Finding a good brake master cylinder?

[pizzaman09]

I am looking to replace the brake master cylinder on my 99 M3 and am having trouble finding a new ATE one for sale. Are the TRW or Centric ones any good?
I have all TRW brakes components on my Austin Healey and they seem to be quality components. The rebuilt Centric calipers on my M3 haven't impressed me.

The back story is, I am having issues with randomly locking up rear brakes. Some days they both work perfectly, others the left or right or both with not retract and get very hot. The calipers are not rusty, and move freely, I replaced them about 12 months ago.

[NeilM]

I am looking to replace the brake master cylinder on my 99 M3 and am having trouble finding a new ATE one for sale. Are the TRW or Centric ones any good?
I have all TRW brakes components on my Austin Healey and they seem to be quality components. The rebuilt Centric calipers on my M3 haven't impressed me.

The back story is, I am having issues with randomly locking up rear brakes. Some days they both work perfectly, others the left or right or both with not retract and get very hot. The calipers are not rusty, and move freely, I replaced them about 12 months ago.

I'm having difficulty coming up with any credible scenario where the master cylinder causes only the rear calipers to stick. That's good news for you, since the M3 master cylinder is pricey.

The most likely cause is the pistons sticking in the caliper bores — yes, despite the fast that that you'd replaced the calipers. This could be because the rebuilds just aren't any good. You may be able to fix them by rebuilding them yourself with new seals. The rebuild is a bit messy to do, but not hard. The caliper piston isn't available as a BMW part (and you might not need one), but there are stainless steel aftermarket versions.

Another possible, but much less likely cause could be internal deterioration of the flex hoses to the rear calipers. In theory that could close up the hose ID and impede brake fluid back-flow when you get off the brakes. However while I have heard of that happening, I've never heard of it on our cars. BMW uses high quality brake hoses.

Neil

[bluptgm3]

The caliper piston isn't available as a BMW part (and you might not need one), but there are stainless steel aftermarket versions.

BimmerWorld for stainless steel or Centric for OEM.


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[pizzaman09]

The thought had occurred to me that it could be the cheap rebuilt calipers. They were the Centric ones off of rock auto. The lines were replaced at the time of caliper replacement with ECS tuning stainless lines.

The fact that it is both rear brakes that lead me back to thinking it could be a master cylinder.

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If it comes down to calipers, I'll probably buy new oem this time as I am tired of having these issues.

I doubt rebuilding them would do much as they aren't even rusty yet.

[bluptgm3]

I doubt rebuilding them would do much as they aren't even rusty yet.

You won’t really know until they are disassembled.


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[frbz]

The Tech editor of the Roundel recommends replacing the rubber lines at 15 years due to issues with internal swelling and dragging callipers. It is way easier to change the lines than to rebuild the calipers. Give that a trial first

[bluptgm3]

Do both, as you have to remove the caliper from the carrier to remove the line.


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[pizzaman09]

So to follow up and close up this problem, such that someone 10 years from now reading this learns the results.

I did start with replacing the master cylinder, that didn't change anything as others pointed out. However it is nice to know it is fresh and new. I did have to buy the Centric one but honestly the quality was fantastic compared to other Centric products I've purchased.

It still amazes me, but both rear calipers did actually go bad at the same time, they were one year old rebuilt Centric calipers with new ECS tuning brake lines. I did confirm with a pressure gauge that the issue was not hydraulic pressure related, the pressure went to zero at the brake bleeder when you let off the brake pedal. The issue was hanging pistons that weren't returning into the cylinder when hydraulic pressure was released.

I decided that I didn't want to deal with the poor quality issues any more or the $1000 for BMW parts. So I went the aftermarket route and purchased Wilwood rear calipers, mounting brackets, and adapting brake lines. The total price was just under $600 shipped to my door and the quality is fantastic. There were a few fitment struggles, I needed to cut away some of the sheet metal shield behind the rotors so the new slightly longer calipers would fit. Also I needed a single washer between the caliper and Wilwood mounting bracket to give it the correct spacing to be centered on the disk.

The caliper I used was the Wilwood DynaPro 4 Piston Brake Caliper Lug Mount. Part Number 950-120-9693-SI.

The mounting bracket was Wilwood part number 950-250-10901.

The brake line with adaptor fittings was Wilwood 950-220-8800.

There is a huge selection of brake pads that fit these calipers. I got sometime BP-10 Performance Metallic pads. Part number 555-63310.

