Thank you for the information! After extensive bleeding and remortgaging our house to pay for fluid.... I had a very solid pedal with the X off excited I fired her up and down to the floor it goes.. This morning I decided to think outside of the box and disconnected the brake booster. Fired her up and had NO sinking pedal. For the life of me I can't figure how a booster could cause the pedal to sink. Only thing I could come up with is the input shaft from the pedal to the master must be split rather than a solid rod as I would imagine it. All I know at his point the booster is on my hit list. I will report back with results. Thank you for all the input!!

My thought is that somehow the booster is allowing fluid backwards. I'm guessing a check valve is stuck open. There are some good rebuild DIY out there


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Did you mean master cylinder? The booster has no brake fluid, just vacuum.

OP find out where the pressure fluid is going. Is the fluid level decreasing after several pumps to the floor?

Fluid level remains consistent. As I think I mentioned petal remains firm with X5 running booster vacuum unplugged. Reconnect booster vacuum pedal disappears. Thinking I may have to swap it out.

I would guess the seals in the master are leaking internally. As Andrew pointed out it seems the fluid is allowed to flow backwards. Did you put the same size master cylinder on it when you replaced it?

Yes. Ordered it by part number. Been through two of them.... I would’ve assumed if the seals in the master were bypassing technically it should not matter if the vehicle is running or not. Super firm pedal I can stand on it and the brake pedal remains firm. Vehicle off of course. It’s very perplexing not to mention super frustrating... I have bled many vehicles in the past only time I ever had a problem was with clutch slave cylinder in a 944 turbo. Second Was a 79 Volkswagen rabbit with a faulty rear proportioning valve. I have a pressure bleeder and from what I can figure decent software to activate the DSC. I use an Atel 908

Yeah, it has to be a leak somewhere. The booster is vacuum only and has no effect on the hydraulic system other than to increase the pressure. Have you bled the ABS pump yet?

Yes I believe I have the air out.

Question
Can you get a hard stable pedal with the engine off?
If yes and the pedal drop only happens with the engine running, then look up stream from the master and booster, vac line, etc
I can not tell what your year, make, model is as there is nothing in your signature line
On the 4.8is application there is a right side cam driven vacuume pump, when it is dry for an extended period of time it fails
See realoem for exploded visuals
On theses cars, a bad booster, vac leaks will give you a soft pedal engine on!

+1
On the vacuum pump (458€) but since I just finished this job last night...

Check closely and see if you hear a noise like gurgling/boiling behind the steering wheel then your pump (4.8iS) is shot and most likely the booster although I am not sure which failed 1st.

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