Stiff, hard to press brake pedal

This is a 69 with rear drums and front discs. I've checked the fluid and topped it off and that had no effect.
The brake pads were replaced a couple years ago (maybe 5k miles on them) and the master cylinder replaced as well so I'm thinking maybe it's the booster?

Is there a good way to test if it is in fact the booster?

And if it is, how do I know which replacement part I need, I'm browsing these online and they have dozens of different kits, some where you have to replace the pedal etc. I really don't want to have to do that if I don't hafta.

thanks

2Blue2 said:

Take some pliers and crimp the vac line going to booster while engine is running.
Did the idle change? If so your booster might be leaking vacuum.


Is the rest of brake system stock?

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Tried crimping off the vac line, absolutely no change in idle

And yes, to my knowledge the brake system is stock. The master cylinder has been replaced a few years ago but that's all I'm aware of.

2Blue2 said:

Do you hear air when stepping on pedal?

Still thinking the booster is suspect but other guesses are

piston in caliper freezing up
Pads not sliding in caliper use brake grease on back side of pad to prevent the binding that can occur
Vac fitting on manifold getting gummed up
Really old/cheap soft lines swelling under pressure using up brake force

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Yeah you hear sort of a woosh when you step on the pedal. Also where the vaccuum hose connects the booster there's a rubber cap over the port (I assume you could connect another hose to this for some other reason?), if I pop that off the idle speeds up