Big brakes big problems [E46 BMW]

[bikealarmblair]

I've fitted bigger brakes to my car (01 325ci with an LS1 5.7l) 4 pot APs on the front & 2 pot APs on the rear my problem is the pedal moves half before the rears do anything (the fronts are working fine) all the calipers are new so their not siezed etc they have been bled lots.... the problem is there with the engine on or off but when the car is running (servo) the problem is worse. The master cyl has been stripped & rebuilt just in case. I was thinking of using an M master cylinder but the rear bore is smaller than the 325 one for some reason.

Anybody had a similar problem?

Blair

[steve6304]

sounds as though the master cylinder does not have enough capacity, hence the long pedal, check with caliper suppliers for the correct master cylinder. If you fit the wrong cylinder not only could you have a long pedal travel, but the front-rear brake balance could be wrong.

[Nick3point2]

If you have 4 pots on the front and 2 pots on the back you have probably substantially increased the volume of fluid in your system.

I'd say switch to twin master cylinders for the brakes and a front/rear bias bar. This way you get sufficient flow and can balance the force generated from each axle, which will bring out the best in what is a high spec calliper set up. Frankly, without twin master cylinders I wouldn't bother upgrading the brakes to the degree you have!

Looking into an AP racing pedal box, they will be the best people to know how to size your master cylinders based on the calliper set up you have.

[RLK500]

I had a similar issue when I put AP's on my e30 M3. I only did the fronts and couldn't get any braking power at all. Spongy pedal, horrible. I think the m/c was already a bit knackered but replacing it with the one from a 750 (popular upgrade - 23mm to 25mm) sorted it.

However, as per the other poster. If like you I had done both ends, I would go for a dual m/c setup with bias adjustment. Pretty important to get them well balanced and to match your driving style.

[bikealarmblair]

I've still not got to the bottom of this, it's not the master cylinder or the callipers that are at fault spoke to a couple of AP Racing dealers & they had done this upgrade many times with no issues. A friend has calculated how much fluid is required by each calliper & the AP's are to within a few mm of the std BMW callipers.

I've spent some time with INPA (BMW software) & there are options for bleading the abs unit which we have to no avail, what I have noticed is there was a very small amount of fluid in the abs wiring plug as we haven't touched the fluid connections for a few weeks on the abs unit I guess it may have a small leak so I'm going to source another unit & try that next

[bikealarmblair]

Sorted!!!

Problem 1 was a faulty ABS module but this didn't change pedal feel.. turns out the braided lines were to big (-4 when -3 was specified) the correct sized lines have been fitted & I now have a brake pedal & decent brakes

[Mr2Mike]

How did the original braided lines give such a large increase to pedal travel? The only way this could happen is if were ballooning under pressure, which tends to suggest not only were they the wrong bore but the wrong specification of tubing. Seems more likely the extra bleeding you had to do when replacing the brake lines got out air that was originally present.

[bikealarmblair]

Mr2Mike said:

How did the original braided lines give such a large increase to pedal travel? The only way this could happen is if were ballooning under pressure, which tends to suggest not only were they the wrong bore but the wrong specification of tubing. Seems more likely the extra bleeding you had to do when replacing the brake lines got out air that was originally present.

Bigger bore means less pressure for the same amount of fluid moved.

[Mr2Mike]

bikealarmblair said:

Bigger bore means less pressure for the same amount of fluid moved.

Sorry but this is irrelevant. A pressure drop only occurs when fluid is flowing through a restriction. Once the brakes have been applied, fluid is no longer flowing so there will be no pressure drop. The bore size of the brake hose will make absolutely no difference whatsoever in terms of the pressure seen at the calliper in this situation, provided the hose does not swell so much that you run out of pedal travel first.

However, a larger bore hose has to cope with a greater force for a given pressure (since it has greater internal area).

[bikealarmblair]

I'm explaining it as it was explained to me, I can tell you 2 things
1. There was no air in the system
2. Changing to the smaller bore pipe has cured the problem