E24 Brake bomb or master cylinder???

[L33 AEX]

Hi all,

I imagine this has been posted many times before, I have a 88/89 E24 635csi and I’m having some issues with the brakes, when cold and first started there’s no issues it’s great, when it gets hot ( engine hot ) the travel in the brakes is a joke, it’s more than half way travel before it actually does anything, now I’ve done all the tests on the bomb and to be honest it all seems ok, also when the problem accurs I’ve tested the bomb & it seems like if I kept pumping the brake the ATF would overfill although I’ve never had to top it up. Then when started it drops quite rapidly. Would you be looking at master cylinder or can anyone give me some advice on what to look at please. Thank you and any help is appreciated.

[Dschart]

There is a solid rod which runs straight through the hyd brake booster. This means that distance travelled between brake pedal and master cylinder is not affected by the hyd brake booster. It only affects force applied, ie it amplifies the force from the pedal to the mc, but does not change distance. If you have too much pedal travel (distance), then you have a problem with the mc pistons not holding, or have air in the lines or calipers or ABS actuator, or you have worn pads/warped rotors, bulging rubber brake lines.

The fact that it changes from cold to warm I believe is a function of the force applied by the booster changing thus making the issue more perceivable. I bet you would get the same excessive travel cold if you pushed harder with your foot.

Sent from my moto g(7) power using Tapatalk

[L33 AEX]

Hello thanks for your reply, it’s a really strange one, its really firm initially but after the cars been idling for a while there’s travel then it firms up, I’ve removed the “bomb” and going to change is as a process of elimination, I don’t really want to be getting to far into the master cylinder as there quite expensive and rather complex to rebuild. Thank you I will update in due.

[carsnplanes]

Hi all,

I imagine this has been posted many times before, I have a 88/89 E24 635csi and I’m having some issues with the brakes, when cold and first started there’s no issues it’s great, when it gets hot ( engine hot ) the travel in the brakes is a joke, it’s more than half way travel before it actually does anything, now I’ve done all the tests on the bomb and to be honest it all seems ok, also when the problem accurs I’ve tested the bomb & it seems like if I kept pumping the brake the ATF would overfill although I’ve never had to top it up. Then when started it drops quite rapidly. Would you be looking at master cylinder or can anyone give me some advice on what to look at please. Thank you and any help is appreciated.

Rising and falling levels in the reservoir is normal when you pressurize or release the pressure in the brake bomb by pumping the brakes. When the engine is running, the hyd pump pressurizes the system including the brake accumulator, aka "Bomb". The pressure works against a rubber or thin metal diaphragm in the accumulator. On the other side of the diaphragm is a certain amount of nitrogen gas ( I think about 1250 psi). So the pump pressurizes the accumulator and the accumulator pressurizes the brake system(and steering). As the internal diaphragm flexes internally with pump pressure, the fluid level in the reservoir will drop. This is the "working pressure" and level. With pump not running, the nitrogen pressure in the accumulator tries to do the pressurizing of the system when the pump is not running and thus pushes what is on the fluid side out to the system and back into the reservoir. This will make the reservoir rise to a final level. This is the static pressure and level. This is the level you keep and maintain the reservoir, which is about 1/2-3/4 in below the rim of the reservoir.
The accumulator can leak its nitrogen gas out over time or the diaphragm can stretch which lowers the gas pressurized side and then you will be using the hyd pump to do most of the brake and steering work rather than the muscle of the accumulator. If this is the case, you can hear the pressure switch/valve working each time you brake and because you have no reserves of pressure from the accumulator, you will not have the 20 brake pump assists from the accumulator when the engine is off.
How many brake assist applications/pumps do you have after you shut the engine off before the pedal gets high and hard again?

Think of the hydraulic pump and accumulator as an alternator and a battery where the battery is the accumulator. Both the battery and accumulator keep a nice steady level of voltage or pressure.

Perhaps your "firmness" is due to no assist for some reason when you first start. It could mean you accumulator pressure depletes overnight or that your pump takes a while to pressurize the accumulator.
When "firm", do you actually get good braking too? ..or just a high pedal?

[1986series6]

Hello thanks for your reply, it’s a really strange one, its really firm initially but after the cars been idling for a while there’s travel then it firms up, I’ve removed the “bomb” and going to change is as a process of elimination, I don’t really want to be getting to far into the master cylinder as there quite expensive and rather complex to rebuild. Thank you I will update in due.

With all due respect, when it comes to a safety issue as brakes no expense or complexity should prevent a fix.

I read that you don't want to but you may have to.

[DesktopDave]

+1, I'm with CnP: I think the hydro system is working properly, believe it or not. When you pump the system pressure up, it'll drain the reservoir to fill the accumulator chamber. When the regulator valve releases pressure, all that fluid gets pumped back into the reservoir. I've been fooled a few times, it's tricky to check the actual level. Weird little system, but I'm willing to put up with it. I like the way it makes my brakes feel. That ATE hydro system is simple in theory, but it's complex in practice.

There's a member here that can advise you on troubleshooting, Jay (JCS?). Hopefully he'll stop by. He also offers quite a few rebuild services to the classic BMW community at large, here's some info about that. Rebuilt hydro components for these cars are eye-watering expensive.

You didn't mention any brake binding, but if that's what you're getting, it could be problems inside the master cylinder. Apparently there's a plastic piece in the middle of the chambers that binds when it gets warm. Brucey (as is his style) posted an outstanding writeup over at BigCoupe here.

[DaHose]

Over on bigcoupe.com we have documented the brake testing/repairing procedure many times. Look at the thread below.

http://bigcoupe.com/phpBB2/viewtopic...ng+brake+pedal

Sounds like you tested the bomb correctly. Proper fill level is about 5mm from top of reservoir lip with bomb fully drained.

Some pedal drop is not tragic, but if you question that the MC may be faulty, I actually bought a rebuilt from Oreilly (with lifetime warranty) years ago. It has worked just fine. Alternately, you can rebuild yours, provided the body is not corroded or somehow damaged. Definitely read closely when Brucey says ANYTHING over on BigCoupe. That dude KNOWS what he is doing with E24's.

The last place to look is scary, because it's the ABS pump. If anything is wrong there, you can't get a new one. You would have to find someone who can rebuild, or find a good used pump.

Jose