Brakes spongy when engine running yet fine with engine off..

[kim0663]

Had same problem in my 101, bleed the hell out of it, did nothing. Turns out my pads where that low

Replaced pads, Mint again

how does that work?... Unless the pads themselves had become spongy

[allencr]

how does that work?...

The pads have a taper & twist/flex the caliper.
There is nothing in the system that can be very 'spongy' except air & hoses that feel & very visibly swell up, everything else is pretty much solidp. Any excess pedal movement beyond squeezing the seal a bit, is there for you to see & feel if you have some one pump the pedal.
Fluid, whether it is old or water contaminated doesn't compress.

[iOnic]

As the pads wear out the caliper piston[s] slide further out of the bore so that the friction face of the pad is always the same distance from the disc regardless of wear (unless the pads are completely worn and the piston has reached max travel or the discs are below min thickness) - so it's quite hard for low pads to have any effect on the pedal feel....That's why Kim asked how that would work

As for old/water saturated brake fluid - Water in the fluid can boil (since water boils at a fraction of the temperature experienced in the braking system) and when water boils it turns into a gas....and gases can be compressed - which is how one can get a spongy pedal from old/contaminated fluid.

with the car off the pedal is perfect but as soon as you turn the car on the pedal turns to mush

Does it build up pressure if you pump it? Sounds like there is an internal fluid leak past the piston seal in the master cylinder. Did it have no fluid/sit around for a while? That can cause the seal to perish and give problems like that.

[pc]

As the pads wear out the caliper piston[s] slide further out of the bore so that the friction face of the pad is always the same distance from the disc regardless of wear (unless the pads are completely worn and the piston has reached max travel or the discs are below min thickness) - so it's quite hard for low pads to have any effect on the pedal feel....That's why Kim asked how that would work

Dunno about your pads, but mine wear on an angle. So once the pads are worn down a bit (on an angle), then when you press the brake pedal one end of the pad touches the disk and you have to push the brake pedal further to get the whole pad touching the disk... hence spongy pedal with worn pads.
I don't think this is the problem here though.

[Girvs]

I get the same thing in m supra, brakes go rock hard with the enigne off, with the engine on they feel spongey. Though they felt less spongey after swapping the brake lines for braided.

I think it's normal.

Had the same issue Snaps. Changed fluid and so forth car drove fine. Next day driving it upto SH2 in Welly and stopping for the lights before entering and brakes went almost to the floor
Re-bled the brakes and it seemed to come fine. Though like you I'll be moving to upgrade my lines in the not too distant future (esp given the last warrant said that the fronts were needing replacement soon).

[Dell'Orto]

with the car off the pedal is perfect but as soon as you turn the car on the pedal turns to mush

Does it build up pressure if you pump it? Sounds like there is an internal fluid leak past the piston seal in the master cylinder. Did it have no fluid/sit around for a while? That can cause the seal to perish and give problems like that.

Yeah it does, $&#$% thing. Had no brake fluid in it for a couple of days, combined with the high k's its done it probably did crap out. *sigh*

[iOnic]

Dunno about your pads, but mine wear on an angle.

My pads don't wear on an angle...In fact I rarely come across pads that wear unevenly and everytime I have there was a problem causing it (I do about 6-8 brake pad changes a week). If your pads are wearing on an angle you'll probably find that you have other problems (eg calipers flexing under braking, uneven piston pressure etc). When you buy brand new pads and rotors, both have a flat surface - if one of them starts to wear on an angle it should indicate that something isn't right.

[sergei]

The most common cause of the pads wearing on angle is the seized calliper sliders. Pull the sliders out, give it a good clean (bores where the sliders slide as well), put some copper anti-seize grease, and make sure the rubber boots are intact.

The best approach is get a calliper seal kit from toyota (they are ~$50 or so) and replace all the seals and check out piston/bore as well.

[B_giB]

Had same problem in my 101, bleed the hell out of it, did nothing. Turns out my pads where that low

Replaced pads, Mint again

how does that work?... Unless the pads themselves had become spongy

They had no pad material left, down to the metal. I changed them, problem solved

[GT101]

did you bleed front and rear brakes? or just the front?

[pc]

Dunno about your pads, but mine wear on an angle.

My pads don't wear on an angle...In fact I rarely come across pads that wear unevenly and everytime I have there was a problem causing it (I do about 6-8 brake pad changes a week). If your pads are wearing on an angle you'll probably find that you have other problems (eg calipers flexing under braking, uneven piston pressure etc). When you buy brand new pads and rotors, both have a flat surface - if one of them starts to wear on an angle it should indicate that something isn't right.

I don't normally have a problem with it, but the race car does it everytime when going to the track even after greasing the sliders etc. I don't think the stock single pot calipers were designed for that type of use, but i've seen it happen on road cars as well.

[matt dunn]

I don't normally have a problem with it, but the race car does it everytime when going to the track even after greasing the sliders etc. I don't think the stock single pot calipers were designed for that type of use, but i've seen it happen on road cars as well.

My race car single pot calipers on the rear used to always wear on an angle too,
even though I made sure all the sliders were ok.

[Dell'Orto]

Ok - rekitted the master - still does it. Now what??!?

[allencr]

Ok - rekitted the master - still does it. Now what??!?

It depends on what the definition of 'it' is.
Mushy pedal? You will have to figure out whether there is air in the system, a vigorous push/kick of the pedal will dislodge/move it out during bleeding, or something that is mushy/twisting/flexing/compressing/expanding/compliant somewhere between the sole of your semi-mushy foot, all the way through the system to the incompressible brake rotor/disk.

[matt dunn]

If the pedal is fine with no vacuum assist,

and then with the vacuum assist there is a problem,

would that not mean the problem is in the booster?

I also had a honda that I had thru wwork that I thought was the same,
good pedal with no vacuum, but pedal was soft as with the car running.
Enough that I thought it was a problem.

Took it to the brake specalist we deal with,
he jumped in it, felt the pedal and said 'nah that's normal'.
Think about how hard you are pressing the pedal to male it feel soft,
and then think about if you press it that hard while driving it is past thye point of lockup?

You may be chasing a problem that does not exist,
only thing with that is that it failed a wof?