Braking issue

[mattyy]

Hi,

Currently got an issue with my brakes where even though the pedal is good and the brakes are good (just passed an MOT) occasionally the pedal will drop to the floor whilst driving.
This usually happens when not accelerating or just after lifting off.
Interestingly I've only got to touch the pedal and lift to get the pedal back and it returns back to normal.

Obviously I can’t drive like this but I’m unsure where to start. The garages bled the brakes at the same time as the mot but it is still the same. I have previously bled the callipers numerous times. Rear lines have been replaced, rear callipers rebuilt.

One thing to note is that I can feel when the brakes ‘let go’ and when the pedal has gone without touching it. Could this be a servo issue or still trapped air?

 

[Retroman]

Assuming no fluid loss, fluid-tight brake pipes and decent hoses that are not "ballooning", I would guess (and it's only a forum guess) at air trapped in the ABS pump. Usually these have to be bled by a software initiation of the pump.

 

[mattyy]

No fluid loss and I can’t find any leaks (nor did the tester)

I made that guess too. The pump has been bled originally but there’s nothing to say more air isn’t trapped.
What I’m trying to work out is whether air trapped in the pump would make the pedal fall to the floor under normal driving when you would assume the pumps valves are closed?
The pedal under normal conditions is really good.

I’m leaning towards bleeding the abs pump and then maybe master cylinder fault - car has been dried stored for 2 years before this.

 

[Retroman]

I’m leaning towards bleeding the abs pump and then maybe master cylinder fault

I agree - could be an internal MC leak