Weak brakes

[Todd Black 88]

I am having a brake issue on my M3, and when I bought my e30 cab that confirmed it, as that car brakes very well

I have always thought the brakes on the M were not as good as they should be, and have trouble getting the abs to engage, even when I try to stomp the pedal hard.

Since I have had the car, all rotors and pads were replaced and calipers completely rebuilt. System was flushed and bled with Ate super blue, and all rubber lines to the calipers and the two over the subframe have been replaced with Goodridge stainless lines.

Fast forward to this season, and I took the car for a warm up drive around the neighbourhood to scrub any surface rust off everything, then went for a run up to grouse mountain from my house out by Seymour mountain. (about 35-40minutes) Got back to the house and pulled the wheels off as the brakes did not sound right to me. When I pulled the wheels off, I found the rear rotors still rusty and the fronts clean. The rear pads look almost new (after 1000 miles on the car).

Is this an ABS pump issue or M/C issue or brake bias valve issue.........do you think?
If I bleed the rears I get fluid fine, so the line or T is not plugged, it just doesn't seem to clamp the rears very well at all. Minimal rust was scrubbed off.

Also, the ATE m/c on my M3 has a circle with the number 30 in it, is that the normal m/c for the car or has it been swapped at some point do you think?

[jlevie]

I'm going to say bias valve and/or master cylinder. Most of the time, a bad bias valve or a bad ABS unit will result in difficulty in bleeding the rear brakes. Since that appears not the be the case, replace the master cylinder.

[Todd Black 88]

Thanks for the reply Jim.

Also, does the fact that I cannot even lock the fronts, or get the abs to engage, if I try to, lead more to the mc also?

[jlevie]

Yes. And once you get the master cylinder off, use a rag tied to a wire to see if brake fluid is inside the booster. It so, stop right there and get a new or rebuilt booster. Brake fluid will destroy the diaphragm in the booster pretty quickly

[Todd Black 88]

Thanks Jim.
I have ordered the 25mm mc kit for the m3 from bimmerworld.

I don't have a loss of fluid anywhere, but I will definately check the inside of the booster.

[Todd Black 88]

Ok, I replaced the m/c with a new 750, 25mm one, with the corresponding new line to the abs block. Bled the system with a buddy who races Pro3. No different. Pedal even feels soft to me, compared to what I have heard about the 25mm m/c.

The car brakes, but doesn't bite hard enough to activate the abs in the dry, and is not remotely as good as my e30 cab was.


History.
Calipers were rebuilt, but the rubber guides and pins were not replaced.
New Zimmerman drilled rotors on all four corners
Goodridge SS lines, all 6
Ate type 200 fluid
Booster did not have any fluid in it when I change the m/c.
Bleeds easily at all calipers, so no plugged lines.


Help!

[jlevie]

Are you still seeing evidence of less than optimal rear braking? With the new master cylinder in place the bias valve could be bad if that is the case.

What pads are you using?

[Wanganstyle]

Are you still seeing evidence of less than optimal rear braking? With the new master cylinder in place the bias valve could be bad if that is the case.

What pads are you using?

Brake bias valve would be my guess; friend recently had one fail and it was sticking the fronts; I'd wild guess yours is blocking flow

Sent from my GT-N7100 using Tapatalk 2

[Todd Black 88]

Are you still seeing evidence of less than optimal rear braking? With the new master cylinder in place the bias valve could be bad if that is the case.

What pads are you using?

Iirc I am using textar pads, front and rear. Have not driven it enough to tell much regarding the rears, but wouldn't I be able to lock the fronts simply even with no rears.

Can the booster be a problem somehow?

Brake bias valve would be my guess; friend recently had one fail and it was sticking the fronts; I'd wild guess yours is blocking flow

Sent from my GT-N7100 using Tapatalk 2

Would this make the fronts stop working also, or weak?

[jlevie]

It may be very difficult to get the ABS to engage with those pads in the dry as they or any other OE type pad have little bite. But it should be no problem in the wet. Try a set of Hawk HPS or Stoptech Street performance pads to see a major upgrade in brake performance. Oh yeah, and lose the cross drilled rotors. The holes in the rotors reduce the area swept by the pads and are stress concentrators. For maximum brake performance you want plain OE type rotors.

The bias valve doesn't have any direct effect on the front brakes. But if is plugged or stuck you'll little, if any, rear brakes.

[AndrewBird]

I know the rear calipers are new, but you might check to see if the pistons are sticking and not moving outward.

[Wanganstyle]

It may be very difficult to get the ABS to engage with those pads in the dry as they or any other OE type pad have little bite. But it should be no problem in the wet. Try a set of Hawk HPS or Stoptech Street performance pads to see a major upgrade in brake performance. Oh yeah, and lose the cross drilled rotors. The holes in the rotors reduce the area swept by the pads and are stress concentrators. For maximum brake performance you want plain OE type rotors.

The bias valve doesn't have any direct effect on the front brakes. But if is plugged or stuck you'll little, if any, rear brakes.

very easy to engage ABS with OE pads; borrow some garbage rubber or wet the surface:)

if the prop valve is angry than its possible that not enough pressure is getting through it

[Todd Black 88]

Thanks guys!

I will source some new Hawk HPS pads and probably swap out or remove the bias valve completely if that doesn't work.

The rotors only have 1000 miles on them, do I need to turn them for new pads, or can I scuff them a little with ascotchbrite or something instead?

[Todd Black 88]

Oh, and it's supposed to rain Thursday, so I am going to jump on them in the wet to test the abs, or at least lock them up.

[monty23psk]

Have you installed the new MC?