Brake Pedal Soft, Poor Pedal Response, Suggestions Needed, Deep Into the Problem Already....

[slyez]

Today/Tomorrow I plan to plug the 4 ABS output ports and see if the issue is wheel-ward or engine bay, then, depending on result, I will plug MC output ports.
Could any of this be booster related? One other piece of info, when I log the brake pressure it goes up with pedal travel, then when the pedal firms slightly the pressure slowly climbs as the pedal slowly sinks. As far as I can tell it then will stabilize at the highest pressure with the pedal at the floor, but I will need to re-check to confirm that, if its important.

 

[slyez]

I just found that we have a brake booster vacuum pump? Could this be an area of concern? It is driven by the cam and under the very simple looking flat block off plate on drivers side.

 

[D1JL]

I have been through several different types. Started with Maxima Racing DOT4, Then EBC 307+ DOT4, then Bosch 5.1, then EBC again. Not using silicon or anything incompatible from what I can tell. The Maxima and EBC are higher viscocity than original fluid, but otherwise compatible. Also, not change in pedal from fluid to fluid.

The brake fluid could be part of your problem.
I would suggest that you first flush your complete system with DENATURED ALCOHOL.
Then use ONLY the specified brake fluid DOT 4 LV High Performance Motor Vehicle Brake Fluid / PM-20 (WSS-M6C65-A2) (ISO 4925 Class 6).

I personally have had similar issues as you when DOT 5 was used.

 

[slyez]

I just checked the brake booste

The brake fluid could be part of your problem.
I would suggest that you first flush your complete system with DENATURED ALCOHOL.
Then use ONLY the specified brake fluid DOT 4 LV High Performance Motor Vehicle Brake Fluid / PM-20 (WSS-M6C65-A2) (ISO 4925 Class 6).

I personally have had similar issues as you when DOT 5 was used.

Thats a good point, but No DOT 5, just DOT5.1. DOT5 is the silicon stuff. DOT 5.1 is same chemistry as DOT4 the only difference being the viscocity? That is my understanding?
But I will flush it and fill with Pentosin DOT 4 LV to eliminate it as a variable.

 

[D1JL]

Flush with DENATURED ALCOHOL as it is very inexpensive and will get rid of all contaminants.

 

[slyez]

Another data point:
Tested the vacuum pump with a vacuum gauge, and it seems to be reading outside Ford spec of 15-22 inHg, I am getting around 27inHg. Now admittedly, the gauge could be off since it is just the gauge on a cheap vacuum bleeder, but I dont imagine it is off by 20+%.?

 

[slyez]

So retested the brake line pressure and it doesn't bleed away. It does do the strange increase worth the brake booster vacuum connected. Tested with and without vacuum assist. The"knee" in the graphs send to be lower the faster i less the pedal. . Maybe that is the vacuum in the servo catching up with the displaced area from the diaphragm movement. Peak pressure 1650psi assisted, 450 unassisted and plateaus. The sinking pedal feeling seems to match the slope after the knee, and stops moving once peak pressure is reached.

 

[slyez]

So..... After a bunch of different tests using plug on the MC and on the ABS Module output, I still dont feel satisfied in the answer.... The pedal now feels better, but initially there were only inconclusive results. I re-bench bled the MC, then rebled the system and there was maybe the slightest improvement, but still long travel and soft with engine running. I then changed all of the brake lines, and rebled everything and the pedal now feels acceptably back to normal. It still feels like it is maybe too soft when engine is running, but super good with engine off. The bite point has returned to high in the travel, which was the biggest issue. For now I will drive around the light pedal resistance, but I am going to look further into the vacuum pump reaching a higher than specified vacuum. Otherwise, thanks for all of the help, and I will maybe say the brake lines were the main culprit....? Not exactly sure how since they were newish and felt fine at first. And they had no visible leaks or loss of fluid....

