Hydraulic system contaminated with dirt causing check valves or pistons to stick - flush system with clean, heavy duty brake fluid.

Faulty check valves in master cylinder - repair or replace cylinder.

Swollen cups on master cylinder (by fluid contamination) - flush hydraulic system with alcohol to remove contaminated fluid and replace unit.

Kinked collapsed, dented or clogged lines, hoses or connections - repair or replace faulty part.

Broken return springs - replace unit.

Binding pedal linkage - free up and lubricate.