Brakes go randomly soft

[Mario323]

A year ago I had the rear disks changed after the brakes had seized and worn them down. About 2-3 months back I was taking an intercity trip and had the brake pedal go soft on me as I came off a high speed stretch of road. The brakes worked. The pedal just kept traveling way further than it normally does. I pumped it (out of habit from my old classic mini) and pressure came back up and the brakes were fine for a while. This continued to happen over the weeks but very randomly. It would work fine for a few days then suddenly go soft. I tried a fluid change and bleed as these were probably overdue but was unable to do the ABS myself so eventually booked it in with a garage for a complete fluid change. Within a week of that, two different garages did a full MOT or Vehicle Safety check and no faults were found with the brakes. No warnings or error codes are showing either. I drove for a few days including a motorway run and really thought the problem had been resolved until it did it again yesterday.

I only have to pump the brake once to get the pressure back. There is no visible leak anywhere and the fluid level is unchanging. Once pumped if I hold the brake pedal down it stays firm. I'm thinking of taking it back to the garage that changed the fluid and ask them to bleed it some more but I can't help think something else is going on.

[bangerhoarder]

How old is the car?

What’s the condition of the front disks and brake flexis?

[E-bmw]

Many years ago I had a car that would occasionally when braking slowly just sink the pedal to the floor.

When it did this, as soon as you tried again firmer it was fine, the piston seal on the M/C was hardening so was sometimes not fully expanded by very gentle braking, but firmer braking was fine.

New M/C sorted it.

[Mario323]

Hi, Its a 2007/8 R56 N12.

I'm told the front brakes are good and there is no immediate or upcoming reason for concern with the disks, callipers or any of the pipework, all of which were given a cursory once over when the breaks were bled. I took the car out again today and drove around the city for about an hour so lots of braking opportunities and there is still no apparent loss of fluid. The breaks only went soft once giving off the slightest judder first - like the ABS was engaging for just a second. One more depress of the pedal and we were back in business.

There is something else - when I was waiting for traffic light to let me through I depressed the brake slowly 3-4 times just to see if it was holding pressure and I noticed that when releasing the brake, just for the length of the return to its resting point, there is noise. More "shhhh" than "hiss" but I'm sure it has always done this and it could just be the spring. No hissing sounds at any other time that I can hear.