Temporary Loss of Brakes

[SOMMERS]

I have a 97 Vectra - about 3 or 4 months ago I was driving on a duel carriageway, at night, and seeing a roundabout in the distance applied my brakes only to find I had none. I pumped the pedal and felt the brakes come up but as I pressed, although they slowed me a little bit, they soaked away again. I managed to get to a petrol forecourt to check for possible faults. The brake fluid resovoir was full and nothing seemed obvious. I could pump the brakes up, but when the pedal was pressed the brakes soaked away. The car was parked for about an hour before I tried again and now they worked fine again. A week later the car was MOT'd and the garage did a full brake test ,& various checks at my request. (twice)They found no faults and passed it.
I've had no problems since, until last saturday night when an exact repeat happened. On the first occassion I'd driven about 18 miles. Last saturday I'd driven 40 miles, parked for 4 hours and was 20 miles into my return journey. Can anybody throw any light onto this as I obviously want to get this fixed. Thank you

[woodchopper]

Don't be driving a car on the roads that has faulty brakes for safetys sake - take it to a garage and get them to sort it.

[cazzz1975]

It sounds like the master cylinder is worn, What is happening is although it is not losing fluid, When the brake fluid is getting warm (ie like on a long journey) the brake fluid thins, thus seeping past the seals causing the brakes to fail because the fluid is leaking past the seals and not getting to the brakes.

also when you replace the master cylinder , replace the fluid as well.

[SOMMERS]

That's the point Woodchopper, it went to a garage and then to a MOT. Despite inspecting and testing and driving it, and as the guy said putting it under a pressure test (presumably to see if there were any leaks) they couldn't find any faults.

[SOMMERS]

Thanks Cazzz1975,
any idea on costs ? Sounds expensive and I guess just a change of brake fluid won't cure it !

[cazzz1975]

trust me sommers the master cylinder is the problem, it will only show up if you have done a long journey, it will be fine when its cold

[cazzz1975]

not sure on cost, I would phone around, Im pretty sure it shouldnt be too expensive

[SOMMERS]

Thanks Cazzz1975 - one final point, could it just be the seals need replacing or definately the whole cylinder ?

[cazzz1975]

I will say, before you go for a new master cylinder, it might be worth just changing the fluid first to see if it sorts out the problem, maybe the brake fluid itself is thinning because it is old, if it still does it when you put new brake fluid in then it is the master cylinder

a master cylinders is normally arounf the 60 quid mark, worth trying the cheaper option first (changing the fluid) just to rule out old fluid

[cazzz1975]

you would need to strip down the master cylinder to check if the seals are worn, personally I would replace the master cylinder and fluid itself simply because its an easier job than a refit). when was the fluid last replaced? if 3 years or over then replace it first, then give it a long drive to see if it would fail again

If you dont mind the work then replace the seals and fluid, a seal kit is only about a tenner

[SOMMERS]

Mmm, I think I might go for the whole thing. If, after changing the fluid, the only way of checking is to to drive 40 miles, slap my brakes on, and hope !
Although most of my driving is very local in town. I have, in the past month, made this last journey 3 times with no problems.

[joko]

did you drive through any deepish puddles just before?

that can make the brakes useless for a short while.

[SOMMERS]

No I didn't JOKO.

Incidently, needs must and I had to make the same journey last night. I had no problems.
I will be booking into a garage tomorrow.

Thanks for all your help on this

[bhg481]

Brake fluid is hygroscopic (absorbs water) and should be changed regularly, normally every 2 years. If this isn't done the boiling point of the fluid is reduced and you can get brake fade after heavy or extended braking (exactly your symptoms). Your brakes would have been cold at the MoT test and, probably, when the garage checked them, so the problem may not have shown itself. Check when your brake fluid was last changed; if it was more than 2 or 3 years ago get it done and see if the problem disappears.