Temporary brake failure Any ideas as to the cause?

[New Forest Terrier]

As the title says I had a very short term total brake failure this morning. Scary,no damage but I need to work out what caused it and make sure there is no repeat. Stopping for a traffic light, not very fast, I pressed the brake pedal. Two mechanical feeling bumps through the pedal and no braking at all. First thought, Oh s**t, its an automatic, how do I stop it. Next action I kicked the brake pedal down really hard and the brakes came back with a magnificent emergency stop. Much relief. Tested the brakes and all was fine. I have had to do about 60 miles today, fortunately with no repeat.

The Bongo is a 95 V6 with ABS. The ABS light is functioning correctly and it feels if the ABS is working correctly. No pulling or unevenness on the brakes. They go on totally smoothly and it stops as well as ever, which is very fast.

I can think of a possible range of causes but am not sure what to check first. It was a boiling hot morning and the Bongo has spent a lot of time on dusty tracks in the last few weeks and it could just be something sticking from accumulated grunge.

There could have been a sensor failure, but if it was ABS I would expect mechanical braking. I think it is too old for fly by wire. A sticking cable perhaps? Any other mechanical fault would have continued to show itself.

Is there a technique for stopping an auto with no brakes? Gears are easy, change down and finish with handbrake. Any ideas welcome.

[wonkanoby]

any signs of leaks

is fluid up to level

[Muzorewa]

For stopping an auto Bongo with no brakes, you should be able to ram it straight into L & Hold, (1st gear effectively) and it will progressively change down as soon as the road speed suits, thus (in theory) not damaging either the engine or gearbox. Then handbrake as you suggest.

[amauvis]

Years ago I had an old E28 BMW 5 series that used to do that. It was a dodgy ABS sensor but I was skint, so I learned to manage the problem until I could sort it. Basically, start up in the morning, drive really slowly and when used, the brakes would crunch away with minimal braking until braking woke up and worked. For the rest of the journey, all was good. Whether the car diagnosed a problem and bypassed ABS altogether, or the dodgy ABS sensor needed some wakeup time, I don't know, but it was a sensor and replacing it solved it.

If it was a proper mechanical issue where loss of braking caused by loss of pressure had occurred, your braking performance would not have returned.

[the laird]

Get the master cylinder checked out sounds like m/cylinder fault to me.if the fluid level has dropped and there are no visible signs of leaks look under the dash where the brake pedal is and look up to the bulkhead you should see where the brake rod is connected to the pedal and at the bulkhead/floor see if there's any signs of fluid marks
Good luck n b carefull

[hijimhere]

Bit of an odd one very rare for ineffective braking as a one off experience - was the feel of the pedal the same as foot on pedal with no engine running??? I am thinking along the lines of servo more than master cylinder - check the master cylinder type is it duel type ie one line for front and one for rear! Look at the servo interaction with master cyl.

[New Forest Terrier]

Still no explanation on this one. Fluid level is perfect, no dropping and no sign of a leak. Brakes are working as well as usual, which with ABS is very good. Pedal feels fine, servo clearly working.

I wonder if it could have some sort of automatic adjustment which stuck, hence the the two thumps. Pedal then felt completely unconnected, none of the heaviness of a failed servo. There seemed no resistance while I floored it, but then all braking came sharply back. No delay in the morning, brakes are fine from the start. No fade either. I hope it will just stay a one off.

[B*Witched Blingo]

Could this happen if the fluid has a high water content. When was the last time you had the flyid changed? I am sure I read somewhere that the fluid should be changed every two years. A mechanical thump, could this have been anything to do with not being able to pressurise I.e. due to air or water build up? Just chucking a bit in there for discussion.

[mikeWalsall]

Could the actuating rod from the brake pedal be an issue ..

I think I once read on the Toyota forum that the rod had unscrewed or the pin had come out of the clevis ..(??) or something like that ..

[Northern Bongolow]

get a fresh or new master master cylinder or refurb kit from mazda mary.
the internal seals harden or wear then they let fluid past.

how long had the motor been running at the time of first brake fail, if a short time the vac hadnt had time to build in the servo, then as it built up it managed to flair the damaged/leaking seal and seal it in the bore, then the brakes worked---------------------maybe.

dont ignore it mary.

http://lushprojects.com/bongopartsmk2/c ... o=.html#18

[the laird]

If there was water in the fluid I think if it was hydroscopic the failure would happen all the time or more often go to halfords or any good factors you can buy a cheap brake fluid tester for moisture in system you dip it in reservoir and get a reading of it ,this cannot be ignored,brakes and steering faults have got to be addressed,!
Dot three brake fluid should be change every two or three years

[hijimhere]

Brake fluid does absorb moisture but will not give you a hard pedal - while brake fluid flows contaminated fluid will compress so you end up with spongey pedal.

If it was a master cylinder (not push rod/clevis) the seals fail progressively and fluid will by pass the seals but braking would loose efficiency and pedal travel would significantly increase over a short time.

In 30 years in the trade I never witnessed a master brake cylinder seize - but I would be expecting to be looking at permanent seizure, the chance of a one off could probably be discounted.

A solid hard pedal comes with the servo or it's link with the master cylinder. The premise is that servo assistance was lost albeit momentarily (one off), and far heavier depression of the pedal gave some braking. Check the pipe connections - where is the vacuum coming from?

[Velocette]

jack up each wheel and make sure they spin. A binding brake could cause the fluid to overheat and boil. Dust could concievably cause a piston to stick I suppose.

[helen&tony]

Hi
I don't want to be the harbinger of doom, but I'd replace all the seals throughout, and put new fluid in....
Cheers
Helen

[New Forest Terrier]

Thank you for all comments and ideas. I agree this is one for prevention rather than long drawn out investigation. The brake fluid will have to be replaced as soon as possible, its due anyway and a new set of master cylinder seals can do no harm. I will take advice from Discount Trucks on this as the master cylinder has the red paint splash of a Japanese rebuild, so they may be OK.

It did happen fairly soon after starting out, although four traffic lights, two roundabouts and other hazards came before. Its was ferociously hot,30deg C and the Bongo had been sitting in full sun all morning. Could this have boiled some water in the brake fluid? Also it was the first longish run after a total dust bath at the weekend. It was covered in orange dust which a shower had streaked everywhere. Add in some blown sand and it was just the sort of stuff to jam anything.

A total check of everything is called for. It was a nasty moment and one not to be repeated.