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MG MGB Technical - Fitting a Powertune Remote Servo on a MGB

Fitting a Powertune Remote Servo on a MGB

I have recently fitted a Power Tune remote servo to my 1976 mgb roadster and thought it would be useful to share the problems I had after fitting it and getting it to work.
Firstly a big Thank You to all the people who have contributed to various threads about MGB servos and brake bleeding. These have been invaluable in enabling me to complete the work particular thanks to Paul Hunt, Miles Banister and Garth Bagnall
The actual fitting of the new servo was fairly straightforward - ensure new copper washers are used when refitting the brake pipe banjo to the servo.
bleeding the brakes was a nightmare. Using a gunson eezibleed I bled the brake, staring at the rear nearside, then near offside brakes and then moving on to the nearside brake calliper and finally the offside brake calliper (nearest the master cylinder). So no problem there you would have thought?
When I tried the brake pedal, it was spongy, but pumped up hard after one or two stokes of the pedal. Thinking there must obviously still be air in the system, I then went through the whole process again, with still the same result spongy brakes, but after one or two pumps, they were fine.
I decided to try the brakes on a test run and although the brakes were ok after a couple of pumps of the pedal, the brakes were sticking on after braking and then released after a couple of seconds. Reading various threads it seemed that this could be caused by a sticking air valve piston. I found these websites really useful.
http://www.mgb-stuff.org.uk/servo.htm
http://www.triumph-spitfire.nl/servoimages.htm
I removed the air valve and air piston retaining plate and then removed the air piston itself. To do this you need to depress the brake pedal gently (engine off) and the piston will be forced out of the servo. Make sure you have a clean rag over the piston aperture as the piston will come flying out under pressure. Also be careful of some leakage of hydraulic fluid.
The threads I had read said that the brakes can stick on if the piston is sticking in the bore and the way to cure this is by lubricating it with a little silicone grease, or in some extreme cases of a piston sticking, by cleaning the bore with fine grade emery paper wrapped around a pencil. In my case I lubricated the piston rubbers with a little silicone grease and refitted it (you have to push the piston in hard to overcome the hydraulic pressure). I then re bled the brakes and tried the car on the road. The sticking brake problem was cured, but I still had the spongy brake pedal problem.
By now the car had a least 3 litres of new brake fluid pumped through it by the eezibleed and there was obviously still air in there somewhere. I had read the tips about cracking open a brake calliper whilst your Attractive Assistant pushed the brake pedal down as hard as they could and I had also read a tip of bleeding a remote servo by bleeding with the engine running and again cracking a calliper bleed nipple, but this time pushing the brake pedal down quickly as the bleed nipple is opened (the idea being that the additional pressure generated by the servo would force out the air that is trapped in it).
I decided to try the latter method and bled the calliper nearest the servo first with the engine running, but without using the eeezibleed. This worked a treat. When I opened the calliper and my Attractive Assistant quickly depressed the pedal a great lump of air came out. I then repeated the process on the offside calliper (nearest the master cylinder) and got no further air out.
I then tried the brakes Eureka, a firm brake pedal at last! The car was MOTd the following day and passed with flying colours.
Thanks again to all those who had helped me with this problem, I hope that this thread helps others in the future.

A T Robinson

How did you orientate the servo - as the factory did it i.e. horizontally with the air-valve uppermost? Or tilted so that the fluid outlet is higher than the inlet and the air-valve low?

When you say "bled the calliper nearest the servo" do you mean 'geographically' nearer i.e. the left-hand side or 'pipe run' nearer i.e. the right-hand side?

The two 'pressure' methods you mention seem to be the same except that in the second the engine is running. I've never needed to do that.
PaulH Solihull

Paul.
The servo was oriented in the standard, I guess factory fashion, with the servo tilted slightly from the horizontal and the air valve at the top. See Photo

I bled the nearest brake calliper to the servo, which is the nearside of the car on right hand drive cars. My thought here was that if there was any residual air stuck in the servo, then that was the nearest place for it to come out.

I seemed to me when I had put so much fluid through the braking system , that perhaps the Gunson Eezibleed just wasn’t up to the job and something with a bit more oomph and pressure was needed. I had two choices I guess, one was to buy a higher pressure or vacuum bleeding kit or the other was to utilise the actual pressure from the servo. I disconnected the Ezzibleed as I was not too keen on using both its pressure and the pressure from the servo simultaniously. So I started the engine and used just the servo pressure to bleed the calliper and it worked a treat.



