1971 Challenger brake booster

My 1971 Challenger has power disc brakes. At idle the brake pedal is very hard. Cruising and applying the brakes...everything is fine. How do you know if the brake booster is bad? I initially thought my cam might be too big and not allowing much low end vacuum. It's an Isky cam...485 lift/280 duration....really not very radical. Any thoughts?

Your initial thought was correct.. but tuning will probably cure your problem..

Try adding more base timing & limit the total to around 36 degrees

I don't know about that.....My cam is a Comp XTQ with 270* @.050 and 584 lift and I have no issues with the booster UNLESS I hit the pedal more than twice at low rpm. I footbrake the car.71 Challenger R/T.
I'm going to say at the very least you may have a poor check valve...other than that I'd say the booster is marginal.
But....I've been wrong before

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I don't know about that.....My cam is a Comp XTQ with 270* @.050 and 584 lift and I have no issues with the booster UNLESS I hit the pedal more than twice at low rpm. I footbrake the car.71 Challenger R/T.
I'm going to say at the very least you may have a poor check valve...other than that I'd say the booster is marginal.
But....I've been wrong before

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I agree coming down from higher RPMs there should be a couple brake applications before the reserve vacuum is used up... So it's a good idea to try replacing the check valve... But tuning for the cam can also effect vacuum signal... & I'm sure you've optimized that...