Odd Brake actions

Morning All!
Got an odd thing happening on our Defender 110.
Drive along, no problem. Try & stop, BIG problem!. Pedal is real hard & not much brake effort...
What i have tried so far is :-
Stop engine & pump vac out of system. Hold pedal down, start up & pedal sinks a bit.
With engine running, hold pedal down & turn engine off. The pedal then rises...
Any ideas?
Many thanks in advance,
Merv!

What age is your defender? Sounds like a servo issue

Get the bonnet open and check rubber pipes etc.

Vacuum pipe on servo or the servo itself

IME, most likely cause is the exhauster (AKA vac pump ) failing - easy to test - and should be the first thing you test IMHO.

IF this is working, then check the pipes for leaks - remember they are letting air IN if they are leaking....

Servo's do fail - but you need the exhauster to work before you can test this - second hand electric "vacuum pumps" from the likes of audi / skoda / etc are often seen on the bay of fleas....

Have you changed the front pads, and chucked a whole lot of extra fluid into the system?
Do you have shoes of the back, and if so, when was the last time they were adjusted?
It has been known for the 'self-adjusters to overcompensate for wear, which heats up the shopes and backs up the pressure in the system.