One of my co-workers wife used to have a geo i think that exhibited similar symptons as yours. After a bunch of work that didn't make a difference, they discovered that the brake fluid has transmission fluid in it from someone who didn't know what they were doing. the power brakes would allow the fluid to be pushed to apply the brakes but the brake fluid collapsed the lines to where the brake fluid wouldn't return and would lock of the brakes at any given moment. so new brake lines and all new componets plus fluid cleared up that problem.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual