replacing wheel cylinder, do i need silicone seal when i install brake line?

No.

Absolutely, positively, most assuredly NO!

Martin-If you're referring to a standard flared brakeline (pipe) fitting, the seal occurs at the compression flange on the end of the line, not at the threads, no sealer is required on the threads and indeed may make disassembly in the future difficult. Always best to use flare nut wrenches when dealing with flare fittings, not standard open end wrenches. If you're talking about a standard thread type brake hose fitting, replace any washers, same with banjo type fittings. Again no sealer should be required.

Dan

No sealant required. However like someone else posted use flare wrenches or you'll likely round off the fitting.

For the future I tend to lube brake fittings in one of two ways. If I feel like spending more time I'll carefully coat the threads, and ONLY the threads with anti-sieze. When it's together I'll put a glob of grease over the fitting. If I want to go faster I'll just do the glob of grease. Lately, the grease glob has been the way for me rather than worry about getting anti-sieze in wrong spot.

The glob of grease keeps the fitting and line from rusting. Same with bleed screws, I glob grease on the outside of them.