Brakes got soft

[JavaMan]

I was driving today, someone cut me off and I stomped the brakes. I noticed afterward that my brake pedal is soft. Still stops ok. If I hold down on the brake when stopped it makes a hissing noise.


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[Vanishing Point]

Maybe first check underneath the dash on drivers side brake pedal (push rod) for leaks (wetness) and hissing noise. Then continue checking outside in engine compartment for wetness from underneath the power booster then master cylinder (fluid level) to all brake lines connections from calipers (Disc) and wheel cylinders (Drum) on back of all four wheels for leakage (wetness) of any kind coming from those areas.Then again while having someone gently but firmly stepping on the brake pedal and holding it down with the parking brake released and wheels blocked. If still no noise (hissing) or leakage present try removing one wheel at a time to physically look at their connections for leakage (wetness) inside on your Disc and Drum brake setup. Good Luck!

 

[PlayingWithTBI]

Check your vacuum booster, check valve, and hose going to it.

 

[JavaMan]

Guess I should have mentioned, it's a 96 Suburban K2500, so it's got hydro boost.
Don't see any leaks, brake fluid level is still in the normal area.

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[Pinger]

I have the same as you (1999 C2500 Suburban).
I had the calipers off working on wheel bearings, pushed the pistons in and pumped them out with the pedal a couple of times to make sure they were free but didn't break into the hydraulics at any point.
Put it all back together, pumped the pedal to bring the pads into contact with the disc (rotor) at which point I should have had a solid pedal but it is soft and mushy and the pedal is slow to return.
Went for a drive and gave the brakes a work-out but still the same. It stops perfectly but the 'long' pedal isn't right.
Feels like they need bleeding but how could air have got in?
The other possibility (in my mind) is that using more pedal travel pumping the pistons the master cylinder piston has encountered a usually unused portion of the cylinder and has damaged the seal (this is something known to happen sometimes during pedal bleeding).
No external leaks - does a leaking master cylinder just leak back to the reservoir?
But, it still seems to stop too well for it be master cylinder related (I want to check this further) and, OP, in your sudden stop - do you think you could have pushed the pedal far enough beyond normal use to have done as I described?

 

[JavaMan]

I suppose its possible, and could explain why its happening.
So does that mean it's just a minor worry, not a stop driving and get it fixed thing?

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[Pinger]

If you're referring to the master cylinder - I've stopped using mine until I'm certain it isn't.
Spoke to a guy today he knows this kind of stuff and we both reckon if the master cyl was bad it wouldn't hold pressure - pedal to the floor. Mine stops short of the floor and by the sounds yours does too. Mine brakes, pulls up fine - just the pedal travel is far too much - and soft/mushy.

Soft/mushy pedal is a classic symptom of needing bled so I'm going to try that and see what gives. If yours is as bad as mine, think carefully about using before getting it fixed.

 

[Pinger]

The more I'm thinking about this, other possibilities exist.

I was driving today, someone cut me off and I stomped the brakes. I noticed afterward that my brake pedal is soft. Still stops ok.

Maybe, with higher than usual pressure you have moved something seized - eg wheel cyl - and you now have extra volume. I'm thinking I have the same but in the flexible hoses - in my case because the were twisted around while I was working on the calipers.
Flexibles can also collapse internally. Then, they usually trap the fluid on the brake side leading to a binding brake.

If I hold down on the brake when stopped it makes a hissing noise.

The same hissing the steering makes on full lock?

 

[JavaMan]

Yes, like the steering full lock noise

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[Pinger]

I think we have different problems. Mine, I think though need to check further, is due to piston seals pulling the pistons back too far. Mine doesn't make the hissing noise.

The hissing noise - isn't that because you have reached full travel - as when it happens with full steering lock?

On another car way back, I had the pedal go long because of a heavy stop. The adjuster in a rear drum had broken. Maybe you have the same.