Is this a servo or vacuum pump problem

Hi there, yet again the brakes on my 300tdi defender 1997 have come to my attention as they are poor. The reason its come to mind is i had to do an emergency stop the other day and only just missed the other driver as it takes so long to stop. When i do an emergency stop i cant lock the wheels no matter how hard i try, and the force i am applying onto the peddle is alot, im nearly of the seat! They are just not up to scratch when braking hard.

I have checked the brake vacuum pump for suction where it attatches to the servo and its sucking a fair amount to feel it on my thumb. Whern the engine is started, the pedal does sink, so does this mean that the servo is working? As soon as i turn the engine off i loose total servo assistance instantly... Whereas i thought the vacuum is maintained for atleast 10 mins?(correct me if im wrong). Another thing i have noticed, is a hissing noise when the brake pedal is depressed and when the engine is turned off after it being running you can hear air escaping from somewhere when the pedal is depressed. I have checked for cracks in the servo and i cant seem to see any and ive also checked the seal between the master cylinder and servo and that seems fine.

6 months ago the defender had a new master cylinder but there are bubbles coming up out of the reservoir when the pedal is depressed (is this normal then). Its also had new brake pipes all round including flexis, new brake pads and i rebuilt the rear brake calipers. The system has been bled that many times that it cant be any cleaner .... (but that was a previous brake problem that is now sorted)

Any help would be greatly appreciated folks, thanks in advance

I had exact same symptoms, replaced all brake lines and new stainless flexes, bled about a hundred times.. Suck from vac pump was ok but not great us oil coming out of tell tale so the diaphragm in pump was shot, on top of this there was a small split in hose between pump and servo which is a common problem, and my servo rubbers were sticking together.. New pump, hose, servo, master cylinder seal kit, fresh fluid and new lines fixed my prob-now stops on a dime :0)