Unless you're visibly losing brake fluid, your symptoms are typical of the master cylinder being bad and fluid leaking past the seals in it. The brake pedal slowly sinking to the floor is something of a giveaway if there's no fluid missing from the mc.
You can rebuild it yourself or buy a new/reman unit. If you do the work make sure to bench-bleed the mc before installing it. That'll make bleeding the rest of the system quicker.
|