I have a 79 Conquista. Brakes were fine one minute. Went to a barbecue. Started the car up to go home and the brakes are very squishy all of a sudden and i hear like a hiss sound as i press the brake down. Could my brake fluid be low? Where do i find the refill tank? What kind of problems could this mean?

Could be a bad booster. It's easy enough to see if you're low on fluid or not.

What does the refill tank look like?

I had a F-brand pick up that the hissing came from the booster and when you kept the brakes depressed the engine would idle rough, vacuum leak.

I believe the hiss is air from whatever pushes the pedal back up. thats where the noise is coming from. That is probably normal? It doesnt idle rough when fully depressed. When driving if i slam hard on the brake to the floor i think the back wheels spin a little.

I bet they're locking up, not spinning. The master cylinder (where the brake fluid goes) is under the hood on the driver side connected to the booster that is bolted to the firewall. The master cylinder looks basically like a metal cylinder with two metal hoses coming out of it and a plastic tank on top.

Why would they be locking up?

What kind of brake fluid should i use. the brake fluid is almost completely empty

I think DOT 4 is OK, but if you're suddenly about out of fluid, you must have a leak somewhere! If it's not in the lines, it could be the master cylider itself. Maybe, take off the the two bolts holding it to the booster and check for fluid (maybe you'll see some leaking from under it.) If there is fluid coming from it, get a new MC. I don't see why this would happen so suddenly unless somebody decided to siphen your brake fluid :lol: but I guess anything is possible.

The fill resevoir was about empty but i could see fluid at the top of the chute. Also it looks very dark and nasty. Brakes seemed to get a little better adding that extra fluid but it could also just be my imagination.

How does air get into the brake lines? Thats my next suspect.

Brakes are about what they used to be now. Really strange! Maybe a tad bit weaker. Still curious how air gets in the brake line. Im stunned how adding so little brake fluid fixed the problem.

When the fluid level gets too low in the Master Cylinder reservoir, air is drawn into the system. The result = squishy feeling brake pedal. to restore the pedal to its original feel, first try bleeding the brakes at each wheel. Right rear first, left rear second, right front third and left front last. During the bleeding process, keep the fluid level topped off in the master cylinder reservoir. In the future, keep an eye on the fluid level in the master cylinder. It should take several years before dropping below the minimum fill level unless a leak exists somewhere.