Front brakes not working

[77c10s10]

I have a 77 c10 heavy 1/2 ton. The front brakes stopped working a couple days ago. When I press slowly on the brakes, the pedal goes almost to the floor before there's any response and it's only the rears working. If I slam on the brakes, the pedal feels normal and I get a response from all brakes but then the pedal slowly sinks until only the rears are working. Any ideas on what could be causing this?

 

[PrairieDrifter]

Master cylinder is what I would guess. Some one else here may have another opinion!

 

[Rusty Nail]

My first thought was master cylinder.

 

[chengny]

X3 on the MC.

If you can develop pressure by quickly slamming on the pedal - but not with a slow steady push - the cup lips are stiff and not properly sealing up against the ID of the cylinder.

 

[77c10s10]

Thanks for the reply and advice. I'll install a new master cylinder and let you know if it fixes the problem

 

[PrairieDrifter]