How can I tell which I have?

I was going to go to the movies today to see Transformers 3(matinee) and when I was on the road and hit the brake pedal it sanked to the floor. I had brakes but a REAL spongy pedal. I pulled over, looked in the master cylinder, both reservoirs full but I cant go to the movies in this car, danger danger.

I get home and I stand on the brake pedal and I still am not losing fluid nor are there any external leaks. Pulled all tires, no busted hoses, or brake lines. Still no underbody stains on the ground. SO I deduced the Master Cylinder took an early retirement on me. I figure a busted seal in the bore. With the lid off I got a spurt from the rear reservoir(front circuit) but not the front reservoir(rear circuit). That is the seal I think died.

Autozone has both JB5, JB6 and JB7 master cylinders for $30 but I dont know how to identify them to tell which one.

I did the same dang thing last month in my Cadillac. The pedal sank to the floor and I replaced the master cylinder and brake booster only to have to also do the calipers when the bleeder valves snapped off in my 10mm wrench. They have a J4 and a J55 brake system. What the hell?


This time I m going to hit the bleeder valve with my steel toothbrush and some KROIL and hope it penetrates down to the threads.


So what are the identifiers between JB5, JB6, and JB7?