[lxndr]

Could be a leak or it could be a bad master cylinder. A bad MC would let fluid flow back into the cylinder as you apply the brakes causing you to loose pressure. Usually you will really notice a bad MC at a stop light as the brake pedal slowly sinks to the floor. If this happens you can pump the brakes a couple of times to build up pressure, but if this is the case, your next stop should either be the brake shop or home (if you are going to fix it yourself)!

I dont know the specifics of your car but I do have an thorough understanding of brake systems. If you could give me the specifics of everything that you swapped, I might be able to help you a little more.

How long did it take you to notice this problem after you did the swap?

Exactly what parts did you swap?