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MG MGB Technical - Brake Servo

My late model rubber bumper car sucked all the brake fluid from the master cylinder into the engine.
Would this be a faulty servo or faulty master cylinder seals?
Thanks your reply in anticipation
Regards Trevor Burnett
t burnett

master cylinder,-- but you'll also need to suck as much fluid as you can out of the booster through the vac. hole with something like a mightyvac to stop the smoke

willy
William Revit

My 79 MGB did the same thing some 10 years ago. The master cylinder seal failed and the brake fluid was sucked into the intake manifold and burned along with the fuel.

Replace master cylinder.

Cheers

Gary
79 MGB
Gary Hansen

I thought this kind of failure was due to the servo diaphragm giving up the ghost. As far as I know it only happens to cars equipped with the remote servo, which is why I won't have one on my cars. I copied this from the MGCC V8 Register site. https://www.v8register.net/subpages/V8NOTE395.htm

Servo failure on the MGBGTV8 model has been an important topic for members with the original Factory V8 as the servos on most cars are now approaching 35 years old! Bob Owen posted a response on the V8BB commenting "the problem described in V8NOTE228 is one I have experienced with a servo failure and loss of brakes on my chrome bumper MGBGTV8. To my knowledge that is just one of a dozen or so similar episodes suffered by other V8 enthusiasts. So far as we know, fortunately all have survived! The fault - servo related brake failure - only happens with remote or indirect servos as fitted to many cars in the 60s and 70s as enhancements to existing non-servo braking systems".

Remote or indirect servos
Remote or indirect servos sense the FLUID pressure and assist this - see the extract from the service manual to see the principle of operation. Their advantage is that they can be placed anywhere in the system and do not have to be at the pedal. The remote type is sensing fluid pressure and assisting via a diaphragm with vacuum from the inlet manifold and the two are separated by a seal.

This rubber seal is overlooked when the brakes are serviced and leakage at the seal produces no tell-tale fluid seeps or brake pull. This is because the fluid leaks are drawn into the engine or stay in the large bowl of the servo. Moreover, the engine vacuum is ALWAYS there to draw fluid through the servo seal, unlike other brake seals which only experience a pressure differential under actual braking. Consequently a servo seal leak can be quietly emptying your brake master cylinder reservoir as you sail along the motorway and the first you know of a serious loss of fluid is when you apply the brakes at the exit - and find you do not have any! This happened to one of Bob Owen's Low Brake Fluid Sensor customers; faced with a loss of brakes at a motorway exit he decided he did not really want to get off there anyway and would go along the hard shoulder for a while . . . handbrake on . . . and a spine chilling cold sweat!

Single or split/dual brake lines
Later cars have dual braking systems each serving two wheels so, even if the car is fitted with a remote servo, this failure mode would still leave diagonal wheel brakes operational.
Mike Howlett

Diaphragm failure doesn't cause fluid loss on either system, 'just' loss of assistance which apparently is much more noticeable on the dual system than it is on the single remote. There has to be seal failure to lose fluid via any route.

"this failure mode would still leave diagonal wheel brakes operational. "

Completely wrong for MGBs including the V8. One circuit supplies the front and the other the rears. If the front fails you really would know about it as rear brake effort is much less, however if the rear fails you may not even realise except from the balance warning - if working.

The problem is that the front circuit is closest to the push-rod, and if the push-rod seal fails it will be the front circuit that loses pressure. There is just a simple rod joint between the servo and the master, so fluid would have to leak into that space first, which may or may not be retained by a seal or gasket between the two units. Vacuum pressure is between the master and the diaphragm, so fluid would have to get into the servo past it's half of the push-rod for fluid to be consumed by the engine. Logic dictates that there must be a vacuum seal at the front of the servo, but whether that's part of the servo so is also leaking, or between the two units, I don't know.



paulh4

Paul is completely right about the last sentence from the V8 Register article. The split on dual circuit cars is front and back, not diagonal. Unusual for Victor to allow an error to creep in when he edits these things.

