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Where does brake servo get its vacuum from? - Suspension/Brakes/Wheels/Hubs/Steering/Geo - The Lotus Forums Jump to content
wildatheart

Where does brake servo get its vacuum from?

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I’m  the go-between between my garage who are working on my car and this forum, but the mechanic notes that there isn’t enough vacuum at the brake servo for it to operate properly; before he starts taking off parts and panels he is hoping I can help him with some research as to the likely source of the problem, ie where the vacuum is created and its routing. Can anyone here advise? 

1985 Turbo Esprit

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 There is a common rail that taps into each intake.   The takeoff pipe for the servo is then situated near the oil filler cap.   On the early UK cars, including mine, there is no additional vacuum pump, so everything (including engine tune) needs to be in tip top order to create sufficient vacuum.   Do you know what reading yor mechanic is getting under what conditions? 

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Many thanks - I am seeing the mechanic later today and will report back. The engine was rebuilt by PNM 1200 miles ago so should be in good shape. I don’t think the brakes are worse than usual (and they are bad, they have some response under light braking but are solidly wooden with much force needed thereafter) and on previous investigation another machnic gave me the ‘they are all like that Sir’ so frankly I’m delighted that the current mechanic (a well regarded if mainly classic Porsche specialist) thinks there is an issue after all.

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Mine can lock up the fronts (or at least  have them 'chirruping' on dry tarmac at motorway speeds).  The most marginal braking is when I am pootling around the local estate at maybe 20mph when the engine is generating the least vacuum  

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When I deleted my belt driven vacuum pump on my  '85, I added a check valve and a large vacuum reservoir  to the vacuum line  leading to the booster. The hope was the reservoir would hold a bit of vacuum from the last period of high vacuum ( i.e. overun in gear)  for the brakes next application , irrespective of current manifold vacuum.  Obviously this won't help with repeated applications of the brakes, and is limited by the size of the reservoir, but it costs and weighs next to nothing to implement and cut down on the incidence of the  "low brake vac" idiot light on my car.

In the future, I plan to add an electric vac pump and an additional reservoir to keep vacuum levels more steady.

This is of course only helpful if your brakes are in good condition to start- no amount of brake assist will make up for glazed pads or sticky calipers etc' check those first!

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The vacuum system is easy to 'half-split'. Minimum vacuum is 18hg. at various points.  Plug the lines half through, see if that makes a difference; go forward or backward from there using the diagram under the hatch.

 

If your car has the external vacuum pump, common trouble points are the rubber T near the front of the engine; and the elbows on the hard plastic lines. The ones located under the dash  (heater controls), and also plumbed over the RH gas tank and sail panel have been known to have duff sealing capabilities.

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Thanks all -

The mechanic didn't have numbers but said that at idle there was 'as good as no' vacuum at the servo and that when he drove it he was convinced the car didn't have a servo fitted...

This may be a stupid question but if there is a leak around the heater controls, this would cause the loss at the brake servo end?

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Our '88 (Stevens) has a low vacuum switch near the servo that illuminates the BRAKE warning light on the dash. I once knocked a fitting askew over the gas tank and the light went on. Not to mention, the vacuum pump started complaining (noise). That was where the pipe split (heater to one side, boost transducer on the other)

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22 hours ago, wildatheart said:

This may be a stupid question but if there is a leak around the heater controls, this would cause the loss at the brake servo end?

Yes, but the heater control "hoses" are of such a small bore that I doubt they would have much effect when the egine is running, just allowing the vacuum to leak away when it's not. :thumbup:

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On 22/03/2019 at 12:36, wildatheart said:

The engine was rebuilt by PNM 1200 miles ago so should be in good shape. I don’t think the brakes are worse than usual (and they are bad, they have some response under light braking but are solidly wooden with much force needed thereafter) and on previous investigation another machnic gave me the ‘they are all like that Sir’ 

He might be at least partly right unfortunately. The LC turbo does not really produce enough vacuum. I have all the parts to fit an electric pump from a VW Toureg on mine, just need to get around to doing it.

Its worth checking for any leaks however minor in this already marginal system. 

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I have done some investigations; I noticed the vacuum controlled flap by the heater fan isn't working. I removed the engine side panel above the RH petrol tank, disconnected the small heater control vacuum tube and found almost zero vacuum there; enough to hold a cigarette paper but I couldn't feel anything on my finger.

