Vapour lock

[Jeff J]

If you get spongy brakes due to vapour lock does it clear itself after cooling down or do the brakes have to be bled.
I'm having an intermittent problem with spongy brakes which lasts for a couple of miles after driving up & over & then down a really very long very steep hill locally. Once it disappears I can then drive 200 miles without any problem. I have checked brakes etc & can find nothing binding, front offside wheel bearing rumbling slightly so I will replace it but I doubt it is going to cure the problem.
The only other thing I can find out of the ordinary is that the fluid level is too high in the master cylinder.(1/2 inch above max.) But again I can't really see how that could cause it.
Any other ideas please? Thanks.

[CycloneMike]

As a start you want your fluid changed and the whole system thoroughly bled through. The symptoms indicate a problem and it won't go away by ignoring it.

[Jeff J]

I've no intention of ignoring it. An intermittent problem is difficult to diagnose even for professionals & I prefer to pinpoint what the problem is rather than just throwing parts at a problem in the hope it is cured. So at the moment I'm just trying to gather as much info as possible & maybe learn from someone else who has had this or a similar problem. Thanks for your input but it does not really answer my original question. If you had said that a vapour lock problem does exhibit those symptoms then replacing the fluid would indeed have been the first thing I would have done.

[pionte]

To me it sounds more like air/moisture in the fluid boiling when it gets hot. My first action would be as CM suggests is to change the fluid.

When was it last done ? It is suggested by most manufacturers that it is replaced every 2 years.... to be honest I dont do it that often , but 5 years plus is pushing it and if there is a problem then its the first port of call.

Easily enough to do on your own, £20 ish for 5 litres from Halfords , an easy bleed pipe and a happy hour pumping the pedal.

Hope you get it sorted.

[CycloneMike]

Jeff, sorry you didn't find my answer helpful, but from what you describe that's where I would start. OK it was brief, but I thought I would try to help before I went to work this morning.
As pointed out brake fluid degrades and needs periodic replacement, but it is often neglected and subsequently contributes to poor brake performance or failure. So for the avoidance of any further doubt, "a vapour lock problem does exhibit those symptoms". Your bearing may be failing because the heat from your brakes has melted out the grease, rather than it being the other way round.
Heat is the killer of brakes and these vans don't have much capacity to deal with heat to begin with, even when all the components are in good condition.
A long steep descent with lots of brake use will put a lot of heat into the brakes and this combined with what you described is symptomatic of poor fluid condition, hence my advice.
You probably also won't want to know that the situation you describe is also exacerbated by worn/thin discs and pads. The fact that sufficient heat has passed into the caliper/fluid through the pad probably means that the pads have exceeded their working temperature range and are also irreversibly deteriorated.
When you come to change the fluid, be prepared for seized bleed nipples, go carefully with them or you will have to throw some more parts at it. All part of old van ownership I'm afraid.
As for not ignoring it, are you not continuing to drive it knowing there is a problem? An intermittent problem is just a problem waiting to happen again, perhaps next time with serious consequences? But let’s hope not.

Good luck with it and to all of those in front of you!

Regards,

CM (Expert in diagnosing brake failure)

[Jeff J]

Thanks that's a lot more helpful. Pads & discs already replaced but wheel hubs, calipers & rear drums do not appear to be excessively hot when the fault develops which is what confuses me. Also I would have thought that if air was introduced into the system it would have stayed there until it was bled out,or is it a case of the brake fluid vapour condensing back into fluid when it cools, which I guess is the question I should have asked in the first place. It's not a regular driver so I guess my next thing will be to change the fluid when I do the bearing.

[CycloneMike]

You are right, once the heat is taken away the water vapour is reabsorbed into the brake fluid, so the sponginess goes away again. It's not air that is in the system but water vapour produced when the water contaminants in the brake fluid boil. The only way to get rid is to change the fluid and flush out all the old grot.
I regularly test brake fluid and in older cars I often see brake fluid that begins to boil off as low as 140 C. This compares to testing new out of the bottle fluid which I have seen as high as 320 C (which far exceeds the quoted spec on the bottle).
You would be surprised how quickly a caliper will heat up to 140 C, particularly with steel wheels as these hinder ventilation.
Also the colour and condition of the fluid in the reservoir is not always a good indicator of the condition of the fluid in the calipers.

Regards,

Mike.

[Jeff J]

Thanks