Two weeks ago pulling up to stop and noticed brakes were spongy. When I pressed hard on peddle, peddle almost went to floor. At first I thought I blew a brake line but after thourogh check all brakes lines were fine.

If I pump brake peddle, peddle gets hard, but then slowly loose's pressure.

Replaced master cylinder, problem still there.
Bleed all brake lines, no air, problem still there.
Bleed ABS, no air, problem still there.

My mechanic connected to ABS system and ran full diagnostic, activated and deactivated internal valves, checked and double checked and found nothing wrong.

Bit of snow on the road the other day so took van down side road and stood on brakes a couple of times doing 40km to simulate emergency stop. ABS works fine.

When we disconnected the rear brake lines and blocked them off after the ABS, problem still there.

When we disconnected the front brake lines after ABS and blocked them off after the ABS, brake pedal nice and hard. Problem gone.

Reconnected front brake lines, (bleed system) problem comes back.

Clamped off both front brake lines before caliper, brake pedal nice and hard, problem gone.

Release clamp on drivers side front brake caliper, brake pedal soft, problem back. Re-clamp drivers side, release clamp on passenger side front, brake pedal soft, problem back.

Kept passenger side front brake line clamped off, replaced driver side front brake caliper, soft pedal, problem still there.

Anyone have any idea what might be wrong?

I'm thinking it has to be the ABS hydraulic modulator, but I can't see why it would act up like this?

 

Anyone?

 

did you ever get an answer?
had same problem

 

Interesting problem. I, too am curious.

 

I had this problem with spongy brake changed the master cylinder. Booster pump. Abs pump. Nohting worked. Then I read on line it could be the rear caliper because the rear caliper has the emergency brake built into the calipers this type of screw piston could give a problem. Sure enough when I changed the right side caliper. The brakes became hard again. Thanks for the advice. Hope's the helps. Also u can teĺl by pressing on the emergency brake paddle it should be hard, no slack. Before I fixed it there was some slack . After there was no slack.