I was losing brake fluid, so I took my 1995 Grand Caravan to the shop. My brake lines were split and my rear brake cylinders were leaking so he redid my rear brakes. Since then, I have lost nearly all of my brake fluid on three seperate occasions. Each time, there will be little to no leakage at all for weeks up to a month or so, and then while driving, the brake pedal will suddenly become very soft, and when I check my brake fluid reservoir, it is nearly empty.
When I fill it back up, it's fine again, with no loss of fluid, until it randomly happens again. It seemed to happen for me twice after heavy brake usage (having to stop suddenly several times in traffic), but it also happened this past week when my brother used it on vacation, and he said he barely used the brakes during the trip with no sudden stopping.

Any suggestions on what might be the issue?

 

What is meant by "brake lines split"?

Are you sure your rear brake cylinders are still not leaking?

Look inside the rear tires to see if in fact they were fixed.
For that matter, check the front wheels too.

Do you see any tell-tale fluid marks on the ground where the van sits?

Look around the master cylinder where you fill the fluid and see if
there's obvious leaking behind and down the firewall.

I'd be watching daily the fluid level at the master cylinder to see if it's going down
slowly. You say it happens all-of-a-sudden. Is this just by feel of lost brakes or are you watching the fluid level stay high, then it dumps after a hard braking?

The fluid is going someplace so should be leaving tracks.

I once had a brake job done. On the way home from the job, the brakes failed. The technician, in his attempt to save me money used the old rear cylinders, new pistons (probably didn't hone out the old cylinder). A hard stop, caused the fluid to bypass the pistons and bleed out onto the drums and tires.

Take the van back to whomever did the job.

I don't believe I'd want to be driving around like that.

 

The casing perhaps? At any rate, I believe they were replaced.

We checked the wheels last time it happened, and they were all dry.

I will have to check this.

I've checked it on numerous occasions. It may go down ever-so-slightly, but remains fairly constant until it suddenly dumps.

I think I will be doing that. Currently, at least from what I've seen, this may be unrelated to the original problem I had with the leaking rear cylinders, but perhaps by getting the rear fixed, something else that was wearing out now has normal pressure amounts on it and can't handle it.

 

I checked the master cylinder, all four wheels, and the ground under my van after pumping the brakes hard for a minute or so, and could find no discernable leakage. Baffling. Guess I will have to have it looked at.

 

Remove the master cylinder from the booster, but don't disconnect the lines. If the master cylinder is lealing you will see brake fluid inside the booster and leak from the rear piston.