Brakes less effective after first use

My 1987 Defender 90 has developed a problem which I need to fix before it gets worse. When I brake, the first press of the pedal works well, but if I immediately lift and then press the pedal again it feels spongy and less effective, this gets progressively worse if the pedal is 'pumped' a few m8re times. The brakes seem to recover a few seconds later, but it's a little scary and clearly needs fixing ASAP. Is it likely to be the master cylinder, fluid needing changing or what? Any ideas where I should start?

Thanks!!

have you bled the system ,if you use a plastic pipe in a jar you will see if theres signs of air or just clear fluid, get someone to pump pedal whilst you watch reservoir with cap off look for splashing can mean theres air or m/c seals are poor

Sounds like servo, simple test, engine off pump pedal until no servo assistance, hold pedal down and start engine, within a second or two the pedal should feel lighter meaning the vac pump is working.
Also look at the pipe between vac pump and servo and the non return valve in the servo to pipe union

poor servo or vac would give a hard pedal though

Good point, maybe flex breaking down?

Check your rear drum adjustment.

I'm getting similar on mine which I'm 99% sure is down to my rear drums needing adjustment.