Hard brake pedal at all times. Replaced 5 pin solenoid but, no change. on 1990 Ford Thunderbird SC

Brakes will work one time out of 100 times. Dash brake lights on all the time except that particular time when it appears to work. What seems to be the problem? The SC brake system has a reputation to be troublesome. There have been sugestions to replaced the SC brake system to a conventional booster and master cylinder. Any help in the subject will be greatly appreciated. Thanks.

Asked by for the 1990 Ford Thunderbird SC
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