Brake booster / master cyl adaptor

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Guys, just something to watch out for. Today during a motorkhana I had a brake failure. My foot almost went through the firewall. Now most of the issue is my fault as I was frustrated by my lack of ability today and took it out on the brakes. However the failure has highlighted a weak point in some brake systems.

 

Some boosters have the bolts horizontally and some vertically.

 

My booster has them horizontally, while the master cyl has vertical bolts. This creates the need for an adaptor. The adaptor I used is OEM off some Nissan (I lost track which parts I got from which car). It is an alloy of some sort and as it turns out not strong enough.

 

As you can see in the attached photo it broke into 3 pieces allowing the master cylinder to float about, supported by the brake lines, not even touching the booster.

 

You can also see I have found in my workshop a piece of steel that is exactly the correct thickness and has enough extra length and width to give me confidence I will never encounter that problem again. I have already done about 60% of the work on it.

 

So this post has dual purposes. Firstly to warn others not to trust these allow adaptors.

 

Secondly to ask a question. Why is it, my brakes locked on when the master cylinder was floating around. Is that a safety mechanism built into master cylinders? Or was it a fluke. All I know is every time I unbolted the master cyl the car would not move, then as soon as I bolted it on again the car drove normally.

 

Any way thats it for now.

[Scoota G]

Aww gawd. Thats unreal.

[Whittie]

Wowsers!

 

I'm not even sure how that is possible!

 

Good to hear you and the car are OK though! Imagine if that had happened at Bathurst in the esses or dipper!

 

My brake booster has the same IDENTICAL adapter. I know this because I just pulled it out and put it back in because the 1" master cylinder I bought mounts horizontally and the bolts from the booster aren't long enough. On my brake booster there are 4 bolts coming out of the booster, in the positions of the holes in that adapter. The vertical ones are long and the master cylinder bolts on easily over the adapter with the long threads. The horizontal bolts are short and are long enough for the adapter only.

 

With the way mine mounts there is no way the adapter could burst like that, there simply aren't the forces involved. In fact with my setup (I believe original) I can't even imagine how this would be POSSIBLE unless the two vertical bolts were part of the adapter and didn't originate from the brake booster itself. Any chance of some pics of the brake booster and the bolts to get a better idea of what is happening?

 

This adapter really should only be a spacer, not a structural component.

 

So, is your adapter/booster or mine the original?

[sexual_sushi]

Looks like you showed that pedal who's boss Greg

[PeterAllen]

Those cut-down washers might have been the root cause. The hexagonal impressions indicate the bolts may have been overtightened and by cutting down the washers the force has not been spread evenly, in fact, they have created a stress concentration at the weakest point of the adaptor plate. I suggest once one gave way the m/c applied a twisting moment on the m/c mounting points which subsequently failed.

 

If you remake the adaptor add at least another 5mm to the overall dimensions to leave move material around the mounting hols - and use a complete washer. 

[stevo_gj]

Greg make it out of CF! Think of the weight saving!