 

[TyphoonFiST]

So..... After a bunch of different tests using plug on the MC and on the ABS Module output, I still dont feel satisfied in the answer.... The pedal now feels better, but initially there were only inconclusive results. I re-bench bled the MC, then rebled the system and there was maybe the slightest improvement, but still long travel and soft with engine running. I then changed all of the brake lines, and rebled everything and the pedal now feels acceptably back to normal. It still feels like it is maybe too soft when engine is running, but super good with engine off. The bite point has returned to high in the travel, which was the biggest issue. For now I will drive around the light pedal resistance, but I am going to look further into the vacuum pump reaching a higher than specified vacuum. Otherwise, thanks for all of the help, and I will maybe say the brake lines were the main culprit....? Not exactly sure how since they were newish and felt fine at first. And they had no visible leaks or loss of fluid....

Brake vacuum booster?

Sent from my SM-S908U1 using Tapatalk

 

[slyez]

Brake vacuum booster?

Sent from my SM-S908U1 using Tapatalk

yes, I have thought that, but it seems to operate properly and passes any test I can think of. I thought maybe it wasnt equalizing the pressures on the sides of the diaphragm properly, but it check out good. I may consider just putting a vacuum regulator in the line from pump to booster. Not sure if this is smart or not though....

 

[M-Sport fan]

I just checked the brake booste


Thats a good point, but No DOT 5, just DOT5.1. DOT5 is the silicon stuff. DOT 5.1 is same chemistry as DOT4 the only difference being the viscocity? That is my understanding?
But I will flush it and fill with Pentosin DOT 4 LV to eliminate it as a variable.

The Bosch DOT 4 LV stuff has the highest wet and dry boiling points of any of the LV fluids, including the also excellent Ate SL.6 stuff, IF you are worried about that because; track car and all.

 

[sonicmn]

The Bosch DOT 4 LV stuff has the highest wet and dry boiling points of any of the LV fluids, including the also excellent Ate SL.6 stuff, IF you are worried about that because; track car and all.

I figured I could just throw my experience in the ring, I have done a number of HPDE track days with my fiesta and tried a couple of different types of pads/fluids.

Until last year I didn't realize there was an actual difference between the regular DOT 4 and DOT 4 LV, after my first track day I started using Prestone REGULAR DOT 4 fluid bled with a Motive pressure bleeder before each HPDE day. After the first bleed my brake pedal always felt a little bit spongy, I could never get it to bleed up and feel like it did from the factory, the engagement point of the pads actually felt earlier in the pedal travel and I would begin slowing down before the cruise control would disengage hence something felt wrong. I found that the OE fluid held up better at the track than the brand new (bled less than one week before a track event).

Well last year I started digging into the actual types of brake fluids (lo and behold there is a real difference! ya learn something new every day). Well last year before a track day I tired bleeding with Motul RBF 600, this was not labeled as LV on the front label on the bottle but I figured it was temp resistance that was causing the issues and I would give it a try

After bleeding the fluid, the brakes immediately felt less spongey, during the track day we I noticed that they felt much better they faded less by the end of a 15min session and generally held up much better (most likely due to the higher boiling point fluid). By the end of the day (4th session) I noticed some fade, this could be as I was driving faster and braking later than the morning. The entire setup sure felt much more solid than the Prestone DOT 4 fluid.

All of this was done on the factory brake hardware (I have used EBC Yellow, Blue, and factory high temp (S) pads)

This is not scientific in any way, just my observations and testing. I guess what I'm getting at is they type of brake fluid seems to make a real difference, and if your not using a LV DOT 4 fluid I would recommend giving that a try before getting out the parts cannon and spending a bunch of money. Good luck on the hunt! It can be frustrating until you get to the bottom of it.

 

[slyez]

I figured I could just throw my experience in the ring, I have done a number of HPDE track days with my fiesta and tried a couple of different types of pads/fluids.

