Brake Pedal recessing

[Rabbitt]

I have an issue with brake pedal not holding pressure. On first press the pedal is firm , and travel is fine - but when keeping a constant pressure on the pedal it slowly and gradually descends over about 30 seconds until it is literally on the floor and braking is lost. A release and pump returns the pedal back to normal. The car had been standing for a considerable time unused prior to my purchase but recently it did have some attention to brakes as the flexible hoses and couplings are brand-new. (advised by seller).
I was just going to try another brake bleed and hope that fixes the problem as it may not have been done quite right, but I noticed in the workshop manual there are suggestions it could be Master Cylinder cups? ... I hope not but the pressure is being lost somewhere. I cant see any signs of leakage.

[67-2door]

Worth trying another bleed, but it sounds like the master cylinder needs replacing to me.

[James k]

I had this exact same problem when I bought my car and it turned out to be a failed master cylinder. I wouldn't drive it until you've had it sorted.

[IslipMinor]

Almost certainly it is a master cylinder problem - either worn seals or bore corrosion or both. The lack of sealing allows a small internal fluid bleed past the seal, which in turn allows the pedal to descend slowly to the floor. A quick pump and the cycle starts all over again!

As above, replace the master cylinder before using the car.

[bmcecosse]

As above - unless there is a slave cylinder leak - needs a new master cylinder.

[Rabbitt]

Removed the MB Cylinder a couple of days ago ... and are currently checking all brake hubs / seals / wheel cylinders and so far so good. Looks like the brake shoes and flexi's have been recently replaced , but whoever - decided not to do the Mcylinder...
The MC is being chrome sleeved and rekitted ..... thanks again for all the feedback that was certainly the problem ...