticky in its bore the pad rubs against the rotor causing premature wear.
To be continued.

 

I'll trade you my NA front and rear calipers and rotors for your SRT6 ones, then maybe you'll get your pedal back. Fair enough ?

 

I am not sure I agree with this statement:

<<Some free play is necessary to allow the expansion of the brake fluid as it warms up otherwise the fluid expansion would start to move the caliper pistons.>>

 

[SIZE="2"]So you're hypothesis is that the brake fluid within the brake lines expands enough when heated that "free play" back to the pedal exists to prevent brake caliper closure & excessive pad/rotor wear due to dragging. Like James, I'm not sure I agree with this hypothesis as any thermal expansion of brake fluid from increased heat should be handled within the MC reservoir which has room for fluid expansion when filled to proper level. Is the brake system a closed system? In theory it is, but if there is a weak link it would be the MC cap which may or may not be an absolute seal. Are there check valves to prevent back flow to the master cylinder? Probably - To keep the brake lines full & minimize pedal travel when braking is required. There must be a pressure relief valve in the system to allow the pressure in the brake lines to reduce to a "steady-state" condition once the pedal is released, allowing the calipers to retract and excess brake fluid flows back to MC reservoir. And if this is the design, I can't see any force being applied to the MC actuator due to fluid thermal expansion. Gotta look at the brake system PFD to confirm. However, the viscosity should be impacted and that could contribute to a need for increased pedal travel to develop braking force pressure. Gotta think about that some more.

But I do agree with your second hypothesis. Increasing the size of the MC would have an adverse impact on required pedal travel to generate sufficient braking force.

Just thinking out loud.

 

There are one way valves in the MC, you prime the MC when you fit a new one and if the valves were not there the fluid would simply drain out of th MC making the priming a waste of time. Fluid gets past these valves to make up losses but cannot get back into the MC or the reservoir.

 

Have you confirmed the srt pistons and na pistons in the calipers are a similar capacity?

That is the other side of the equation to your force diagram. Woody

 

They are not the same, the SRT pistons have a total area greater than the NA. That means if the MC is the same on both the pedal travel is greater on the SRT.
If the pedal linkage is the same as well then we ar more or less on an even footing.
The psi on a piston does not change because there is a greater piston area, the piston just needs more oil to move it the same amount.

 

DUH!!! I should have known that!!!

 

My brakes stock are great, when at Summit point doing fast friday , my instructor kept telling me to brake early????????? I said why I got lots more braking and were going into the corners way slower than this car can handle. SRT bigger calipers makes for more braking effort and it seems more travel on the pedal. Enjoy, Woody

 

Then explain how the piston will release after brake application... or how do you push in the piston to change the pads?

 

I use a c clamp and old brake pad to compress the pucks. WW

 

The one way valves are on sensor lines not on the brake lines themselves. the brake lines are open otherwise we would not be able to bleed them or change 100% of the fluid. The one way valves are muddying the water.
We all know that if you compress the brake cylinders too much the fluid will overflow the reservoir.
I am still trying to understand why no free play in the brake system would cause a slow brake lock up as the fluid warmed and expanded. It is mentioned but I cannot wrap my feeble brain around it.
That ABS system can get air in it and it is cleared using the dealers computer or sometimes by getting the ABS system working by braking on a gravel road and bleeding the brakes. I do have SS brake lines and changing out the MC did not change anything.
I see nobody else has anything worthwhile to add, so no interest I guess.
People disagreed with what I said but never offered a different opinion.

 

onehundred80 - Don't know if you saw my post on "soft brake" or not, but I'm soliciting you input. I have a 2005 Limited Coupe that has about 74K miles on it. Two months ago I had new brake pads and rotors put on. The pedal seemed just as low as it did when it needed brakes and very soft pedal. I brought it home from the shop and it has been garaged since then. I got it out a couple of days ago and the ABS light was on in the dash and the brake pedal went almost to the floor. I switched the ABS switch several times, the light went out and the brake pedal came back to about half pedal. If I pump the brake 2 or 3 times it comes up good and is nice and firm, as I think it should be. I took it back to the shop and they re-bled the brakes and checked for adjustment, so they told me. There is no change in the pedal feel or how far up the pedal comes. I don't think a brake pedal should feel soft and "mushy". It stops but I have no confidence in the brake system because of the way it feels. Every car I've driven has firm brake pedal and little travel after applying the brake. Shouldn't the Crossfire be the same?

I'm wondering if I have a master cylinder problem or perhaps a caliper problem. There is NO sign of brake fluid anywhere. Can't be a leak or I and the shop, would see fluid. I talked to the Chrysler dealer and they said they would replace the master cylinder for $460 plus labor. They don't know if that is the problem or not. That's an expensive guess, I think.

If you have any ideas or suggestions, I would love to hear them. I almost afraid to drive the car with the way the brakes feel. It's driveable, but, I worry about stopping in an emergency.

Thanks for your help and your expertise. If anyone else reading this has had a similar experience, I'd love to hear your opinion and what you did to isolate the problem and fix it.

Jim Holian
jpmlholian@aol.com

 

I have what I would call a soft brake pedal, the car stops fine. I would just like the pedal to be firmer. I continue to search for the source of the problem.

 

Just curious when was the last time you changed/flushed your brake fluid?

 

philoaophico - I just bought the car last September. As far as I know, the fluid has never been flushed. The shop that did the pads and rotors bled the brakes and added more fluid, but did not flush the system.

Your opinions are solicited and appreciated.

Thanks,

Jim

 

Last September..

 

Testing brake feel:

Posted in other section also:

Try this, engine OFF. pump the brake several times to bleed off the vacuum assist.

Now press the pedal to see the unassisted pedal feel. It should be solid and unyielding.

It should also not drift, go to the floor or other odd thing.

Has the brake fluid gone down?? have lid on when pumping brakes or expect a fountain to erupt.

Let us know, WDY

 

Gentlemen - Looking at the diagram, it looks like a longer push rod would increase leverage and provide a taller brake pedal and better feel. A longer push rod would not interfere with the brake light switch. Just a thought!

Jim

 

OK, did the engine OFF step and the pedal was firm and it had the amount of pedal depression I would expect it to have, no sinking pedal at all.
When I turned on the engine the pedal sank to where it normally is, pumping it did not raise the pedal by any noticeable amount. Where did that oil moved by the extra stroke go?
I think that the brake lines would have to be open at the time so new fluid can get in as the air gets pushed out. The reservoir would have to be filled as the operation continued to prevent air getting in via the reservoir. It