Until last year I didn't realize there was an actual difference between the regular DOT 4 and DOT 4 LV, after my first track day I started using Prestone REGULAR DOT 4 fluid bled with a Motive pressure bleeder before each HPDE day. After the first bleed my brake pedal always felt a little bit spongy, I could never get it to bleed up and feel like it did from the factory, the engagement point of the pads actually felt earlier in the pedal travel and I would begin slowing down before the cruise control would disengage hence something felt wrong. I found that the OE fluid held up better at the track than the brand new (bled less than one week before a track event).

Well last year I started digging into the actual types of brake fluids (lo and behold there is a real difference! ya learn something new every day). Well last year before a track day I tired bleeding with Motul RBF 600, this was not labeled as LV on the front label on the bottle but I figured it was temp resistance that was causing the issues and I would give it a try

After bleeding the fluid, the brakes immediately felt less spongey, during the track day we I noticed that they felt much better they faded less by the end of a 15min session and generally held up much better (most likely due to the higher boiling point fluid). By the end of the day (4th session) I noticed some fade, this could be as I was driving faster and braking later than the morning. The entire setup sure felt much more solid than the Prestone DOT 4 fluid.

All of this was done on the factory brake hardware (I have used EBC Yellow, Blue, and factory high temp (S) pads)

This is not scientific in any way, just my observations and testing. I guess what I'm getting at is they type of brake fluid seems to make a real difference, and if your not using a LV DOT 4 fluid I would recommend giving that a try before getting out the parts cannon and spending a bunch of money. Good luck on the hunt! It can be frustrating until you get to the bottom of it.

Sorry to ask, but are you saying that the Motul non-LV fluid was better or the factory LV fluid? The viscocity of the 2 is quite different. DOT 4 LV requires less than I think 700mm^2/s vs the Motul is rated at 1,750mm^2/s at -40C.....
Or maybe I missed the point, and what you're saying is that some fluids just work better, and if thats the case I agree.

 

[sonicmn]

Sorry to ask, but are you saying that the Motul non-LV fluid was better or the factory LV fluid? The viscocity of the 2 is quite different. DOT 4 LV requires less than I think 700mm^2/s vs the Motul is rated at 1,750mm^2/s at -40C.....
Or maybe I missed the point, and what you're saying is that some fluids just work better, and if thats the case I agree.

I was saying that some fluids work better than others (like Prestone DOT 4 did not work well at all), I haven't completely gotten to the bottom of things yet, that was at the end of last year and I have not been out to an event yet this summer.

However, I have been quite pleased with the Motul (Non LV) as that's still what's in my car and working well.

I guess it was more of a story of what NOT to use than an outright recommendation for Motul RBF600, just in case you had not gotten to that point yet.

Just for my own knowledge, where are you looking up the viscosity of the fluids at? That seems like a useful thing to know where to find.
Any chance they have viscosity measurements at any higher temperatures than -40C/F? That's useful in the winter in MN but a bit out of scope for track use (or AZ weather) and may not tell the full story of the fluid comparison at operating temp.

 

[M-Sport fan]

My guess would be that if you have decent venting to the rotors/calipers, a large heat dissipating BBK, and of course depending on the track configuration itself, one could use one of the (lower dry boiling point than the 'racing' fluids) DOT 4 LV fluids, and not experience any fade/boiling if it is totally flushed out, or at least bled, before each full open track session.

 

[M-Sport fan]

Just for my own knowledge, where are you looking up the viscosity of the fluids at? That seems like a useful thing to know where to find.

Some of the technical sites, or even the better manufacturers' sites will give specific gravity/viscosity specs for their fluids.

Then contrastingly, other sites, or manufacturer's data sheets will not even state the WET boiling points, but only the (higher, and therefore better looking) dry.

 

[slyez]

Some of the technical sites, or even the better manufacturers' sites will give specific gravity/viscosity specs for their fluids.

Then contrastingly, other sites, or manufacturer's data sheets will not even state the WET boiling points, but only the (higher, and therefore better looking) dry.

And yes, you are right that the -40C value doesnt mean much, but many of the better fluids also list the viscosity at 100C, then I think you can draw a line on a graph to get rough viscosity at any interpolated point?