Brake headache

[hairyape1]

Scratching my head with a brake issue. Over the last year the brakes have progressively got firmer. It has got to the point where I nolonger want to drive it till its safe.
I holds vacuum overnight so I assumed there was no leak, after pumping the brake pedal once it becomes firm and needs 5s to build up vacuum again. So i had two theories
1 the vacuum pump was weak and not achieving a full vacume, thus allowing a partial vacuum to let the pedal feel firm, but the brakes are still assisted.
2 the brake servo is knackered and leaking.

This morning I replace the vac pump with one from the scrappy (condition unknown :/ ) after alot of cursing I got it back together. On a test drive, the pedal still feels hard, but if you leave the car on tickover for a min the pedal goes much softer but not every time.

So it could be a dodgy replacment pump. orrrr could it be a vacuum leak, but only above a certain level, if this makes sense? so it holds a small vacume but leaks as the vac increases.
Or possibly its the brake servo?

How can you check for a vacuum leak on a diesel?

Im getting abit desperate... has anybody else experianced this?

Many thanks

Ben

[hairyape1]

Update: Solved ish

Rebuilt the old vac pump, machined up to reduce tolerances to improve vacuum. The extra vac helped but still didnt solve the issues. Swapped the vac pipes as one had a small split. This made v little differance. Finally bit the bullet and replaced the servo. Reading online it said you have to break the plastic clip between the servo and pedal and bleeding brakes. Investiagtion showed you can get round the clip by winding out the adjuster and the master cylinder can just be pulled out the way without disconnecting brakelines - thus no bleeding necessary.
Replaced, went to test, pedal jammed, realised the servo and master cylinder were misaligned doh! (before I realised this, I disconnected the brake lines so now need to bleed :( ) Fixed that issue and now the brakes work a treat. ALthough they are a little sensitive, not as much feedback as I would like. Is this adjustable?

Cheers
Ben