128i brake master cylinder

[rowsdower]

2011 128i Sport package with 80,000 miles. Another day, another $200 part to replace!

Today while in line to get on track at a HPDE event, I noticed the pedal went down further than usual. I wasn't totally sure so I brought back to the paddock and sure enough, my pedal slowly sinks to the floor if I hold pressure. My day was done early, as I won't mess around with brakes on a race track. Not 100% = not going on track.

Seems like I'm due for a new master cylinder. This doesn't appear like a super difficult job. The master cylinder is right there when you open the hood. Is there anything else I should be looking to replace while I'm in there? I don't think there is anything wrong with the brake booster but this is my first time messing with the master cylinder side of a brake system so I thought I'd ask before diving into this.

[02Pilot]

Your life is going to be a lot easier if you have a pressure bleeder. Getting all the air out of the master and the downstream ABS pump can be annoying (general statement - I haven't dealt with it on my 128i yet). If the booster holds vacuum it's fine.

How often have you changed your brake fluid? Just curious about expected longevity of the m/c.

[rowsdower]

Every couple of years. Most recently was in July of 2018, as I was prepping the car for track days. Prior to that was March 2017, then May 2015. I do use a Schwaben pressure bleeder so it shouldn't be a result of pushing the pedal all the way to the floor during bleeding.

My best guess is that I've started doing HPDEs recently, about 8 in the past 6 months. As part of that I'm likely pushing the brake pedal farther down than I had been doing on the street the prior 5 years and messing up the internal seal.

Thanks for your help about the ABS module and booster.

[juld0zer]

You are supposed to replace the two locknuts and the oring seal between the master and booster.

Are you replacing it with a new master or a used one? New ones don't usually come with reservoirs so you may need seals there too.

Here's the official instructions, for a 135i but the procedure is identical:
https://www.newtis.info/tisv2/a/en/e...linder/GMvJzza

If your master is brand new then I would bench bleed it before fitting, but it will be messy to install if you don't have the right sized plugs.

I have done master swaps without a DSC bleed before. Just unmount all your calipers, insert a piece of wood the same thickness as the discs between the pads and pump the pedal to extend the pistons. Swap your master (donor or brand new, bench bled). Tighten up the hydraulic lines, fill the reservoir to minimum level, then push all the pistons back home. Top up as required and check the pedal. Usually the pedal is good but be prepared to power bleed and DSC bleed it using INPA if it isn't.

[rowsdower]

Originally Posted by juld0zer

You are supposed to replace the two locknuts and the oring seal between the master and booster.

Are you replacing it with a new master or a used one? New ones don't usually come with reservoirs so you may need seals there too.

Here's the official instructions, for a 135i but the procedure is identical:
https://www.newtis.info/tisv2/a/en/e...linder/GMvJzza

If your master is brand new then I would bench bleed it before fitting, but it will be messy to install if you don't have the right sized plugs.

I have done master swaps without a DSC bleed before. Just unmount all your calipers, insert a piece of wood the same thickness as the discs between the pads and pump the pedal to extend the pistons. Swap your master (donor or brand new, bench bled). Tighten up the hydraulic lines, fill the reservoir to minimum level, then push all the pistons back home. Top up as required and check the pedal. Usually the pedal is good but be prepared to power bleed and DSC bleed it using INPA if it isn't.

Thanks for the advice! I ordered a new one and it appears to include the locknuts and the seal to the booster. Really good idea about pushing the pistons back in to drive fluid back up into the master cylinder. I hadn't thought of that! I'll give it a shot.