I have a 64 Pontiac Catalina, I purchases recently without brakes. It needed a new master so I installed a new one. Think I bled the system ok as I am not used to power brakes. Took the car out for a test and as soon as I touched the brakes, the tires locked up and I went into a slide. When I push the pedal down, it goes real easy then suddenly the brakes lock. No middle ground. Really don't know what to do and don't want to pay a mechanic. Need help.
You might want to look at the brakes themselves and check on they're condition. Three things you dont want to screw with. Tires, brakes, and steering. That stuff has to be right.
If the car has been sitting for a while the brake hoses could be clogged internally. The pressure put out by the master cylinder is very high and can overcome restictions in the hoses but the return pressure (return springs) provide minimal pressure. A easy check is to apply the brakes, allow the wheel to become "locked" and open the bleeder valve. If the pressure is removed (the wheel is un-locked) it may indicate a clogged hose. Make sure you do your testing in the driveway and not on the street!
When brake shoes become contaminated they often cause the brakes to be super grabby and lock up easily. Most often the culprit is brake fluid leaking into the drums, usually from a worn out brake cylinder. Try to determine which wheels are locking up (probably not all), then check the shoes. Hope this helps.
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