Where does brake servo get its vacuum from?

[910Esprit]

Why not check the engine vacuum with all 'accessory' hoses disconnected and blocked off at the manifold rail (apart for a take off for the vacuum guage) - If you have still have a low vacuum, then you can rule out the 'accessories'    

[drdoom]

2 hours ago, 910Esprit said:

Why not check the engine vacuum with all 'accessory' hoses disconnected and blocked off at the manifold rail (apart for a take off for the vacuum guage) - If you have still have a low vacuum, then you can rule out the 'accessories'    

Put a "tee" fitting where the disconnect is to be tried so you may check vacuum both with and without accessories online. If there is a difference then you will know with certainty that air is leaking in somewhere. You can then similarly isolate branches downstream in order to locate the fail point. A significant vacuum leak very near the engine may be checked by careful application of VOC aerosols such as brake cleaner at the suspect points. If that is drawn into the inlet it should register an effect on the idle conditions.

[wildatheart]

Thank you gents, I have just ordered a vacuum gauge so I can try see what's occurring here with greater accuracy. I'll report back.

[wildatheart]

Took a few more days for some tubing to arrive but finally I have a better sense of what I'm dealing with - NOT helped by the classic Porsche/Ferrari specialist in Edinburgh who seem to have given me a total bum steer on this.

There IS decent vacuum at the manifold;  0.45 /15 inches on overrun - It's nicely half way up the dial, so I take it that's within the correct range?

When I blank off the brake servo line -and with a new non return valve fitted- the heater flap works now - and there is no vacuum loss.

There IS a leak between servo and manifold (again contrary to what the garage said) as there is only .20/6 inches on overrun at the brake servo end.

If I put the car on a couple of axle stands at the front, can I have a go at tracing the servo line myself or is it a nightmare and should this only be done on a bridge? Is there a non return valve in the brake servo line,  if so where?

I think the front under tray needs to come off is that right?

Spending 25 quid on a vacuum gauge is clearly 'the best thing I've ever done'  

Thanks as ever for your help!

Edited May 20, 2019 by wildatheart

[drdoom]

Jaap,

The next point at which you will want to isolate a leak is the servo itself, as it does contain a sort of non-return valve. There is also the possibility that the diaphragm material has aged sufficiently to have developed a perforation, through which vacuum may be lost. So see about disconnecting the vacuum line at the servo, checking readings right there rather than tracing the vacuum line physically along its length.

Cheers

[wildatheart]

Thanks - I checked the vacuum at the end of the hose where it connects to the servo, and that's where I got the low reading - so there must be an issue between manifold and servo...

[wildatheart]

I have been under the car and found no issues between the rubber hoses and their connection to the metal pipe that runs through the chassis. But, I could move the metal pipe where it goes into the chassis at the front in & out by an inch or two, whereas on the engine side it is held firmly in place by a grommet.

Is this steel pipe is just a single piece (ie. I should not be able to move it at the front when it is static at the back)?

If it is broken (how could it break..?) then I wouldn't be getting ANY vacuum reading at the servo end (currently it's around 30% of what it should be)? 

Any idea what's going on here...? And has anyone replaced this pipe?

Thanks!

Edited May 21, 2019 by wildatheart

[wildatheart]

I went for it and I have the skinned knuckles to prove it; I put the car on axle stands and eventually managed to pull the vacuum line out of the chassis from the engine side. From here of course there was much anxiety as to whether I'd ever be able to get a new line in, though  I was happy to see that my faffing around with the vacuum gauge (and despite miss diagnosis from a professional) had led me to the right place

 

...clearly the decline will have started many moons ago and I have obviously rather got used to less than optimal brakes..! The replacement pipe from Lotus is no longer available, so I had one made up by Pirtek, a hydraulics company in Edinburgh. In the end, despite the minor injuries and some expletives along the way it was no worse a job than I had feared and it went in, eventually, as the old one had come out. 

And yes, my braking is much improved. Now that I'm on a roll, I'm thinking I might fit a non return valve in the servo line.. can't do any harm right..? Near the manifold?

 

 

[Chillidoggy]

I would.

 

[Smithy111]

Thread resurection, i have similar problems with mine not a new problem its always been like that the last year i have owned it and just got used to rubbish brakes. Got some same tests to do first to check amount of vacuum but would this kit solve the poor vacuum generally found even when working correctly?

https://www.ebay.co.uk/itm/12V-Vacuum-Pump-Kit-Brake-Booster-Electric-Vehicle-Hotrod/192877251317?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

[Chillidoggy]

19 minutes ago, Smithy111 said:

Thread resurection, i have similar problems with mine not a new problem its always been like that the last year i have owned it and just got used to rubbish brakes. Got some same tests to do first to check amount of vacuum but would this kit solve the poor vacuum generally found even when working correctly?

https://www.ebay.co.uk/itm/12V-Vacuum-Pump-Kit-Brake-Booster-Electric-Vehicle-Hotrod/192877251317?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

Yes, if you can find somewhere to fit that bracket, but it's a little expensive in my opinion. Have a word with @top-plumber