The 05/06 masters have a reputation for not being that great. That said, if you just did a rebuild, I'm pretty sure you replaced anything that could be bad, unless the inside of the MC is actually pitted, causing air.
On my 06, I had a similar issue. In my case, I resolved it by finally putting all new crush washers on the lines, top and bottom (Yes, I know, I could have saved myself the trouble if I'd done that when the lines were changed!). That said, many many people prefer the larger diameter 07+ MC to your original once. I have an 07 and it's quite good.
I would say, if you're going to replace it, go with an 07 or better. Any MC will fit: some folks like the Brembo ones off an R6, but you could pretty much pick up any new radial MC and it'll work.
However, if you just rebuilt it, you may not need to replace it. It could be air leaking in somewhere else. The two culprits here are the crush washers and the caliper seals. You could do just the washers, but then if you need to pull the calipers off to rebuild them, you need to do the washers again. You could rebuild your original calipers, but, again, many folks like to upgrade these. The 07+ are much better.
If you're doing all that, might as well do SS lines, you're pulling the lines off anyway.
Logically, I ended up here when I worked on my 06.
It's not worth rebuilding the 06 MC. I bought an 09 MC. I bought 07+ calipers. I bought used SS lines from
which were sized for an original 06. I put the lines on with the new MC and Calipers.