My 69 GTX 440/727 sometimes has a rock hard pedal and very poor brakes. Scarey when that happens! Just started doing this in the last few weeks. How would I pin point if it's the booster or master? Vac is 12 inches in park, 10 in gear. Comp Cam hyd HEL275. Used to work fine. Suggestions appreciated

Master won't cause that.. But your vacuum signal is right in the borderline area... Has anything changed in the tune-up? A few degrees less timing could drop the vacuum past the minimum threshold... The booster could be failing but I'd look at vacuumm signal first..

Does the problem occur mostly after a few stops & not when stopping the first application after a cruise period? In cruise mode the vacuum signal will be stronger, idling around at low speed it will be weak...

Check the seal around the check valve sometimes they crack and leak periodically.
Turn the idle up a bit and don't pump the peddle sometimes coming up to a light/stop you can be on it and off it without realizing using up your reserve vacuum.
Also check your check valve.
then you can check the booster to see if it leaks after reving up shut off get out and see if it holds vacuum.
Check valves
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I ran a 263/271 @ .050 cam on the street for years with no problems but kept my idle at 1100 4000 stall.

I now run a 272 @ .050 and still run my power brakes but the may get removed this season idle at 1250 5000 stall.

And if all seals check out you could always add a vacuum reserve canister to help with the braking duties...

The only time that I've experienced a rock hard pedal it was the brake booster. Master cylinder has always been a soft/mushy pedal that goes to the floor with very weak braking.

My current vehicle idles at 18 inches in park, and about 11 inches in gear, and I never experience a hard pedal, FWIW.

roe

Another thing you can check is the hose from the booster to the engine. Had a similar problem on my fathers Charger. The inside of the hose was collapsing cutting off the vacuum. Drove me crazy trying to figure it out.