GM-GMC: Sinking Brake Pedal, disc brake calipers, caliper piston

Question
Thank you for the quick response.The system was bled at all 4 wheels.According to information I have found the problem was an internal leak inside the master cylinder,or in other components in the braking system.Seeing as the fluid level in the resevoir never decreased,I went for the master cylinder first,but am not sure how to diagnose the other components for internal leaks  without "throwing" more parts at it.-------------------------
Followup To
Question -
I have a '92 Chevy Blazer,full size that has a sinking brake pedal.I have just replaced the master cylinder which has not solved the problem.It has a 4 wheel abs system.I properly bled the master and have a good pedal until steady pressure is applied to the pedal? What is my next course of action?
Answer -
Did you bleed the brake system at all four wheels?

Changing the master cylinder, whether it was bled or not before installation, requires that the entire hydraulic system be bled.

If (after bleeding all four wheels)the pedal still sinks, you should start looking for leaks.  Usually a wheel cylinder or caliper is at fault.

Answer
Rear brakes, if not disc brakes, use a double-acting wheel cylinder.  Remove the wheel and brake drum.  Pry the lower end of the rubber dust cap off each end of the wheel cylinder, if brake fluid runs out, that wheel cylinder is leaking.  Sometimes, with a bad leak, brake fluid will drip down onto the drum and out the bottom of the drum onto the tire and/or ground.

Disc brake calipers can leak around their piston(s).  Remove the wheel and caliper.  Remove the pads and look for signs of leaks around the circumference of the caliper piston(s).  Do not try to pry the dust covers off the piston(s).

Also check the two front brake hoses and the one that connects the rear axle to the metal brake line on the chassis for deterioration.