Got this prob recently with my '92 corolla. Initially the pedal only sinks after abt a min or so but got more frequent. Once i was pressing the brake & the pedal sink but i didn't realise until the car behind honked at me...
When i release the pedal fully & pressed again, it worked fine, but after abt 10-15secs the pedal sank again...
Anyone knows what's the prob? Is it the master cylinder? or the brake booster?