Hard brake pedal and comp code 1406

[Ralph50]

WOW,
my daughers car is giving me gray hair.
96 Lesabre, 3.8L, 115K miles.
I was getting a 301 code so I replaced wires, AC delco, Plugs AC delco the OE plug. Then she says the car has a loud tick and we get a 301 code again. I figure if the valve is not opening then it could send a misfire code. So I put Marvel Mystery oil in the crank and no more ticking. Today she says starts the car and the brakes would not go down far enough to shift into gear and the brakes were really hard. I check the codes again (CEL on) and get a 0300 and a 1406 EGR issue. I just replaced the oring in the UIM where the brake vaccumm line goes into the UIM so I dont think it is leaking vaccumm. I go out and start the car and everything is OK but CEL lite is on. So my million dollars question, could a stuck open EGR valve cause such a lower vacumm in the UIM that the brake booster is not providing the assist? I have not checked the vaccum reading in the engine, that might be next.

Thx