Brake Problems

I have a 1988 Pontiac 6000 Station Wagon. We just had a huge snowstorm yesterday and I had to lock up my brakes to stop. I slammed on the brake pedal and the brakes started to engage and then my brakes went out. I'm not sure what part needs to be replaced. I have been told its the master cylinder and the brake booster and the proportioning valve and the metering valve. Three different mechanics told me different things. If anyone knows what part I need to replace let me know please.