The brakes on my '96 Disco are dreadful - is this normal?

It does sound like the pump's iffy - but it's difficult to be sure "remotely".

Richard

Richard Do you know how hard the vacuum should be please? Graham

On or around Mon, 02 Apr 2007 19:16:09 +0100, Graham Bowers enlightened us thusly:

good question, that. I've never tried measuring it.

The 300 type pumps are of indifferent quality and non-serviceable (or not easily) and I imagine you#'d have trouble getting parts for it in any case. If I can find a vacuum gauge I'll see if I can measure it. In fact, measuring the vacuum in the servo would be a better use for such than measuring vacuum in the inlet manifold.

According to my internet perambulations a petrol engine pulls 17 to 21 inches of mercury at idle. I'm guessing that the servo in a diesel and petrol disco is the same part and that there is a factor of safety to allow the brakes to work effectively under poor vacuum conditions, so my bet is 10 inches of Hg vacuum should be OK. In proper units that's roughly a depression of 5psi. I think I'll try to find a vac gauge tomorrow to check it out. The alternative is to park on a bridge and dangle a long tube in to a puddle. Graham

Where are you going to find a puddle of mercury? Water's no good for that job. :-)

Martin

Why would you say water isn't any good to make a manometer for this purpose, short term? It needs to be 10 metres high for a perfect vacuum although I accept it'd want to evaporate if it could get enough heat fast enough (steam tables not to hand right now btw). I suppose dissolved gases could come out of solution but one could use a boiled puddle that had been re-cooled that would have got shot of most of them. I would expect the vacuum from the pump to be 20 inches Hg max, which is around 10 psi depression, or around 7 metres of water which would make the endeavour more do-able. Have you tried this experiment?? Cheers Graham PS you know what they say, never dare a fool :-)) PPS when I were a lad a puddle of mercury could be found in the palm of the hand of many a schoolboy.

I remember playing with mercury in Physics, if got a gobful of mercury fillings too!

I've found this from an old Haynes manual (Peugeot Diesel) and may be helpful, the pump in question is a diaphragm type:

'500 mm Hg should be recorded after 1 minute' [of operation]

HTH Julian.

That's the top end of what I'd expected, and is corroborative. Cheers Graham

'Cos it would suck it into your engine and knacker your pistons as you say, unless you were 10 metres above the puddle to be safe don'cha know. Don't tell me, you've got BIG wheels on it. A tall story :-)

For a short while anyway!

Martin

There's a potential way of reducing the risk of sucking water into the pump. Insert a tee in the vacuum line from the pump to the manometer and have the branch open to atmosphere. Don't make the bore too small. You can then gradually increase the vacuum seen by the manometer by incrementally throttling the connection to atmosphere until you reach the limit of your manometer. You should be able to measure about six feet of water gauge without the apparatus becoming too unwieldy.

On or around Tue, 3 Apr 2007 22:59:05 +0100, "Oily" enlightened us thusly:

I think there was mention of a bridge... and he's talking about a diesel with a vacuum pump.

Mind, I've had a diesel with a throttle valve in the inlet which as far as I could see was there only to allow it to generate servo vacuum.

"Austin Shackles" wrote.......

Yebbut, where does the outlet from the pump go to quieten it?, into the engine of course, either block or manifold on most, petrol or diesel, not usually to atmosphere.

Martin

I think it does go in to the sump if memory serves me well but rest assured that if this theoretical experiment does become reality (a fairly low chance btw) the water will not end up in my engine. A bottle placed in the vacuum line between said manometer and pump with a volume exceeding the manometer contents and with the inlet and outlet at the top would ensure in the event of a bigger suck than expected, the manometer contents just end up in the bottle. Prime path is to find a vacuum gauge at no cost to me, though. Cheers Graham