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MG MGB Technical - Brake Servo test

just while im thinking about it, this may sound daft but how do i test the brake servo on my 1974 gt. I never heard anything like a hiss from it but the cars has always been the same pedal feel wise and stops the car OK.

I just dont know how to see if the servo actually does anything.
whiskeyfrank

To test your servo's effectiveness, first with the engine switched of, pump the brake pedal several times, 2nd, keep pressing the brake pedal and then start the engine, if the pedal dips then the servo is working. This is the method that MOT inspectors use to test servo's.
Hope this helps.
john wright

With the single circuit brakes and remote servo you do have to press the brake pedal pretty hard to feel further downward pressure when you start the engine. It may be easier to feel on the later combined master and servo as that gives more assistance.

The other test is of the vacuum circuit and that is after the engine has run, switch off, then pump the pedal as if for the test John says. You should hear hissing and wheezing from the servo as the pedal is operated and the pedal should gradually get higher as you use up all the vacuum in the reservoir. This should still happen several hours after switching off. If John's test works, but this fails, then there is a leak in the vacuum circuit and it will be weakening your mixture (which the carbs may have been tweaked to 'compensate' for).
Paul Hunt 2010

I have just fitted a replacement servo on my '73 B but on testing it using John Wright's method the pedal rises slightly when I start the engine. Any ideas on further tests please?
Graham Gilmore

Both mine go down slightly when the engine is started, so yours sounds it might have an issue. If you have replaced like for like has the brake 'feel' changed when driving (carefully!)? Although having driven with and without and one with that had been disabled the standard in-line servo makes very little difference anyway.
paulh4

Thanks for the response Paul. I hreplaced the remote Delphi unit with a remote PowerTune from Moss. Unfortunately I have not had much chance to drive it in anger but the pedal is as hard as I would expect after many 'bleeding' sessions. It is in the same location as the previous one, on the nearside shelf where the LHD pedal box would be. The Air Control Valve orientation is at the correct 30 degree downwards angle. However, the slave cylinder is only about 15 degrees rather that the 25 to 45 above the horizontal. Could that be the issue?
Graham Gilmore

Try these two as well.


Allan Reeling

Graham - when you say 'slave cylinder' I presume you mean the servo cylinder? My roadster is horizontal and the V8 maybe 10 degrees at most, with both having the air-valve uppermost, and both test as they should.

Air-valve positioned low has been mentioned as a way of stopping the servo locking on in hot weather but those who have tried it say it made no difference.

I can see that angling the cylinder up to the outlet will probably aid brake bleeding as it gives less space for air to gather above the outlet port. But I have always achieved the same thing with high-pressure bleeding which involves someone pressing hard on the pedal while I open each caliper nipple in turn, which always blasts the remaining air out.

On that basis I can't see either having an effect on your problem, at the moment I can't see how starting the engine would force fluid back into the master to push the pedal up.
paulh4

Thanks for the diagrams Allan. The new unit does not pass the first test, the pedal does not go down slightly when the engine is started. Having just driven the car the brakes certainly do not feel as effective as previously with the old servo.

The article you posted was headed 'Brake Booster Operation without a Tester'. Do you have any details of such a Tester.

Paul,

Yes, the slave cylinder I referred to is the servo one. The brakes have been bled endlessly and a firm pedal always obtained. With brake pedal wedged down overnight it maintains position and there are no leaks. As mentioned above I am convinced that this new servo is not working as the brakes on the car have always been very good.
Graham Gilmore

Forgive me, but did you change the pipework, or were the connections in different places to the old one? I only ask because I can really only think that the pedal being pushed up when you start the engine might happen if the inlet and outlet are reversed.

