1999 Volkswagen Caddy brakes

You don't say if the pedal goes to the floor, then pumps up, or if it goes solid.
Goes to the floor - Either a fluid leak or air in the system. Check all the unions for leaks. Check for wet carpets inside the car, around where the brake linkage goes through the bulkhead. The master cylinder can leak into the car.
A trick that can help to get the last of the air out. Wedge the brake pedal down as far as it will go, and leave overnight. Check the fluid level is OK before and after you remove the wedge.
Solid pedal. Check the servo works. With the engine off, pump the pedal 5-6 times until the pedal goes rock hard. Keeping a constant pressure on the pedal, start the engine, the pedal should sink noticeably. If not, servo faulty.
A more obscure one is that the flexible lines can collapse internally, restricting flow. This is more usual when the brakes stick on

Ian

Assume there is no leaks , master cyl.level not dropping ? When he bleeds brakes , is there air in system ? If front brake locking up you have got to find out what is causing imbalance . The easiest way is to put car on brake tester and check out all readings . Also lets you check /test each wheel individually. This should show what is working and what is not under normal working conditions .If your mechanic doesn't have brake tester it's a case of asking nearest MOT centre to run through brakes when they are quite . Lets you see exactly what's working.