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Sounds like your booster. mine was. Same conditions on mine. Pull your brake booster vac line off and plug it. Drive it and test the brakes . If it's the same.....booster. If it changes it's probably not the booster. You might want to change your brake fluid too, and check your calipers to make sure your brake pads aren't hanging up. If the calipers/pads are ok, THEN check the booster.
Good luck
Bryan
I got 1 on ebay for $65. 2 bolts to remove the master, I took my seat out(4bolts) for more room, take panel off above drivers legs, use an extension with a swivel, 2 bolts, undo the rod and it's out. Going back in is a PITA to get the nuts back on, but if you've ever done a heater core, this will be a breeze.
There is a rebuild kit for it I'm going to get to rebuild mine....if I can remember where I seen it.
A good 1/4 inch rachet drive set will be a great help with the inside nuts.
Just on prinicipal, check the vacuum, hoses and check valve just to make sure. Is there an RPM increase when you step on brake and can you hear any vaccume leak.
As said, lots of places to buy a booster, look around.