I mounted this all around rear disk stock sized Power Stop slotted and drilled rotors.

Overall I'm very pleased with the brakes, the brake bias has moved rearward in the car but I'm not going to judge it too much until I break them in a little and get to autocross with them. I am running M3 stock brakes up front with Power Stop Z26 ceramic pads so the imbalance could be largely due to pad type.

I'm extremely happy to be able to drive the car again and happier that I have quality brake parts that aren't going to jamb up like I've had so much issue with in the past. I don't know why oems use cast iron for their calipers, I'd much prefer aluminum for those of us that live in the rust belt.

[bluptgm3]

Should have simply rebuilt the rear calipers with ATE rubber components and BimmerWorld SS caliper pistons, then load all four corners with PF ‘Z’ pads.
Powder coat the calipers if you need rust proofing.


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[pizzaman09]

True, I could have rebuilt the factory calipers for less, but honestly I've had the same rusting and binding issues on the e39 M5 I had and I originally replaced all the calipers/soft lines a year ago because I was having binding issues on the front calipers, just to deal with it a year later on the rears. I just wanted the steel floating calipers gone.

I've really always loved the solid feel of the factory fixed dual piston calipers on my Austin Healey Sprite, so I wanted so see if I could impart some of the awesome firm pedal goodness into the M3. If I really enjoy this setup I may try to adapt some Wilwood fixed calipers to the front as well.

[trading10]

Does Wilwood preset the brake bias? Or is this something we can do correctly ourselves? Did you or will you have toe swap out the master cylinder for a larger bore to accommodate the extra pistons?

[pizzaman09]

Does Wilwood preset the brake bias? Or is this something we can do correctly ourselves? Did you or will you have toe swap out the master cylinder for a larger bore to accommodate the extra pistons?

I would recommend a larger master and possibly a proportioning valve if you want to maintain the factory balance. I actually prefer the rear bias that I gained as it helps me rotate the car into turns when I autocross. However I am not certain the bias I have is due to the piston area or the fact that the rear pads are aggressive and the fronts are low dust ceramic pads that don't have a lot of bite.

I've been fighting a soft pedal with this setup, and I'm fairly convinced it's a result of now having an undersized master cylinder.

[nick325xit 5spd]

I would recommend a larger master and possibly a proportioning valve if you want to maintain the factory balance. I actually prefer the rear bias that I gained as it helps me rotate the car into turns when I autocross. However I am not certain the bias I have is due to the piston area or the fact that the rear pads are aggressive and the fronts are low dust ceramic pads that don't have a lot of bite.

I've been fighting a soft pedal with this setup, and I'm fairly convinced it's a result of now having an undersized master cylinder.

I'm not aware of a readily available larger master cylinder for the E36 M3.

Also, I see absolutely no reason to believe that your soft pedal is due to that. I'd bet that it's a combination of the extreme pad mismatch, and poorly bled rear calipers. Multipiston calipers are MUCH harder to bleed. I can tell you that when I did my 4-wheel Massive kit on my E30 M3 (wilwood based), it took me a few tries to get it bled. In particular, I find that I have the greatest success doing a two-person bleed WITH a pressure bleeder. I generally am not happy with the results if I try to do one or the other by themselves.

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OK, I just looked up the caliper you bought. WTF?!?!?! Those pistons are WAY too big! 1.75" pistons are larger than the stock 40mm, AND you have two of them!* So now your rear calipers have more piston area than the front!

So yes, I take it back. You probably do have a "too small" master cylinder for those calipers. But the problem is that you need to buy different calipers, not a different master cylinder!!!

Edit: * You only use one side of the caliper to calculate bias. A single piston on one side of a floating caliper exerts the same force as opposed dual pistons in a fixed caliper.

[nick325xit 5spd]

So I looked up the Dynapro Lug Mount options.
Your choices are the following:
4.8" piston area (what you have), which is incredibly bad
3.0" piston area, which is 50% more piston area than stock
1.58" piston area, which is somewhat less than stock

For reference, stock is about 2" of piston area.

None of these are particularly good choices. What you want is either 1.12" pistons (stock bias), or 1.25" pistons (a little extra rear). I think that the least bad option is to run the 1.58" caliper with an aggressive pad, but I don't like any of these.

I highly recommend the Wilhelm Race Works brake bias white paper here, which also includes an excellent downloadable excel brake bias calculator spreadsheet:
https://wilhelmraceworks.com/brake-bias