Servo vacuum

Technical advice Q&A

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Stevejaguar
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#1 Servo vacuum

Post by Stevejaguar » Sun Oct 17, 2021 8:24 pm

Hi, my series 1 2+2 is having problems with the new servo. The suppliers swear it is a vacuum problem elsewhere in the circuit.
Today I put a vacuum gauge in each line to the servo.
Started the engine and it pulled 10 inHg in both lines.
Depressing the brake pedal fully caused the vacuum in the line to the rear of the servo to drop to zero. Release the pedal and the vacuum returns.
That's how it should work isn't it?
If you switch off then BOTH lines drop to zero. I would have expected that the line to the front of the servo to retain some vacuum and that is what you can hear ( in my case you cannot) as you pump the pedal Is that correct?

Am I correct ? is 10inHg about right?
Steve

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christopher storey
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#2 Re: Servo vacuum

Post by christopher storey » Mon Oct 18, 2021 9:18 am

It sounds to me as though you have either a leak in the vacuum Reservac tank or its one way valve or a fault in the sequencing device on the front of the master cylinder . When the engine is switched off, you should still get vacuum , in decreasing quantities , to the front of the servo for about 6 to 8 pedal pushes

The level of vacuum rather depends on your engine setup - I would have thought more like 15 - 18 in would be typical but 10 in is better than nothing !

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mgcjag
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#3 Re: Servo vacuum

Post by mgcjag » Mon Oct 18, 2021 9:30 am

Hi Steve...there are good brake systen diagrams in the service manul (early & late) like this....should help you Steve
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Steve
69 S2 2+2 (just sold) & Building a C type replica, 1960 xk150fhc

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#4 Re: Servo vacuum

Post by Stevejaguar » Mon Oct 18, 2021 8:31 pm

Thanks all for your response. At least I know that the first part( getting vacuum on both the front and rear of the servo) is correct and that the master cylinder is working correctly in releasing the vacuum to the back of the servo. Now to chase down those missing 6-8 'hisses' as the stored vacuum is exhausted. In the last 25 years of ownership I have never had this problem despite several new master cylinders and servos. I will swap the valve tomorrow and check the tank for leaks.
Thanks
Steve

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Stevejaguar
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#5 Re: Servo vacuum

Post by Stevejaguar » Mon Nov 01, 2021 8:06 pm

Hi. I have still got disappearing vacuum when I switch the engine off.
I have connected two vac gauges to the two pipes into the servo. I have joined together the two pipes that go to the master cylinder to eliminate any leakage via the master cylinder. The new valve works just the same as the one that has been on there for years. I have checked the vac tank and it has no leaks. I have also tried bypassing the tubes that run through the bulkhead.
As soon as I start the engine both gauges show a vacuum. As soon as I switch off both loose their vacuum. The vac tank has no reservoir of vacuum.
Any thoughts ?
Steve series 1 2+2

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abowie
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#6 Re: Servo vacuum

Post by abowie » Mon Nov 01, 2021 8:24 pm

I'll admit that I'm not totally sure of what's going on here, partly because I'm not totally sure what levels of residual vacuum should be left in the system with the engine off.

From what you've done it does sound like your valve and tank are working properly, and that in isolating the master you've eliminated it as a cause.

If you suspect the servo, I'd disconnect both of the vac lines to it, then connect a couple of pieces of hose to the ports with your gauges in them.

Find some way of attaching suction to the hose ends (big 60ml catheter tip syringe, buy from chemist and an inline tap) and evacuate both sides of the servo chamber and see what happens. I think that both sides should hold negative pressure. If they don't then there is a leak somewhere in the servo.

If there is a leak, try pressurising the servo, then mist soapy water over it and see if you can find an external leak. I've seen one leak where the metal vacuum pipe is pressed into the canister.
Andrew.
881824, 1E21538. 889457. 1961 3.4l Mk2. 1975 XJS.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

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Stevejaguar
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#7 Re: Servo vacuum

Post by Stevejaguar » Mon Nov 01, 2021 10:05 pm

Hi andrew.
When the engine is switched off I expect t get 6-8 presses of the brake pedal that evacuates all the vacuum from the tank. But I get nothing as all the vac has gone. Also if I press the brke and start the engine the brake pedal does not go down slightly.
Confused to say the least.
Steve

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abowie
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#8 Re: Servo vacuum

Post by abowie » Mon Nov 01, 2021 11:19 pm

I'd be interested to see what the outcome with testing the servo for leaks is.

Despite the vendor's comments, it seems to be the only bit of the system that you haven't been able to confirm is good.
Andrew.
881824, 1E21538. 889457. 1961 3.4l Mk2. 1975 XJS.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

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#9 Re: Servo vacuum

Post by Stevejaguar » Mon Nov 22, 2021 9:41 pm

In the end the problem was simple.
After walking away from the problem for 3 days I looked at it again with fresh pair of eyes.
I blew into the braid covered pipe connected to the front of the servo. This is something I am sure I had done before but I may have bee using a temporary hose. There was a very slight hiss where the pipe joined the servo. I tightened the hose clip and the slightest of hisses was still there. I removed the hose. Blocked one end and blew into it. No leak UNTIL I bent the hose like it is on the car. A very small leak appeared. I cut off the braiding with a knife . No sign of damage until I bent the hose that's when multiple splits in the outside of the bend appeared. Once stripped off it was clear just how much the braid sealed the leaks so there was only the slightest of hisses.
Just goes to show that its the simplest of things that can trip you up.
Both servo hoses were the same. New ( unbraided) hose ordered and apologetic phonecall to the servo supplier.
Thanks for all your input.
Steve

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christopher storey
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#10 Re: Servo vacuum

Post by christopher storey » Tue Nov 23, 2021 8:06 am

Steve : that also is likely to be the explanation of why your manifold vacuum was so low at 10 ins . It may be worth now checking engine tune, as that leaking pipe may also have been causing weak mixture

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Stevejaguar
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#11 Re: Servo vacuum

Post by Stevejaguar » Tue Nov 23, 2021 10:24 am

Yes Christopher I have no doubt that thats why I struggled to get a smooth tick over last time I adjusted the carbs. Its on my list of things to look at. I may have to remove the rear carb to route the pipes that go under it so that they are not rubbing on anything first.
Steve

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#12 Re: Servo vacuum

Post by Stevejaguar » Thu Nov 25, 2021 2:08 pm

Hi, does anyone have a photo of how the rubber vac pipes run from the bulkhead to the servo on a series 1 RHD. 2+2
Thanks

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