Master Cylinder problems??

OK, so I replaced my leaky wheel cylinders (lousy mechanic wanted to do) and I still am having a problem with my brakes... when I drive around town and come to a slow stop, my brakes basically sink into the floor (slow stop). But when I jam on the brakes, they work fine. I have tested the booster with an old method I found in the AF:

To check your booster, heres what you do... Dont start the car, just sit in it and pump the pedal a few times so it gets nice and stiff (dont laugh) when it does get stiff (if it never does its a bad check valve), hold it down and if it gradually sinks or goes to the floor, you may have either a leaky hose, leaky cylinder, or a bad master cylinder. Now, with your foot on the brake, start the car and it should sink a bit. This is to test your brake booster. If it doesnt sink, the booster or check valve is bad.

According to this my booster is fine. (the pedal sinks a bit as it says it should) but it also says my master cylinder is fine (it never sinks initially, it pumps right up). I have run out of options, but I'm fairly certain it's the master cylinder. Is there anyway to know a little more sure before purchasing a new one and replacing it? Thanks.

I Ran In To The Same Problem I Went Ahead And Replaced The Master Cylinder And Thats Was It.

yep, master cylinder. When you replace, make sure you get the old seal out of the booster before you put the new one in.

Yea, theres two valves (seals) in the chamber of the master cylinder. The primary and the secondary. When your pedal sinks to the floor when their pressed gently but stays pretty stiff when you jam on them, it means theres fluid getting by the seals. The slower you press it, the easier it is for the fluid to squeeze by them. Anyways, get a new MC and you'll be set.

Mine seems like a similar problem, but it seems worse when I use a lot of electronics (ie. lights, stereo, etc.). Brakes work so I guess I won't worry about it for now...