Odd spongy brake problem

[aitch]

Hey everybody,

My wife picked up a used 2011 Fiesta SE recently and after driving it around for a month or so with no problems, suddenly the brakes became spongy. They would stop the car, but had to press the pedal almost to the floor. I've bled fluid completely through the system (3 times, though I know that was unnecessary...) and no longer believe it's an issue with air being in the system. My next thought is the master cylinder, but the confusing thing to me is 1) there is NO noticeable fluid loss. Brake res. is always topped off at the max line, no fluid leak noticeable on any brake lines, calipers, etc. the cylinders compress fine, and the caliper slides are not seized. Brake booster is working properly, and hose has no cracks/leaks. Furthermore, when I bled the brakes those 3 times, there seemed to be plenty of pressure being generated by pressing the pedal (fluid would spew out quite far, to put it into simple terms...). So, my thinking is that a failing master cylinder would block, or at least lessen the fluid/pressure to some or all calipers. Am I wrong in thinking that? There are no codes showing up on this car, pads and rear shoes have plenty of thickness left on them. What else could it be?

Thanks in advance.

 

[LionsTooth]

Sounds like it only happens when driving. If you have consistent pressure when bleeding/motor off/no booster helping then the only difference is the booster kicking in to help, right? When you bleed them or after (without the motor running) can you stand on the brakes and not feel any bleed down? If that is consistent, then that points to the booster to me.

 

[Handy Andy]

This is a 2011, so age may have added a complexity to corrosion and rust hindering the ability to apply brakes and the rear ones - if drum - may be out of adjustment.

How are the rear brakes - does the E-brake (the Emergency or Parking Brake) - try using them in TANDEM - apply brakes and then lift handle to assist? (this can be done - it's done is some races for steering control and less ABS and skid control engagement for handling a "slide" in a curve - so this may not be for the timid or a cup of tea to dress up your car driving for sport - but the issue is to find if the rear brake adjuster is "adjusting" or hung up - this is an 2011 - so if the adjuster is binding, the pedal has to travel farther and (perhaps) one of the rear wheel cylinders' finally meets the shoe to drum, that pedal has to travel that far - it's supposed to do this adjustment automatically - so have someone pull the rear brakes and examine the hub, and rear brake adjuster and check for fitment to the drum.

One way to check is bring the car up to speed on a straight non-curvy road - and simply put the car in neutral to coast - and apply the E-brake - if things are fine - the brakes will work equally and if not - you'll feel a pull in one direction - the direction it WANTS to pull you is OPPOSITE of the wheels Manual Brake lever that is not working, needs adjustment.

A simple way to check "clearance" is to chock the front wheels - jack up the back end - and do it for both rear wheels, lift up by putting the floor jack in the middle of the crossmember between the rear wheels and jack it up so it is even and lifted equally on both sides so the wheel / tire can spin freely.

Don't need to remove the tire (yet) just spin the wheel by hand and feel the force needed to spin the tire.

It should be pretty easy...

Now, start clicking the E-Brake one click, at a time - then spin the tires on BOTH sides - do this one click and spin method repeatedly until you find out the tires are not spinning so freely - and note which one stops sooner than the other.

With the rear wheels - the bad adjuster will show up as a sloppy braking - requiring A LOT of force or clicks to stop that wheel from spinning - meaning the adjuster on that side needs help. So if 5 or 6 slicks and one wheel stops, 5 to 6 clicks is about the ideal "pull" of the handle, but if the other wheel still turns - the "other one" that still turns is the one needing the most adjustment or service.

Because of extra travel of the cylinder - will force more fluid from the master into the lines to make that cylinder "catch up" to the others - so it feels spongy - because one side is not "engaging" at the same time the others are.

It's a 2011 - it's 2022 - 11 years and if not service history, lots of times the shoes get "Frozen" in one place and they develop a wear pattern of "a lot of dust" in one spot of the shoe with a thin spot of brake material left on the other side of the shoes - so that means they froze in place and only pivot to a point because the backing plate they rest on bears the weight and force of the braking and the hub then "bends" or twists to accommodate the brakes instead of the brake shoes floating and mating to the drum every time you apply the brakes.

So this may indicate also a worn hub caused by worn brakes forcing the bearing to bend to the will of the wheel cylinder.