Hey guys, been a while.

Got a problem. 2000 cherokee sport.

Got in the truck to drive home from work, hopped on the freeway... Brake light goes on.

Pulled off into a gas station, checked brake fluid. Bone dry in reservoir.

Checked around all wheels for leaks, didn't immediately notice any.

Brake pressure is pretty normal far as i can tell though.

It's stopping just fine, pedal does not go to the floor.

I know you're not supposed to, but i added brake fluid to the full line, and tried pumping the brakes in reverse. Did nothing. Light still on.

Towed it home. Checked further around reservoir, master cyl, and booster. Has a bit of moisture (tiny bit) from between the master cyl and booster.

What gives?

Where did all my fluid go, and what caused the light to trigger?

 

Master cylinder piston seal is gone, it's leaking brake fluid into the vacuum booster, replace the master cylinder.

 

Mmm Kay. That's what i thought. Light will go out after the master cylinder is replaced and the lines are bled right? Anything else i should know?

 

Bench bleed the new M/C before installing it. Might have to stomp the pedal hard a few times to re-center the differential pressure switch to turn the warning light off.

 

^^YUP what they said^^

 

And booster fluid will eat up rubber diaphragms ...

 

Had the same problem on my '78 Power Wagon yrs back. Problem was caused by water getting in between the m/c and booster causing the rod to pit and eat the seal. Found a new seal but before I put it back together I smoothed the rod and made a gasket to go between the two units. Never had another problem. It's not a common problem by a long shot but it does happen.

 

The proportioning valve was stuck open from the fluid running out, and looks like the mc is def leaking from between the reservoir and mc. There is a small amount where the mc and booster meet up, but seems like not where all the fluid went.

Replacing the mc right now. Got the light to go out already though by bleeding the system and stomping the pedal and holding it until it clicked .

 

Just curious, did you actually hear/feel it click?

 

You kind of feel it in the pedal, and hear it. Like a pop of air.

 

Thanks!