Working on it now and new master cylinder squealing?

[ubtripn]

I have power drum brakes, master cylinder was leaking, bought a new one and I put the rubber cup over the end of it (The old rubber cup stayed on the old mc) but there was still some rubber around the metal pushod coming out of the booster at the base and it looked like it belonged so I installed the new mc and now it squeals when ever I apply the brakes. High-pitched coming from booster-sounds like it maybe where the mc mounts against the booster.) Anybody ever have this happen?