Break goes to the floor/no leaks
 

So my bf bought this van and the brake goes straight to the floor ,no pressure at all. No leaks anywhere, any suggestions

Internal master cylinder leak. Fluid level fine? the fluid is passing around the seals, and back into the MC.

filled up the break fluid pump the brakes and all the flyid is gone.

There must be a leak somewhere if the fluid is gone after pumping the pedal. If you can't see any leaks under the van then the master cylinder is leaking into the brake booster and you might see the leak under the hood.

If i replace the master cylinder will that fix it, or do i replace the brake booster as well

If you are sure there are no leaks anywhere, then I would replace the master cylinder. the booster should be ok if it was ok before.

Are the bleed valves tight?

the master might be leaking into the booster

Maybe the slave cylinder seals leaking, but you won't know until you take the drum off, you'll probably have to replace the shoes too, best to do both sides whilst you're about it for even breaking.