Look at the seats inside the blocks. If they look good, you should have no problems.
I have re-used brass blocks on two restorations and have had no problems. On the first restoration, there were several leaks using new stainless steel lines/fittings. I found that the fittings had to be turned snug tight, loosened, and turned snug tight again (the tubing manufacturer informed me that a significant amount of tightening was necessary).
I used no sealant.
