2000 XKR
Need some help in which direction to go with this repair... brake pedal grabs, but slowly goes to the floor, like there is a leak somewhere.
Calipers seem ok, I've gone over the entire system and don't see any fluid leaks from lines or connections.
I replaced the master cylinder with no change to behavior, all pads are new, even had the system bled at the shop.

Has anyone else confronted this? Or knows brake systems better than my novice level?

ABS valve block?

Never heard of a faulty block with symptom like yours but?

You’ve checked all the lines and calipers for leaks ?

a faulty power unit can leak internally and give the poor braking you described.

Z

Are you having to add fluid? If its not leaking out the only place it can be is inside the brake servo or under the carpet.
There is no fluid inside the brake servo so it could be leaking out the rear of the master cylinder.

My 97 pedal did that, it may have been expansion of the old rubber brake lines. But it was only noticeable when at a stoplight.

I don’t see where the OP has mentioned if he’s losing fluid or not. It would be helpful to know that…..

Z

Thanks about the ABS block idea, didn't think of that unit... but I will look into that section of the system.

I'm not having to add any fluid to the reservoir, it stays consistent. I've checked all the lines in the engine bay and underneath (plus wheel cylinder) no visible leaks.
I just changed out the master cylinder, so there are no leaks back into the vacuum booster (servo) unit.
Pistnbroke - you mention "under the carpet". Are there lines that run under the carpet in the interior of the cabin?

Seems if no visible external leaks on calipers, ABS Controller, and hoses (could be swelling under pressure), plus the lack of any fluid loss in res. one would think the (new) master cylinder is bypassing internally. This a sealed system so either a component is bypassing fluid or the fluid is being dispelled by leak(thus requiring additional fluid, NOT) With system properly bled (no air in system) and no leaks at the 4 corners, only master cylinder( think pump) is left. The ABS controller is sealed and modulates pressure when signaled as needed by the sensors. Stopped or at rest there should be no signals to modulated the pressure. Above thoughts are my understanding of how hydraulic brake systems function. Please share your thoughts regarding my theory.

Is the power unit holding a vacuum and/or pressure ?

Z

K stevusa ...operation theory correct.
If there was no vacuum the pedal would be hard and high.
If the hoses are not ballooning I would go for a faulty master cylinder.
When the pedal goes down does the level in the reservoir go up or down ? That would indicate a faulty master cylinder.

re question ..if the master leaks often the fluid runs down the fire wall to go under the carpet.

The vacuum booster is holding its pressure, no problem there. When I replaced the original master cylinder, I didn't see any leaked fluid into the system, but I'll check the interior firewall carpet.
I am wondering if the shop I took it to for a full fluid bleed actually did the job correctly, I trusted that there was no air in the line after this but...

this is the first thing I’d be rechecking. Jaguar has an oddball sequence for bleeding the brakes. Not the usual furtherest wheel first, etc.


Don’t know how much it matters to follow their unique sequence, but it’s detailed in the shop manual

Z