Brakes poor brake performance diagnostics
soft brakes, pedal to the floor, pumping brakes.
Vapour lock.....when the brake fluid starts to boil with massive heat resulting in gas bubbles which are compressable. This could be caused by several things....excessive amounts of water in the brake fluid (brake fluid has a very undesirable property, it absorbs moisture from surrounding air (Hygroscopic) it HAS to be changed every two years regardless of mileage. Sticking seizing calipers....leading to over heating and vapour lock (as described above) Master cylinder seal failure: normally has the symptom of a slow progressive wander downwards of the pedal (pedal creep) as fluid forces it's way past the master cylinder. Total master cylinder failure where seals fold back on themselves (extremely rare) but can happen after vigourous brake bleeding where the old worn master cylinder seals have been forced over the worn ridge (where the seals would normally travel up to) during old fashioned pedal bleeding (working the brake pedal through it's full stroke)
flexi brake pipe ballooning: where the internal structure of the flexi pipes has been compromised causing fluid to escape into the outter layer of the flexi causing a 'balloon' to appear. Fluid leak: from rear wheel cylinders, front calipers or burst pipe...this would be permanent. Air in the system: would also be permanent.
Soft pedal cant be to do with vacuum and servo.. If you have no vacuum/leak/faulty non return valve, then the pedal will be hard and no assistance will be given.. it will also be much harder to stop as you will need to stand on the brakes to create the same hydraulic pressure on the master cyl..
A soft pedal would normally be attributed to air in the system, but yet you say the level hasnt dropped.. I would presume you have checked the rear wheel cyls for leakage as these tend to go before any issues with the calipers arise and may let a small amount of air in before problems are evident.
Another option is the seals in the master cyl are bypassing.. not common at all.. and i would expect to see some leakage (onto your shoes) at some point..
A flexi hose has damage and is balooning.. but this would be consistant and WOULD fail at some point.. leaking fluid away...
If the fluid is old and absobed water, then it will boil at a much much lower temperature giving the same symptoms as air in the system and a soft pedal..
Can you say under what conditions the pedal failed?? were you coming down a hill with the brakes on all the way?? (VERY bad practice) Do you have a sticky caliper causing the brake to heat up, thus boiling the fluid?