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MG MGB GT V8 Factory Originals Technical - Brake servo

Hi all,

quick question

I have done a V8 conversion on my 1969 mgb roadster. I am using a Edelbrock 500 carb
I have fitted a Lockhead servo (secondhand but i was told it was off a running car) as fitted to later MGB's but he problem I am having is that when the engine is running gradually the brakes come on and will not release until I stop the engine. This can take upto 10 minutes to happen but as i say when it does the brakes will not release. The servo is connected into the back of the carb

Any ideas please

thanks
Steve
Steve D

Shouldn't make any difference where it is connected, although normally they are on the inlet manifold, don't know how that compares with the back of the carb.

Is this the remote servo on single-circuit brakes or the later dual master with integral servo? I know the former can stick on, but that is a different issue. I think there was also a recent issue with the latter type where the actuating rod was too long and operating the servo at rest, fixed by a shim between (I think) the master and servo. The remote servo operates by fluid pressure operating a piston first closing off the passage way between the front and the back of the diaphragm, further pressure opening the back of the diaphragm to atmosphere allowing manifold vacuum on the front of the diaphragm to pull it forward and assist braking. If that piston or its valves were faulty you could get this.

I suppose the first thing to do is disconnect the vacuum and see if it still happens. If it does then it isn't the servo, so could well be the pedal not coming back far enough to allow the master piston to clear the bypass holes, heat from braking causing fluid expansion and the brakes to be applied. This could be from a number of causes, one of which is the later mechanical brake light switch being screwed too far in. But if the brakes release the instant the engine is turned off then that is definitely a servo problem, as the servo should retain vacuum for several pumps of the brake pedal after the engine is stopped, but that is achieved by a one-way valve between the servo and the inlet manifold normally, which you may not have.
Paul Hunt 2010

Paul,

at the moment I have the servo disconnected and have no problems with the brakes. The servo is the early single circuit type that is remore and mounted on the bulkhead

thanks
Steve D

Hi Steve
There are 3 hyd connections on this type of servo. If you have added this as a mod, it is possible you may have the hyd connections wrong.
As Paul mentioned, vacuum is normally from the manifold. Depending where you are taking from the carb... you may be getting positive pressure instead of vacuum..long shot, but worth checking?
b bourne

Three hydraulic connections? Only one in (at right-angles to the cylinder near the diaphragm chamber) and one out (on the end of the cylinder) on my two, plus the vacuum connection.

I'd suspect the air valve then Steve, or maybe its piston isn't fully going back into the bore.
Paul Hunt 2010

Paul,

do you mean the white valve with the diaphragm that is on top of the servo?

The hydraulic pipes are correct

cheers
Steve
Steve D

That's it, under the cover is the diaphragm, under that the 'diaphragm support' that lifts the diaphragm, then the valve housing, a joint washer and the piston. It's the piston sticking that causes the servo to jam the brakes on to various degrees. My roadster tends to do it in hot weather, some say to put a spot of silicone grease on the piston (although) mine already had fluid on it, other say that diaphragm should be pointing down not up to keep it 'wet' (and it can trap an air bubble, the cylinder should also be angled up for the same reason, and some Lockheed documentation shows it fitted this way), yet another has said he had to polish the piston and bore with fine emery. However all these were to do with sticking on, not coming on by themselves.
Paul Hunt 2010

I've had this problem many years ago. The servo case was slightly out of round & the diaphram would jam. This has to be a very rare situation though. I would suspect the air valve allowing air in to operate the servo, or there might be enough residual pressure from the master cylinder to open the air valve. Some of the single M/Cs had a valve in them . I was once told that this valve had to removed if a servo was added. I have had no practical experience of this & I don't know if it's true or just an urban myth. Barrie E
B Egerton

I've bit the bullet and bought a new servo - 66
I'll fit this weekend and let you know if it sorts the problem

cheers
Steve
Steve D

Hi all

new servo sorted the problem
thanks for the suggestions

cheers
Steve
Steve D

Steve,

You said the servo is connected to the back of the carb which would mean you have the carb on back to front so the primaries are not feeding the manifold as intended.

The centre large port on the carb is normally used for the PCV system and there is normally a vacuum take off on the manifold which is used for the servo.

Kevin





Kevin Jackson

Kevin

I have the same carb on my car as Steve, the Edelbrock has a port on the back meant for the EGR valve, that's where I am attaching my servo too, the PCV will attach to the front port.

The Wiz

Wiz,

Odd I've never noticed the EGR on my 500 and I had it to pieces last yesr to set floats and recalibrate for the new 4.35 engine, also it's not mentioned in the handbook.

I'll take a look next time I have the car out.

Kevin.
Kevin Jackson

Hey Wiz Kevin might have his carb on backtofront so which end should he connect his egr valve when he gets one do you think. Only joking Cheers Willy
WilliamRevit

On Edelbrock carbs I've seen, there is a nipple for the PCV valve on the front between the idle mixture screws. There are two vacuum ports on the front also; one for the vacuum spark advance and the other for EGR vacuum control. On the back, there is a threaded port for the power brakes. I don't think you would want EGR gases going directly into the carb and heating it up.

Wayne
Wayne Pearson

This thread was discussed between 09/05/2010 and 29/05/2010

MG MGB GT V8 Factory Originals Technical index

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