Brakes went soft...

[defblade]

... just once.

A little drive off to the local country park, the brake pedal went very soft as I got there. No hints before hand (on that drive - there has been the occasional noise when using from time to time previously, usually shortly after starting, which I have taken as something being a little bit seized - I only use the car a couple of times a week) and it came back afterwards - firmer, if anything.

Definitely not ABS - no pulsing.

No obvious leaks, no super hot discs, plenty of brake fluid, all discs/pads appear in OK condition looking through the wheels. Previously, recurring trouble with drivers sticking on - it's had a new caliper and flexi hose and no trouble for last year or two since.

However, now paranoid about it - anyone got any ideas what's going on?

[SteellFJ]

defblade said:

... just once.

A little drive off to the local country park, the brake pedal went very soft as I got there. No hints before hand (on that drive - there has been the occasional noise when using from time to time previously, usually shortly after starting, which I have taken as something being a little bit seized - I only use the car a couple of times a week) and it came back afterwards - firmer, if anything.

Definitely not ABS - no pulsing.

No obvious leaks, no super hot discs, plenty of brake fluid, all discs/pads appear in OK condition looking through the wheels. Previously, recurring trouble with drivers sticking on - it's had a new caliper and flexi hose and no trouble for last year or two since.

However, now paranoid about it - anyone got any ideas what's going on?

Air in the system?? worth having them bled?

[aka_kerrly]

SteellFJ said:

Air in the system?? worth having them bled?

Do the brakes only feel soft when you apply reasonably amount of pressure but feel fine on light braking? The above about air in the system is true, old brake fluid is also poor performing to so a bleed/fresh fluid may help but I'd have hoped that was done when you had the caliper replaced.

Another possibility is the seals in the master cylinder or servo are failing. Also worth checking the vacuum system that the servo requires for leaks/damage.

[backwoodsman]

You say the pads look ok through the wheels.

Have you checked the pads at the rear of the discs?

[996TT02]

Could be master cylinder seals failing, won't leak, the oil just gets past the first seal.

[slipstream 1985]

bmw by any chance? iv had this on my z3. just once in a blue moon it does it but its very unnerving.

[HD Adam]

How old is the brake fluid?

If there is water in the system, it will boil before the brake fluid does thus becoming compressible.

Flush the entire system with new fluid and see how that goes if there isn't anything else obvious.

[With these feet]

All discs or drums rear? If you have rear drums the pedal can go soft if they need manual adjustment.
Test the servo with engine off, pump brake pedal and hold pressure. If the pedal doesnt go hard and hold then theres an issue with the hydraulic side - either caliper/wheels cylinder/master cylinder.
Start the engine and the pedal will drop a bit feeling less firm but should find a point where it stops. If the pedal stays firm then theres no servo assistance.
Check for bubbles in the master cylinder res. which shows failing seals in the cylinder.

[Rich_W]

As mentioned above. Almost certainly the master cylinder. Get it changed asap by someone you can trust.

[Boobonman]

Under what circumstances? Modern ABS systems don't pulse as noticeably as ones of old. Reason I ask is I have noticed my VW transporter doing a strange trick but only under particular circumstances.
Single lane road with passing places, I've noticed that often if I'm braking as I pull into one of the passing spots the braking causes the NSR wheel to drag on the loose surface, causing the pedal to go soft and the vehicle to brake all the other wheels. The car obviously thinks it is keeping me out of the bush, feels very weird though...

[defblade]

Pretty sure it's not air as it was just the once; came back immediately.

Certain it wasn't boiled fluid either as I'd not exceeded 30mph and the journey was about 3 miles... I'm not that hard on the brakes! Also, wheels/discs not hot.

Discs all round (with internal drum on the rear for the handbrake).

Yes, it is the BMW (318 touring, e46, '01). ABS is definitely old school feel-able (is that a word?!) as I have tested it after having the pump replaced 2 or 3 years ago

Servo is working OK; no sign of bubbles.

I've not looked at the back of discs as no time so far to jack up and take the wheels off; but they are all single pot sliding calipers with the pot to the inside of the disc, so if the outside pad is being pulled and released from the disc face OK, the inside should be as well - that pad should still be moving back and forth with the pot even if the sliders were seized.

Slipstream's comment sums it up best - just the once, very unnerving.

[Pete Eroleum]

After a drive, feel the centre of each wheel in turn. If any feel very hot, then you may have a binding
sub-caliper that could boil the fluid.

Some garages don't bother cleaning up the sub-caliper before fitting new pads. I've even seen people hammer
the new pads in rather than clean the rust off, to save time. Worth a quick check anyway.

[Chicane-UK]

Could be the start of a master or slave cylinder going.. just keep an eye on it I guess.

[Lowtimer]

I would suspect the master cylinder as next port of call, following the checks you have already done.

[McWigglebum4th]

highly unlikely here BUT

I get this with the landrover

A lose wheel bearing batters the pads away from the disc and the first push of the pedal is long

[wiliferus]

I had an old Ovlov V40 that did this. Got progressively more often until the car was unusable. Turned out to be the servo unit, that wrote the car off as it was my shed.