Spongy / Soft Brake Pedal after system overhaul!

[SuagrElf Creations]

Hello Everyone,

I am having a soft brake pedal that I can't seem to get to the bottom of.

2012 Mustang GT with Track Pack (4 pot brembos in the front).

I swapped out the brake lines with SS lines, put rebuilt rear calipers in the rear and bled the system. The pedal was really soft. I tried rebleeding the system a couple more times. Still had the same issue. I drove the car for a bit to see if things would settle in, No change.

Decided to replace the rear calipers with new ones. Did that and again no change.

I thought maybe the master cylinder was going bad. I replaced the master cylinder with a new one. Bench bled it, bled the whole system with a motive power bleeder, activated the ABS pump with forscan, and bled the system again, and activated the abs pump again, and rebled the system again. I got some brake feel back for a day, wasn't perfect, but was better.

Let the car sit for a day and the pedal is softer than it was before!

I checked for leaks everywhere.
I bled both the inner and outer bleeders on the brembos.
I bled in the following order: RR, LR, Inner RF, Outer RF, Inner LF, Outer LF.
Using Liquid Molly Dot 4 Brake Fluid.

My question is; could this be related to the brake booster at all? My thought is that a bad booster or diaphragm would cause a hard pedal, not a soft one, but IDK... Is there anything else I can check? Outside of changing the ABS pump or rebuilding the brambos, I don't know what could be wrong. Outside of replacing the front brake line with SS lines, I did't touch the front calipers and they were fine before.

 

[86GT351]

Boosters can possibly give a soft pedal. It is not a general symptom of a booster. Suggestion is to get 4 clamps and clamp off all 4 brake lines. Is the pedal still soft? You have a master cylinder issue. If not remove one at a time RR, LR, RF and then LF. See how the pedal responds removing each clamp. If you have a major pedal change on any wheel that will isolate the issue for you to that wheel.

 

[Brian Luc]

Boosters can possibly give a soft pedal. It is not a general symptom of a booster. Suggestion is to get 4 clamps and clamp off all 4 brake lines. Is the pedal still soft? You have a master cylinder issue. If not remove one at a time RR, LR, RF and then LF. See how the pedal responds removing each clamp. If you have a major pedal change on any wheel that will isolate the issue for you to that wheel.

This!

 

[SuagrElf Creations]

Okay, so I clamped all four corners. Made no difference in brake feel, so it has to be somewhere in the system.

I rebled the MC, then acutated the ABS pump and then rebled the entire system again. I didn't get any air at any point coming out of the system.

I disconnected the vacuum line to the booster. The pedal was stiff as all hell, but there was brakes near the top of the pedal and the pedal didn't sink to the floor. I drove it a couple blocks like this. I reconnected the booster and the pedal goes to the floor. I still get brakes, but the pedal is soft as hell and if I hold it with the car running it slowely sinks to the floor. The weird thing is that the pedal changes from time to time. If I drive it around it will seem okay, and then all of a sudden it starts to feel weird again.

I've pretty much isolated it to being the booster at this point, but before I go out and buy another one I am trying to understand the logic. I just don't understand how a booster could cause a pedal to sink to the floor?

 

[86GT351]

• Brake Booster Function:
  The brake booster, also known as a power brake booster, enhances the driver's braking effort by providing additional force to the brake system. It amplifies the pedal pressure, making it easier to stop the vehicle.
  
  
• Faulty Booster Symptoms:
  When a brake booster fails or has a leak, it may not provide the necessary assistance, resulting in a harder-to-push brake pedal or a spongy, soft pedal.
  
  
• Other Potential Causes:
  Besides a faulty booster, a soft pedal can also be caused by:
  • Air in the brake lines
    
    
  • Low brake fluid
    
    
  • Leaking or damaged brake lines
    
    
  • A worn-out master cylinder
    
    
  • Issues with the ABS system
    
  • Damaged brake calipers
    
    
  • Misalignment in the rear brake shoes
    
    
  • Booster pin gap issues
• Diagnosis:
  To pinpoint the cause of a soft pedal, a mechanic would typically check for the above-mentioned issues, including inspecting the booster for leaks or signs of failure.

Have you verified the ABS is 100% functional? No codes?

 

[SuagrElf Creations]

There are no codes, and the ABS works as it should. I am going to swap in another MC and see if that fixes the issue. I think I got a dud.

 

[86GT351]

There are no codes, and the ABS works as it should. I am going to swap in another MC and see if that fixes the issue. I think I got a dud.

It is possible that you have an internal ABS issue that even though you think it is functioning properly, it is not.

 

[SuagrElf Creations]

It is possible that you have an internal ABS issue that even though you think it is functioning properly, it is not.

Could you elaborate on what the issue might be and how I might isolate whether it could be the ABS pump?

 

[86GT351]

Could you elaborate on what the issue might be and how I might isolate whether it could be the ABS pump?

What year is the car?

 

[SuagrElf Creations]

What year is the car?

2012 Mustang GT, Track Pack (4 pot brembos in the front).

 

[86GT351]

This is directly from Fords Workshop manual. You stated you think there is no air in the system and the Master is good. What is the last suggestion? HCU which is the ABS Hydraulic control unit.





Message me an email and I can email you the testing procedures.

 

[SuagrElf Creations]

This is directly from Fords Workshop manual. You stated you think there is no air in the system and the Master is good. What is the last suggestion? HCU which is the ABS Hydraulic control unit.



View attachment 105789

Message me an email and I can email you the testing procedures.

Can't figure out how to send you a message directly.

 

[86GT351]

On the top right of the forum click on the envelope. When it opens you will see something that says start conversation.

 

[SuagrElf Creations]

I'm a new member so it's not giving me that option

 

[86GT351]

I'm a new member so it's not giving me that option

[email protected]

 

[Juice]

Inconsistent pedal, likely due to the master cylinder bypassing internally. I had a company car do this. Got progressively worse with the pedal sinking to the floor.

With the engine at idle, car stopped, Stab the brake as hard as you can and hold, Does the pedal still sink to the floor?