K30 master cylinder question

[K85 Octane]

My local parts store has given me 4 options for a K30 M/C. I don't have a K30, but I do have a K5 with rear disc brakes and I want to try it. So I don't have a clue which to get. The master cylinder I got ('99 3500) works but leaks, brand new and it leaks. Bought so long ago, don't even know where I got it.

The guy named off these codes:
LB1
LB3
LB5
LB7

Either he said LB or JB, can't remember

What say you?

 

[nvrenuf]

Those will be "JB" codes, JB8 being dually hydro boost iirc.

Why not just go with another m/c from a 99 C3500? I used one on my vac boost k5 and it works great.

 

[K85 Octane]

I went ahead and picked up the JB7, since it was the largest number M/C they had. I don't have it installed yet but I like that it's solid metal. Comes with a lifetime warranty also.

My '99 3500 M/C works fine, but it feels like I run out of pedal. It's not soft, actually quite the opposite. I push down and soon after it gets solid. Not sure if that's a M.C thing or if I'm losing vacuum. I also had to "make room" in the hood for the '99 cylinder. The very back was hitting. A quick hammering on a support area made it work. Here's a pic.