I have the common header rail above the manifold from which the vacuum pipes for heater controls and brake servo go off; suspecting a leak somewhere between the rail and the brake servo, I removed the vacuum line for the servo and found that I could blow through it with relatively little resistance.

Am I on the right track?  If so, how is the vacuum pipe for the servo routed and is there a most likely spot where it has sprung a leak?

 

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Thats definitely a leak then. 

The rubber hose goes down to the rear end of the chassis backbone in front of the bottom of the engine, then there is a metal pipe through the chassis backbone then rubber again where it exits at the front of the crossmember. Maybe a split hose.

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Jaap,

Review of the Service Manual for Turbo/S3 confirms that the ventilation flap actuators are served via a small diameter plastic feed tube which terminates aft end at the vacuum reservoir canister in the engine bay above the right side wheelarch. Andy is correct in that the vacuum to the servo connects via rubber hoses at each end of a metal pipe fitted into the backbone. At this age it's possible you have leaks at any point in these two circuits, due to cracks in the tubes/elbows/actuators if in the HVAC section, rubber hoses or servo if in the brake section. There is limited vacuum to be had in your carbureted manifold so any loss will be meaningful.

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He mentioned blowing air into the hose to the servo, so this confirms leak in that circuit and it cant be the servo as there is a non-return valve on the servo.

The heater flap circuit is completely separate. Note that blowing air into that will give a false positive as there is a non-return valve in the line from the vacuum rail to the reservoir.

 

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Further investigations have revealed that there is no leak in the pipe to the brake servo. But rather, that there is simply not enough vacuum created ‘at source’ ie the rail above the manifold, to operate the heater controls, let alone the servo. The engine recently had a complete rebuild by PNM and seems in very good shape otherwise.

Before we take the manifold off, does anyone have any suggestions?  Is there some kind of valve in the manifold that could be faulty?

thanks team!

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There is no valve in the manifold but each of the 4 ports have small restrictors. Its unlikely to be those though. This is a known issue and I, and others, are working on a solution:

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Thanks Andy - yes I looked at that already with interest. Thing is, not even the heater controls are working now, when I have had these working in the past; so there must be some kind of fault..?

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Yes....  Have you got a vacuum gauge and a Tee?   then you can see what vacuum the engine is producing and see if you are losing any vacuum as you isolate the servo and heater control circuits  

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There is a non-return valve in the heater control line, if you follow the hose you will find it and on mine there was so little vacuum it was not opening the valve, it was sticking shut, So there was no heater control. I managed to free it up, but the operation of servo and heater is still too marginal.

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Thanks gents - The garage checked for vacuum at the header rail above the manifold and found ‘practically no’ vacuum there. I don’t think he used a tee to test each of the two circuits; just found close to zero vacuum to begin with. Is the non return valve in the heater circuit the dome shaped thingy behind the side cover above the engine?

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Well that doesn't sound right....   I know the engine has been rebuilt, but maybe worth  checking compression, ignition timing, valve timing, carb balance, leaks or basically anything that would affect idle vacuum

The the 'dome thing' is a reservoir.  As Andy mentions, the valve is a brass 'thingy' inline in the rubber hose, should be fairly near the manifold rail.

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I'm back on the case - I spoke with Pete (who rebuilt the engine) at PME and he was very helpful; he recommended sealing off one circuit (brakes, heater controls) at a time and seeing whether that improves operation of the other. I forgot to ask him how to go about that though.. hose clamps near the cambelt seems a bad idea... Any tips?

Meanwhile, my non return valve looks ancient.. (Andy), how can I test it please? Is it easily replaced with new? Thanks!  (hose at the bottom of picture looks like it has seen beter days but it's not the source of any leak, sadly)

IMG-3661.jpg

 

 

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You can test the valve simply by blowing through it one way then the other. They do come apart. New ones are very inexpensive on Ebay. 

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Thanks Andy - I could only blow through it one way by blowing very hard and it made a sound whilst doing so which I can't imagine is to spec.. I have ordered a replacement. I'm thinking though, if the current valve prevents the vacuum going to the heating system, if anything, the brake servo should have a little *more vacuum to work with so this doesn't appear to get to the bottom of the low vacuum issue..

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