A T Robinson

Ah yes, with rubber bumpers it changed from a single pipe from the servo to a 4-way manifold on the right-hand inner wing feeding the rear and both calipers, to two pipes from the servo one feeding the left-hand caliper, and the other feeding a 3-way manifold for the rear and right-hand caliper.

The EeziBleed *is* only a low pressure device, you wouldn't want high pressure with all those plastic parts, but I've always found high-pressure bleeding from the pedal is needed as the final step.

Inverting the air valve is said to cure the sticking on, which I get quite noticeably in hot weather. But rather than doing that, I think I'll opt for the polishing and silicone grease first, as the latter at least seems to have fixed yours.
PaulH Solihull

I guess your "B" must be similar to mine,I have a 1974 1/2 LHD and it is fitted with a tandem master cylinder, so a single remote servo can only work on one circuit, which surely make the brakes unbalanced front to rear and not pass the MOT test. Can you give us some more information how you fitted the servo into the brake system.

John
john wright

The brakes aren't balanced front to rear anyway, there is much more braking effort on the the fronts than the rears. If they were equal the rears would lock much sooner than the fronts and you would lose control. The brakes do have to be equal, within limits, side to side of course, and give a minimum braking effort front and rear. You *could* fit a servo in the front circuit if you wanted to, but paradoxically it would *reduce* overall retardation as the effect of boosting the pressure just in the front circuit is to reduce pressure in the rear.

John's is obviously an export model reimported, whereas ATs is an RHD - now at least. Clausager sez: The remote servo was standard on UK cars from August 73 (so perhaps a PO of ATs removed it because it went faulty). The unboosted dual-circuit master was never fitted to UK cars, export models got it from 1967 to September 74 depending on market, although some continued with the single circuit and no servo until 1976. North American cars got the boosted dual-circuit system with rubber bumpers, UK cars didn't get that until the 77 model year. Clausager doesn't say when other export markets got it.
PaulH Solihull

Paul, I beg to differ on brake balance, when car are in the design stage the size of discs,drums, calipers, wheel cylinders and friction levels of the brake material are optimized to get maximum retardation for sensible pedal efforts and a hand brake that complies with current regulations. I believe the MGB GT has larger rear wheel cylinders to compensate for increased rear axle loading, this is what brake balance is, yes I know the braking effort has to be equal across the axle.

Yes Paul my car was imported from the states and is a 1974 1/2 rubber bumper and is fitted with a tandem master cylinder but is unboosted.
john wright

Yes, the GT has larger rear cylinders than the roadster because the greater weight of the GT allows it (although the V8 had roadster cylinders because of its harder springs, and the roadster did originally have these larger cylinders but they were found to cause locking so were made smaller), but that is a completely different issue to front to rear balance. Look at almost any car with discs front and rear and it will be immediately obvious that the discs and callipers are smaller at the rear than the front. The 50/50 static weight distribution of a well-balanced car - and the MGB is very close to this - changes to about 70/30 in favour of the front under heavy braking. Have a look at this http://tinyurl.com/clf3jv9 from the section entitled 'So why is brake biasing necessary?' which explains how retardation is dependant on the weight on the tyre at each corner, and how weight is transferred from the rear to the front under braking, and therefore why rear braking effort is much less than front braking effort.

The MOT doesn't test front to rear balance in any case, the two ends are tested separately, and they only check that it exceeds a certain level. In fact more MOT stations are going back to testing classics and older with the retardation meter in the passenger footwell rather than the static brake test rig, so that checks 4-wheel braking as the first part, and handbrake as the second.
PaulH Solihull

I didn't explain that very well, and we may be talking at cross-purposes.

I took your comment "make the brakes unbalanced front to rear and not pass the MOT test" to mean that the brakes are balanced front to rear, which they aren't. Yes changing the wheel cylinders does alter front to rear balance, but only fractionally, the change in balance between the two types of cylinder is tiny compared to the difference between the front and rear overall.
PaulH Solihull

This thread was discussed between 31/05/2012 and 10/06/2012

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