Brake peddle went down in one nite maybe vacumm line gone where is it on master cylinder thx

Check the brake fluid level in the Master Cylinder. If it is good, then the most likely problem is a failed master cylinder. If the fluid is low or there is no fluid, you will need to track down and fix the source of the leak, bleed the brake system of air, and retest.

I'd start by checking calipers make sure their not sticking

Brake peddle should be hard, if bake peddle soft check master cylinder or replce, try bleeding brake system too. Ceck if check valve is bad this is located on vacumm line from intake manafold to booter pump body. brake seal at check valve booster pump should hiss if not check vaccumm line for leaks if no leak in line replace check valve they don't cost much.

Master cylinder needs replacing .

Yes and No...Replacing the master cylinder is a good idea, however, giving the vehicle that much pressure with old lines attached may blow out the old rusted lines. If you are going to replace the master cylinder, you may just want to replace the brake lines as well.

Did you bench bleed master cylinder? It takes a lot of bleeding the system after replacing master cylinder.

sounds like the break booster or master cylinder. Also maybe a vacumm issue with the break booster make sure the line is hooked from the booster to the engine intake

If the pedal has sunk further than usual or gradually sinks then usually a Master cylinder gone bad, but also can be a ruptured barke hose or line. It can also be leaking brake caliper or wheel cylinder (will show a fluid level loss in Master Cylinder)

Make sure the reservoir has fluid. A 1/4" drive ratchet and correct metric socket work good on the bleeder. When bleeding, never press the peddle more than 1/2 way down. When you go all the way down the master cylinder seals travel into territory with tarnish buildup. If you do/have you will probably be needing a master cylinder soon. Antilock brakes complicate bleeding sometimes. Just filling the reservoir and letting it set with the bleeder open allows the fluid to gravity down. Also, screw the bleeder clear out, maybe the hole in it is plugged.

The master brake has gone totally bad