But if you are saying that after running the engine and switching off, repeated pedal presses are not making any hissing noises and the pedal getting higher with each press, then as long as the one-way valve on the inlet manifold and the hose are sound then the servo must be faulty. The second test and driving experience would seem to confirm that.
paulh4

Have I found the reason? Having removed the cap to the Air Valve I was astounded to see that the filter on the PowerTune servo was a flimsy ring of very soft foam mounted on a hard plastic ring. I fail to see how any air could get into Air Control Valve. I replaced the PowerTune arrangement with the filter from the Dephi servo that I was replacing. The Delphi ring is a much more substantial foam ring that did not have, or need, a inner plastic ring to support it. A test drive showed that the braking was much improved. Now, after the engine is shut off, you can hear the vacuum being release when you press the brake pedal a few times. I remember that the Dephi unit also did that, but the new PowerTune one did not. I am hoping that this has resolved the problem, but will continue to drive the car with caution until absolutely sure.
Graham Gilmore

Sounds promising, and well done for going under the cover. But surely they can't all be like that? Should the hard plastic have been removed at manufacture, I wonder? And with a fault like that I'd be wondering how good i.e. safe the rest of it was.
paulh4

Paul. Yes it a worry about the rest of it. I have raised the issue with Moss and will wait to see what they offer as an explaination.

If you removed the plastic collar I think that the foam on its own would collapse, possibly obstructing the air inlet valve. A very poorly executed filter.





Graham Gilmore

Does look pretty flimsy and ratty.

I do see holes in the plastic though, so that should let air in to the back of the diaphragm when the master-cylinder pressurises the fluid. It's that which gives the servo assistance when there is a vacuum on the 'fluid' side and air at atmospheric pressure on the back. Several holes perhaps, like the air-valve cover.

It's a long time since I had either of mine off, that was on the roadster to try and cure sticking on in hot weather.
paulh4

I note the hole in the plastic "ring". Does this need to be aligned with something to work? (I don't have a servo!)
Michael Beswick

I bought a Powerstop remote servo to put on my V8 Roadster, when the Original Lockheed unit started leaking, due to a rusty slave. It was dangerously flawed. I had to blank off the vacuum pipe otherwise the car was underivable. I've re-built several Lockheed servos and know exactly how they work, but I couldn't tell you why this thing behaved the way it did, and I didn't investigate.
I removed it straight away and returned it for a refund, and I had the Lockheed sleeved.
Allan Reeling

I doesn't need to be aligned with the holes in the cover as there is a space between the two. As I say I can't recall the proximity to the air-valve itself, you would need to look at the suspect item in-situ.
paulh4

I doesn't need to be aligned with the holes in the cover as there is a space between the two - mine anyway. As I say I can't recall the proximity to the air-valve itself, you would need to look at the suspect item in-situ. Maybe it uses closed-cell foam!

Looking back at notes Roger Parker suggested it could be an internal seal leaking back, such that the 'servoed' fluid was pressurising the master cylinder fluid. Maybe not in this case if just changing the filter has solved the problem long-term. But that's three strikes against Power Tune servos.
paulh4

Have agreed with Moss ffor a refund and it looks like I will have to payout the extra for a genuine one which appears to be from a comapny called Shiftec which used to be AP Brakes.
Graham Gilmore

I've not found that one but AP/Delphi are about 2.5 times the price of Power Tune which is a huge difference for a significant item in a single-line braking system. I note that although B&G list a non-OE item there is no price and it directs you to the AP/Delphi item. Many others show the cheaper item.
paulh4

Paul, I have just finished taking the PowerTune one out and noticed a sticker that shows that it was made in Tawain........!

Yes, the 'proper' one is £150 more, but that is the price of safety, expecially on a rally Car.
Graham Gilmore

Most things are probably made 'out east' these days and that has a long history, I can remember my mother being very sniffy about items labelled 'Empire Made' in the 50s which was a euphemism for that part of the world. AP components quite possibly are as well but one would hope there is decent inspection and testing closer to home. Other Famous Names no longer do, Lucas for example are head quartered here (not a million miles from me) but stuff for our cars is neither made here nor tested here based on my experiences with their indicator flasher modules. They've never been the same since they changed their logo to an arrow going down-hill and backwards.
paulh4

This thread was discussed between 12/05/2010 and 16/02/2023

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