Brake Master Cylinder Options

[R3VM3UP]

I’ve been battling a sinking pedal for some time now and I suspect it may be the master cylinder. I have a 2002 MK20 car. The reasonably priced TRW option is sold out everywhere, not sure if it’s NLA or what.

Are there rebuild options or master cylinders from a different model that will fit?

[AWE46M3]

#MK20Life... haha. Ran into the same issue when I installed my BBK and it was super costly chasing the culprit. I ended up replacing my master cylinder, brake booster, and bled the brakes 5+ times and also got the ABS bled before I learned about the MK20 braking system vs MK60. Once my brake pads were bedded in my pedal feel improved quite a bit.

Are you running a BBK or stock brakes and rotors? Has your brake booster been ruled out?

[dl.m3]

Mk20 life indeed. Changed master cylinder to TRW, running megane/996 bbk on Ferodo pads. Pedal is squishy no matter how many times I bleed.

Buddy with similar setup as me changed everything including booster also, MK20. Same shitty pedal feel.

Friend's MK60 cars have no issue. I've learned to live with it now, kind of gotten used to the longer pedal travel at this point.


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[R3VM3UP]

Calipers and rotors are stock. I’d like to hear an explanation for how the booster could cause a soft pedal.

[AWE46M3]

A brake booster increases the force applied to the brake pedal, amplifying the hydraulic pressure delivered to the master cylinder. The brake booster is 21 years old... IRRC the master cylinder is relatively cheap. If you replace the master cylinder and the pedal doesn't improve, replace the brake booster. If you replace both and nothing improves, #MK20Life

The booster works off of a vacuum and if the part deteriorated with age you might not be getting the vacuum pressure you need, resulting in a sinking feeling. Do you hear a hissing noise if you pump your brakes when the car is off? If so, it could be your booster...

Here is a good Youtube video:

[fattycharged]

A brake booster increases the force applied to the brake pedal, amplifying the hydraulic pressure delivered to the master cylinder. The brake booster is 21 years old... IRRC the master cylinder is relatively cheap. If you replace the master cylinder and the pedal doesn't improve, replace the brake booster. If you replace both and nothing improves, #MK20Life

The booster works off of a vacuum and if the part deteriorated with age you might not be getting the vacuum pressure you need, resulting in a sinking feeling. Do you hear a hissing noise if you pump your brakes when the car is off? If so, it could be your booster...

Here is a good Youtube video:

SO I haven't driven mine for a VERY long time, just moving in and out of garage when I needed the space. I have the straight hissing squish noise when I hit the pedal and I was really hoping that it was just a check valve and not the entire booster that needs replacing...I had read these were pretty robust and it wasn't typically the entire booster, but more likely the valve or gasket that needed replacement.

[R3VM3UP]

How would a leaky booster possibly cause a soft pedal? I’m familiar with how they work…

A leaky booster provides LESS assist, resulting in a firmer pedal.

[bigjae46]

There should be a rebuild kit for the master
cylinder somewhere

[Arith2]

SO I haven't driven mine for a VERY long time, just moving in and out of garage when I needed the space. I have the straight hissing squish noise when I hit the pedal and I was really hoping that it was just a check valve and not the entire booster that needs replacing...I had read these were pretty robust and it wasn't typically the entire booster, but more likely the valve or gasket that needed replacement.

I doubt you have a smoke machine but you could somehow smoke test it. Vape into the line or something. The check valve should only work one way. Also take a long zip tie and check the bottom of the booster for fluid.

[R3VM3UP]

There should be a rebuild kit for the master
cylinder somewhere

I would be very interested in this but I have not had any luck finding one. If anyone has a lead definitely let me know.

[bigjae46]

I would be very interested in this but I have not had any luck finding one. If anyone has a lead definitely let me know.

Sometimes, the rebuild kit is based on the aftermarket part. If the master cylinder is a TRW, then look for a rebuild kit using the TRW part number.

I think it's rare for a master cylinder to go bad. Worst case put a used one on. BMWs get more brake fluid flushes than most manufacturers so most should be in good shape.

[fattycharged]

I doubt you have a smoke machine but you could somehow smoke test it. Vape into the line or something. The check valve should only work one way. Also take a long zip tie and check the bottom of the booster for fluid.

I definitely do not have one in the home garage. The M is fully pulled apart at the moment, won't be addressing this for a bit as I manage to get an hour here and there to fix things when the fam goes to bed.