Why don't my rear brakes work?

[fronton]

Cliff:
My front brakes are strong and the pedal feels perfect but rust is forming on my rear discs. Bad MC, calipers, or user error?

Details:
Ever since my swap, there has been a braking issues, it just keep changing almost every test drive. For spacing issues, I started with an iX booster and iX MC. The brakes were great but my idle was finicky due to either a dying booster or poor vacuum connections related to the booster. Often I had to keep giving it gas while idling or slowing down or it could cut out.

I took my car in for and alignment and when I got the car back, the brake pedal was rock hard and obviously my booster was completely toast. Idle issues were gone though. They claimed no responsibly and said it was working great five minutes earlier. right.

So I switch to an e32 double booster and keep the iX MC. I swap the old iX booster's clevis on because the e32's was too long.

At the same time, again for spacing issues, I deleted my ABS by simply removing the unit and putting a bmw T connector (like the rear brakes have) for the two front hard lines to each end and the center connection to the front of the MC. NOTE: upon checking the rear T connector today I see the the left rear caliper line is connected to the bottom of the T and the right rear caliper line is the the right side of the T while the left connection goes to the MC. This is a different orientation than how I set up the front. Relevant?

So with the iX MC/e32 booster combo, new shorter lines and a T connection, I power bleed the brakes with full liter of new super blue. The brakes felt decent and I put about 2 miles on them. A week and zero miles later they were feeling great in the driveway so I take a longer spin. About 5 miles later I can smell brake pads and feel serious fiction trying to coast to a stop sign. The driver rear brake seems to have the strongest smell and feel the warmest but its hard to tell. I less it cool down for a couple minute then fight at least one locked caliper about a mile home and just park it, very frustrated.

Next morning the car rolls smoothly in neutral with all the calipers playing along innocently but I don't take it out until a week later, being today. After driving for a few miles I have no issues and pull into the driveway only to feel the neutral friction again but not too intense. No smells so I check all the wheels and the fronts are normal temp but the rears are not even warm. Then I notice there is rust dots forming on both rear discs, clearly not getting contact.

Any thoughts on my problem?

My guess is a bad MC but there are no leaks and the pedal is firm and effective, for the fronts anyway. I have not done a second bleed yet but since the pedal isn't squishy at all, I don't think there is air in the lines. If there was, wouldn't my rears still work just all 4 not being great? Is it possible for a bad bleed to cause a caliper to stick, or not allow the rears to work at all?

Could it be that a iX MC is not compatible with the e32 booster? Perhaps the push rod isn't going into the MC deep enough? I find this unlikely because everyone uses a E32 MC with a iX booster, and the boosters are identical dimensions.

[bmwstudent]

sounds like you might of had a sticky caliper on the five mile ride. It shouldn't make any difference what orientation the lines go into the t-block, pressure is pressure. Can you have someone step on the pedal while you watch the brake lines or calipers for moment/deflection? That will tell you if your getting pressure to the rear brakes. Can you jack up the car and spin the wheels freely?

[fronton]

I ordered a new MC hoping that will fix it but am curious about it possibly being bad soft lines. Has anyone had these issues and fixed them with new lines, or SS lines? They are about the same price so I might as well go SS.

Any preference of Stoptech over Bimmerworld's or ECS tuning's ss lines?

[jlevie]

Since you deleted the ABS, the cause just about has to be a bad master cylinder or a bad bias valve.

[fronton]

deleting the ABS wouldn't cause the MC to go bad though would it? I've don't see how it would affect it at all. If so, am I just going to keep blowing through them without ABS?

[jlevie]

A bad ABS unit could cause either of the front brakes or both of the rear brakes not to work. But with it gone the culprits are more likely to be the master cylinder or bias valve. Well, a blocked line to the tee over the rear subframe is a possibility.

[AndrewBird]

As far as the brakes coming on over time, check to make sure the clevis at the brake pedal is adjusted correctly. If it is too far out (too long) it will slowly engage the brakes until they lock. Letting the car sit and letting the vacuum pressure bleed off fixes it.

[fronton]

As far as the brakes coming on over time, check to make sure the clevis at the brake pedal is adjusted correctly. If it is too far out (too long) it will slowly engage the brakes until they lock. Letting the car sit and letting the vacuum pressure bleed off fixes it.

I did mess with the clevis when I swapped the iX booster our for the e32 booster. Since the e32 had a crazy long clevis I put the iX's shorter clevis on, but then my brake pedal was 0.5" lower than the clutch pedal. So I adjusted it all the way out until it was flush with the clutch pedal and there is zero play if I pull the pedal toward me from behind.

Do I need to back it down a little?

[fronton]

I have now put on a brand new girling M5 master cylinder on the M5 booster and power bled the brakes. There is no squish, and I stop very well. However, after a 10 mile test cruise, there is no sign of the rust specs vanishing or being touched at all on the rear calipers. I thought if they are now working it would take maybe two or three good brake applications at speed to make them shiny.

The rotors were about half as warm in the rear as the fronts where after the drive. Can it take a while for the rust specs to come off or are my rears needing rebuilding?

I did adjust the booster rod clevis down about 3mm which may have solved the sticking, or maybe it was the new MC. Either way, they feel great now I just want to verify that they are functioning.

[2761377]

when you say you power bled the brakes you mean with a pressure bleeder, right? when you did this did you get a solid stream of new, clear fluid out of both rear calipers?

assuming your car is like my '91 318iS there are a total of 4 rubber hoses in the rear circuit. they collapse after time and need replaced.

if you get weak or no stream while bleeding, those hoses likely need replaced.

also, you might help diagnose by pedal pumping to bleed. if you get a solid stream with pedal pressure it's likely your problem(s) is/are the calipers themselves.

hth
robert w.

[fronton]

yes, clean new super blue was flowing steadily out of all four calipers with the pressure bleeder as well as when pumping the brake with each bleeder open. This is the second liter of super blue I have run through the system in two weeks trying to troubleshoot.

[jlevie]

With a new master cylinder in place I'd suspect the brake bias valve. It's purpose is to reduce the pressure applied to the rear brakes to balance out braking bias.

[fronton]

someone please confirm that I have my lines to the MC correct, not crossed. the lines from the front brake's T valve is going to the valve on the MC that is furthest from the booster while my rear brake line is connected to the valve closest to the booster. Please tell me I just have them the crossed!

[fronton]

Bentely says the front chamber of the MC is for the front brakes and rear for rear. But which end is the 'front' of the MC? If it is the first chamber, closest to the booster then its diagram is misleading and I have mine crossed. Anyone?

[fronton]

I have switched the MC lines so that the fronts now connect to the port closest to the booster and the rear lines from the bias valve goes to the port on the end of the MC. Rebled, 10 miles test drive, no rear brakes still.

Now I don't know if I should switch the lines back the other way and keep trouble shooting or leave them and go from here. So frustrating.