07 xk with about 82k miles. Newish brake pads and discs. 6 months after brakes replaced brake pedal began to become spongy. Brakes would still lock on hard application. Abs works fine. Blead brakes and pedal returned to normal. Within 2months and 3k miles pedal is again soft. No apparent leaks. Fluid level has not gone down. After braking hard enough to engage abs brake pedal is harder for a stop or two then reverts to sponginess. OEM pads and discs.
Brakes were bled exactly per shop manual.

Has anyone experienced this?

 

I've had a lot of problems at work using the blue coloured dot 4 specifically the bendix brand , not only does the fluid go dirty very quickly (6months) but I've had to replace to master cylinders due to the rubbers deteriorating.

So check the fluid quality and master cylinder is my advice .

 

Sounds like the master cylinder is leaking internally.

 

Even though they were bled, there may be some air or water in the system. Cheapest fix first, replace ALL the brake fluid. That should be done every 2-3 years anyway.

 

Moisture content of brake fluid was less than 1%. Replaces master cylinder and brake performance has been restored. Thanks for pointing me in the right direction.

 

Glad we all could help 😁