brake booster hard pedal

hello mopar friends
i have a 71 charger power brakes has a crate hemi motor been together for many years.always worked fine.

for the last year the brakes have been a stiff pedal. after driving a mile or so it would be fine like the booster was kicking in. sent it out to power brake booster exchange formally booster dewey.
thougth for sure the boot was bad. got it back and still have the problem. i know my vacuum is low around 10 pounds . tried a canister did not help.. anyone ever have this problem?
maybe a caliper ?
thanks for any help

I have this issue as well, except I have a vacuum canister and a vacuum pump. It is worse at colder ambient temperatures. At 70F it is ok. At 30F it doesn’t have power assist for a mile or two of driving. I’ve verified the pump is working as intended, my next step was to put a vacuum gauge at the line going to the booster.

I have a hunch it’s the booster not sealing.

If it was working before and not now did you try replacing the check valve in the face of the booster?

Like Blusmbl did,, I have a pump and reservoir.

I have an inline check valve before the pump T's into the brake booster line. All the pump does is draw a vacuum for the booster and the check valve keeps it from leeching down.

I got rid of my noisy diaphragm vaccum pump and got a vane pump from a Volvo turbo car out of the junkyard.

There are a lot of turbo & hybrid cars in the junkyards and many have electric vacuum pumps. They are quieter and cheaper than the old diaphragm pumps.

You may have to wait for it to vacuum down before you drive. I added a vacuum switch to my pump circuit to cut it off when it isn't needed.

Hope it helps.

i don't think they'll work at 10". i have a midland ross on one of my cars and it doesn't like much of anything below 16". you may need a vacuum pump if an extra reservoir doesn't help. you also may need to play with the distributor centrifugal curve (and vacuum advance if you're using one) to help the low rpm vacuum.