Intermittent brake fault

[Scouter]

Greetings. Today, whilst under gentle braking on a level surface, I felt and heard a buzzing from the brake pedal. It did not feel like an ABS activation and I must stress it was very gentle braking. The pedal seemed to travel a short distance and there was a reduction in the brake effect. I released the pedal and re-applied. These symptons were repeated about three times. The brakes have since seemed as normal. I do feel that I have the beginnings of a bit of disc warp as I have noticed a bit of pulsing lately but it is not in any way severe at this stage. I have searched the forum but am not sure if I recognised these symptoms in other posts.

I intend to get it checked straight away but if I can give my friendly garage guy an informed steer as to the likely fault, that would be good.

[Tony x]

I don't know if it is disc warp or not that's causing your issue. I suspect it probably is.
If your Bongo was a motorbike, I'd say the warping you have is pushing the pads back a little into the caliper as the disc rotates and the pedal is kept in the same place and thus lessening the braking power.

However, your Bongo is not a bike and brake systems between Bongos and bikes are very different so I don't know if this holds true.

It's worth knowing (again drawing on biking knowledge) that once a disc starts to warp, it heats up unevenly as you brake, different pressure in effect applied to different parts of the disc, and this in turn causes more warping...it's a slippery slope.


One reason for disc warping is that, as the disc wears, it forms a kind of T section, the cross bar at the top of the T being the very edge of the disc. Such a shape, when heated will inevitably warp. I can't understand why the edges of all discs don't have a knobbly pattern or a series of cut-outs to prevent this.

[Northern Bongolow]

could be the diaphragm in the vac unit / servo on the master cylinder, these can be tested, but its a bit harder to test an intermittent fault.

heres how they work and how to do a quick test.

http://www.my-cardictionary.com/brake/brake-servo.html#

[Scouter]

Thanks for the replies and suggestions. I feel that the disc warp is not relevant as it is at a very early stage. I will pursue the possibilty of the servo issue. I described the symptoms to my garage man who started his investigation around the brake pedal area as the buzzing sound came from there. If someone were to suggest that a solenoid was malfunctioning in that area, this would make sense of the symptoms (at least to me). My garage man looked up above the pedal and identified the brake light switch but couldn't identify a black component imediately above it with two cables coming out of it, one going up towards the dash and the other into the engine compartment. Any suggestions gratefully received. The fault has not reappeared since so, as Northern Bongalow says, may be difficult to diagnose.

[Scouter]

When I said 'solenoid' I meant 'relay' - sorry.

[Northern Bongolow]

what year and engine have you got.

[Northern Bongolow]

sounds like yours is the later model with the throttle position sensor on the pedal.

http://lushprojects.com/bongopartsmk2/c ... f02.html#9

[Scouter]

Mine is an 'R' Reg ('97/98) V6 with LPG conversion. The component he was wondering about is directly above the brake light switch above the brake pedal. It is black, about 3cms long and cylindrical in shape.

[Simon Jones]

Is it an electrical cable or mechanical one like a bike brake cable?

[Scouter]

The cables are mechanical, like cycle brake cables.

[teenmal]

Scouter , if your vehicle has automatic transmission the cable will probably be for the gear selector interlock.

[Scouter]

Thanks for that teenmal. Is it safe to assume that this is not related to my problem as described above?

[Simon Jones]

Yep, that mechanism is to prevent gear selector being taken out of park without foot on the brake so won't be related to what you have been experiencing. To me, it sounds like the ABS is kicking it at the wrong time possibly due to a faulty sensor/wiring or corroded ABS ring. Alternatively, if the road was wet or there was difference in wear in some of the tyres then it could just be the ABS working correctly for the conditions at the time.

Yesterday I was gently applying brakes while driving down a grassy slope. Two wheels were on gravel while the other two were on grass and there was a buzz and vibration that could be felt thru the brake pedal as the ABS compensating for the different traction on each side. Sounds like like the effect you were getting.

[Scouter]

Thanks again for the replies explaining the unidentified component.

The 'confused ABS' possibility would be feasible except my van doesn't have ABS fitted I don't think. There is no ABS warning light on start-up. I think the fact that these symptoms are not described elsewhere in the forum suggests that this is a rare fault. At the moment all is working normally again. The frustration with all intermittent faults is that they are so difficult to diagnose.