Not enough vacuum?
P.S. AJ/FormS said: choppy cam and 15" vacuum are not usually found on the same engine....
I agree and am a little perplexed at what you have.
With the new information, I have an idea;
Your idle speed is way too high for a 4 speed,
OR the engine speed is not returning to idle properly.
OR, you are leaving it in too high a gear, while simultaneously braking,
OR, you are just unfamiliar with how to operate a manual trans.behind a torquey 340
Here's my thinking;
It stops properly when in neutral with a high hard pedal. Is that correct?
But when still in gear, it does not; but the pedal is still high and hard. Is that correct?
Well then it seems to me that while in gear, the engine is fighting the brakes, and that would be because it is pulling, and the only way that could be is if the idle-speed is too high; and there could be several reasons for that..
Ima thinking the choppy idle is pointing to vacuum leak and someone has compensated for it by cranking in the speed screw.
Ima thinking; fix the idle and your brake issue will go away.
To that end;
Prove the booster is not sucking air and that the check-valve is working. OR just disable it for the following adjustments.
Make sure the PCV is hooked to the base of the carb on the nipple provided for it,
Close your secondaries up tight but not sticking,
disable the Vacuum advance,
and back the initial timing down to 10/12 for starters.
Prove your fuel level is correct and stable.
set your T-port sync,
I'm guessing your idle speed should fall in the range of 650 to 750, and when set to that, you should be able to stall the engine by closing one or both of the mixture screws. Those screws should, when the T-port sync is properly set, fall into the sweet spot at around; 7/8 turn for Holleys; and 2.5 turns for all others.
Then watch the engine smooth out and the vacuum climb to maybe 16. NOW; go looking for a vacuum leak, including from the valley to the underside of the intake ports.
If the idle speed does not fall into the zone of 650 to 750 with the above settings, do not change it with the speed screw....... because
that will mess up the T-port sync again!
Instead you will set the idle speed with timing and Idle Air Bypass. However, with your vacuum level, I doubt you will need to get into that.
Just in case you missed it, make sure your PCV is properly plumbed and working; it is your factory idle-air bypass.
In the event that I'm wrong about your idle-speed....... IDK,lol
