Brake bias

[greatoldone]

I had some issues on the way back from Stoneleigh. I believe I have a brake imbalance, which is only coming to light on longer runs since I fitted the dual booster.

Vikkis rear brakes where binding, and where - if not red hot - bloody hot and smoking when I got home. There was a distinct lack of oomph from her, as she was continually fighting the friction of the brakes.

This could be down to sticky calipers - and I won't know for sure until I take a look at them... But with the mileage I've done, and the lack of problems prior to the booster change, I think it could be residual pressure building up in the rear lines, keeping them engaged.

I'm wondering if I need to fit the brake pressure regulator that was on the donor, to reduce the pressure to the rear brakes. Thing is, I didn't keep mine - so I need to source one. Does anyone have one to hand?

BMW part # 34331152494, part 5 on the diagram below:

http://www.realoem.com/bmw/showparts.do?model=1232&mospid=47263&btnr=34_0152&hg=34&fg=15

Everything I find online is in the states - so assume that the only 'new' source will be at the parts desk.

[Mike]

Re: Brake bias

Jason

Can you call me please.

Mike

[greatoldone]

Re: Brake bias

Right. I attempted to fix this yesterday, but to no avail so far - I believe I need to fit some return springs.

My problem seems to differ from everyone elses, in that my brake servo rod and clevis pin lines up exactly with the pedal - it's not offset. The clevis fits correctly either side of the pedal lever... It so spot on that I wouldn't be able to fit the rose joint I now have with out taking it out of alignment, or radically modifying the pedal.

I've adjusted the clevis on the input rod now so that it's not so difficult to engage the pin (it means the pedal sits a little lower, but not so much that the brake lights are on) and adjusted the output rod so it's not bang up on the master cylinder. The latter adjustment hasn't had any affect - apart from making the pedal travel more before the brakes engage... so I'll be setting that back the way it was.

The issue is manifesting itself exactly like Mike has mentioned, so I do know it's the servo / pedal combo causing this, not the calipers. If I feel any hesitation from her, I know the pedal is still slightly down even if I've taken my foot off it. Putting my toes underneath the brake pedal and lifting it up when this occurs has an immediate effect, and she takes off like she should.

I've done some Googling on this, and apparently it's a well known issue with these brake servos in the US hot rod scene, where they're fitted to wildly differing vehicles, much the same as we have. There isn't a flaw in the servo, it's just that some residual vacuum in the servo can keep the pedal slightly depressed. The fix these chaps do is to have a spring on the pedal to pull it back up to it's normal position. To this end, I have two land-rover brake pedal tension springs that I'm going to fit (somehow) to do this.

If they can pull the pedal up in the same way I lift the pedal (and it doesn't take that much force), it should be fine.

The only problem is I'm going to have to take the pedal box out, as I can't (even with my angle drive on the drill) get in to drill the pedal / box for the springs with it in situ. Joy.

I'll report back after much swearing.

[jon_cox]

Re: Brake bias

Could you not fabricate a bracket arrangement such as this. This would be a lot easier than removing the pedal box. To be honest, I am surprised that everyone else's systems work correctly WITHOUT a return spring. It is standard practice to fit one to in the OEM market.

[Mike]

Re: Brake bias

This is really useful information Jon, and the solution looks simple and elegant, thank you.

I have fitted a weak barrel spring (from a spare throttle body), and found it is not strong enough. Maybe with a pedal ratio of at least 5:1 a stronger spring will still not be noticed compared to the forces generated by ones right foot!

It will be interesting to see if Jason needs both springs, or that one is sufficient.

Could you not fabricate a bracket arrangement such as this. This would be a lot easier than removing the pedal box. To be honest, I am surprised that everyone else's systems work correctly WITHOUT a return spring. It is standard practice to fit one to in the OEM market.

Mike

[greatoldone]

Re: Brake bias

Thanks for that pic Jon... That's essentially what I planned on doing, albeit with a twist.

I did a bit of thinking over my lunch hour, and came up with the idea of fabricating a longer clevis pin that will extend either side of the pedal & clevis, with a spring leading to both sides. Then they just need anchoring within the pedal box somewhere, so they pull back on the pedal.

But this is something I can try first, with a single spring. If it's not sufficient, I'll switch to my plan B.

[jon_cox]

Re: Brake bias

Hi Jason,

For testing purposes you could just hook the end of the spring through the hole in the clevis plan inplace of the R clip. This has been known to be an OEM solution. Only issue is if the spring snaps and the clevis pin falls out !!!!!!!!!!

[greatoldone]

Re: Brake bias

It's fixed

It only took one spring, and I basically followed the same idea as in Jon's picture. I made a small plate that sits on the clevis pin that has a hole for the spring to sit in, and then hooked the other end of it to a bracket I mounted on an existing bolt:





The plate on the clevis pin sits in between the clevis forks. I couldn't get a photo from the side, so here's a quick sketch through the side of the pedal box:



The clevis is in yellow, the pin in blue, the spring 'plate' and bracket are in green, and the brake light switch in purple

Anyway - it works like a charm. After fitting it after work today, I blatted round town for 45 mins in all sorts of traffic (fast dual carriage way, town centre, etc etc), and it's been fine. Pedal comes up fully and immediately after taking your foot off, and there's no noticeable extra effort in pushing the pedal down now there's a spring on it. No more binding / squeaky brakes, takes off like missile now, probably doing the world of good to the fuel economy. Happy