Brake Pedal Slowly Sinking When Braking

[workingonit]

Originally Posted by Peezen

Thank you so much for the input. I have a master cylinder on the way and will install it. I have the BMW specific Foxwell reader, any specific way to bleed the ABS? Do I still prime the MC with these cars?

I have both BMW and General Motors programs on my NT510 Elite; ABS bleeding was included in the GM software, as it is also in the BMW software, as Ben Fenner stated in this thread https://xoutpost.com/bmw-sav-forums/...oids-help.html.

Note on 10-23-23: I can confirm that the brake bleed function is on my NT510 Elite; I saw it while testing my scan tool today. https://xoutpost.com/1234098-post8.html

Ben got it to work, he says, and I sorta got the ABS bleed to work (somewhat) on my wife's old Cobalt. Oh, you do need to bench bleed a new master cylinder before installation https://youtu.be/qnsgCZVww54, though since I had no bench to work from, I did the MC priming via the brake pedal (with help from my neighbor).

[Peezen]

So I used ISTA to go through the brake bleeding procedure a few times. The pedal still slowly sinks to the floor when braking. A few pumps firms it up, but it will sink back down. I have run almost 3 litres of brake fluid through now.

One odd thing is that I will fill the reservoir to full and then it will drop slowly to half, then I fill it and it drops again. I did that three times before it stayed at full.

There are no visible leaks anywhere, and it doesn’t look like it’s leaking into the brake booster. I have replaced the MC, the check valve, and the booster.

I have no idea what to do now. I made an appointment at a local shop to bleed the system and give it a look. I hate doing it but I am at a complete loss.

Any further tips would be helpful.

[80stech]

Make sure you explain to the shop what the problem is and what you have done.

[Peezen]

I will, thanks for all the advice and help so far. I have never had this much trouble with an MC and brake booster replacement!

I’ll report back when I know more

[80stech]

I think that one of the problems that happen with these brake threads is that people think that the scan tool is actually activating the ABS pump to bleed the brakes. The brakes still need to be bled the normal way with the scan tool activating the ABS only to get rid of the air that might be trapped in that portion of the circuit. Not saying that is necessarily the case here but just thought I would get that out

[Peezen]

Originally Posted by 80stech

I think that one of the problems that happen with these brake threads is that people think that the scan tool is actually activating the ABS pump to bleed the brakes. The brakes still need to be bled the normal way with the scan tool activating the ABS only to get rid of the air that might be trapped in that portion of the circuit. Not saying that is necessarily the case here but just thought I would get that out

Could be the case for some people for sure.

I ended up following the service process in ISTA that activated the pump as well as instructed pushing down the pedal with valves open.

It says to pump the brakes until no air comes out. I always thought that you were supposed to only have the valve open when the pedal was being pushed down but it’s doesn’t mention that distinction in the instructions.

[80stech]

Yeah, pumping brakes till no air comes out is wrong. That can be done as a bit of a cheat to get by when you are alone and know what you are doing (pumping the pedal progressively less) but definitely not for a system that is giving trouble. What you are thinking is right.

I think a lot of that kind of thing is translation nuances. Still need to use some common sense and experience usually wins

[Peezen]

Ok so I took it to the shop and they said the bled the brakes and machine bled them but the pedal is still soft.

Said that they suspect that the MC is faulty or wasn’t bench bled properly before install. It is a brand new MC and I did somewhat bench bleed the MC though I’m not sure if I did it enough.

Is this not something a shop could take care of?


How aggravating!

[80stech]

Yeh, that doesn't sound right, maybe try a different shop. "Machine bleeding," whatever that means, is not the answer. Maybe find a shop with some older guys who know how to bleed brakes and will know what is going on if there is more to it.

[80stech]

Another thing to check is that there is nothing keeping the master piston from going all the way back, shop should know that though.