Okay, just got a 1991 F250 from my Uncle. It was parked in his field for the last 5-10 years.
Got the engine running, but now I find there are no brakes whatsoever.
Pedal goes straight to the floor with no resistance. The linkage is okay. I can hear some booster hissing as well.
No leaks noticed and the reservoir seemed a little low but that could be just worn pads.
What would cause the pedal to do this? First I thought booster, but if that was the case wouldn't I still have some pedal resistance?

 

I'd suspect the master cylinder, internal leakage.

 

I've also seen some people mention the ABS module. Could that cause no brake pedal at all?
It does have rear ABS.

 

Booster hissing is a vacuum leak. Test it to make certain. Parts house should have a tester on hand and allow you to use it right there in their parking lot. Also, sitting up like that, may want to check the wheel cylinders and all the rubber lines.

 

I don't think that ABS can cause the pedal to drop all the way to the floor. I think the brake system is designed so that you can ALWAYS operate the brakes manually even if the brake booster and/or ABS fails. My guess is that your fluid has leaked out or (mostly likely IMO) bad master cylinder. That's not saying that the booster and ABS are ok but I don't think that they're the cause of the problem that you described.

 

I'm on the failed master cylinder bandwagon. If it isn't leaking anywhere and the pedal goes straight to the floor odds favor that the seals have gone bad in the master cylinder.

As mentioned, after sitting that long unused you should make sure all the brakes are moving freely and the lines and hoses are good before you get it out on the highway. A stuck caliper or bad wheel cylinder isn't something you want to find out about at 55-70 mph

 

I agree with all the above about the hissing meaning vacuum leak, Checking hoses. Every abs that has failed or caused issues for me make it so you can not press the pedal down so I would not suspect the abs to be your issue.

Any updated on what you found?