replaced brake Master Cylinder, but need rubber seal between MC and booster (72 GMC)

[centuryhouse]

I spent over $1000 on brakes today

At the end of it all, my brakes felt far worse than they did when I wen to the shop to begin with.

The engine is revving higher than normal and running rough, instead of low and smooth like it was before taking it in. And pushing really hard on the brake pedal is required to stop the car now.

There is a rubber seal between the MC and the booster, which they tore when removing it. Air is hissing from there, like a leak.

How can I find a replacement?

(72 GMC c15 2wd, 305 engine, TH350 trans.

Parts replaced today: New front rotors, new pads, greased calipers, repacked bearings, new drums, new shoes & hardware kits, new wheel cylinders, new brake line, replaced axle seal, new master cylinder, flushed refilled & bled brake system, new emergency brake cable system.