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Soft brakes in spite of new master cylinder

Lost braking power. Replaced master cylinder. Brakes went soft again. Shop says it is something to do with the ABS Control Unit (?). Took it to dealer who said it is a leaking master cylinder. Back to shop which claims nothing wrong with new master cylinder and it i the ABS blah blah. Dealer says it can’t be ABS. Went to another shop. Replaced master cylinder again. Brakes still soft. No one can figure this out and I am stuck with very soft brakes.

Hard to say. It’s entirely possible that the shop just isn’t bleeding the brakes properly. You could also have a leak somewhere. Are you losing fluid?

It surely would help to know the vehicle’s year & mileage.

One often overlooked cause of soft brakes is deteriorated flexible brake lines at the wheels. As they get old and weak the brake fluid pressure can basically start going to ballooning out the sides of the brake line rather than clamping down on the brake hardware.

If it is an older van with reasonably high mileage replacing these things is actually just a good basic maintenance/safety move regardless of whether they currently cause a problem or not.

Drum brakes on the rear? Worn or out of adjustment rear drum brake shoes can mimic a brake hydraulic problem.

It could be the ABS unit especially if someone has “slapped a set of pads on it” compressing the caliper piston without opening the bleeder and shoving old brake fluid back into the accumulator getting a piece of dirt in it.

+1 for rubber brake hoses being the problem