The comment about the servo diaphragm was my faulty understanding. In the article it is the servo rubber seal that is implicated. Whatever the cause there have been a number of cases of total brake loss on cars fitted with the remote servo in which that component was found to be the culprit.

However, all this may be irrelevant anyway because Trevor says his car is a late rubber bumper model, in which case it wouldn't have a remote servo, but an in-line one which is mechanical rather than hydraulic. I should have picked that up before posting.
Mike Howlett

Mike
I had one of the remote boosters on my v8 playing up a bit once , it had sucked in some moisture and rusted the shaft inside unknown to me at the time,
The result was the brakes were dragging slightly for a little bit after braking, it was only minor so didn't worry too much about it---big mistake
Like a twitt I went to a race day like it, got up into the hairpin which is an almost standstill from entering at full hook about 200kph in top gear ,let off the anchors and they stayed hard on--them behind were not amused
willy
William Revit

I fitted one of Bob Owen's Low Brake Fluid Sensors for my remote servo on my 76 Roadster many years ago. it's a great safety feature and well worth fitting. Details on this website:

https://www.v8register.net/subpages/V8NOTE331.htm

Andy

Andy Robinson

That web site hasn't been available for a while, I believe. It also seemed pretty expensive to me so I made my own.

I've also had the roadster brakes sticking on, in hot weather and slow-moving traffic. There have been several things mentioned to fix it like lubricating the air-valve, turning the servo over like the after-market kits show, and polishing the bore of the air-valve piston, but none of them are a guaranteed fix. A pal's V8 was so bad we had to disconnect the hose from the servo and plug it with a spare spark plug, and even though I was looking for a difference in brake feel it was barely evident. I also went straight from driving my own roadster to a pals and didn't notice any difference, only realising afterwards his doesn't have a servo. It's one of those things that if you don't have it's not worth fitting, and if you've got one that's causing problems take it off.
paulh4

I think Paul is right in that the remote servo at least makes little difference to an MGB's brakes. After all, the actual brake discs and calipers didn't change at all (apart from the V8) from the earlier non-servo cars to the end of production, and the early car's brakes always worked perfectly well without undue pressure from the driver's foot. To me the remote servo was just unnecessary added complication.
Mike Howlett

I did think about 'upgrading' to servo spec, but decided against it in the end.
Dave O'Neill 2

Many thanks guys,

I have ordered a new servo from Moss, not too expensive and probably a lot cheaper than having the servo overhauled in South Africa, I have also ordered a master cylinder kit so will send the forum members the results when the units are removed for renewal and overhaul.


Thanks again for all the positive feed back.

Rgds Trevor burnett
t burnett

It'll be the master cylinder causing the problem---
The booster would be ok drained out but a new one is better I guess----This has been a case of people talking about the wrong type of booster
William Revit

Not so much 'wrong' as both types and indicated as such.
paulh4

Trevor--A question -are there many MGZT260's in SA
William Revit

Morning William, only one that I know of, it was sold a while ago, do not know who the past or current owner is.

Rgds Trevor
t burnett

All good Trevor -thanks
i was just interested if there were heaps of them about or not many--sounds like not many
A good friend here has one that came from SA when it wasn't very old (about 10-12 years ago) It was one of about a dozen cars that were modified for or by a dealer in SA with uprated cams, larger throttle body and -? and are rated at 300hp instead of 260, -it slips along quite well,,now.
We've had a real go with it, unknown to me someone had tried to reflash the ecu and filled it up with bugs. It's been a bit of a marathon getting it sorted, but now, at last, it's a bit of a weapon--
Cheers
willy
William Revit

The newly supplied servo fitted, obviously made in China, the servo was supplied through the local mg spares centre, The Sports Car Centre whom I suspect gets his spares from Moss.

The only problem was that the mounting studs pitch was 0,5mm smaller than the the original so I had to open up the mounting plate holes to 8,5mm, no big deal BUT WHY !!!

Overhauled the master cylinder and as we all know these later master cylinder are a bitch to bleed but managed, brakews are good but will give a bleed again in a few months as there is always SOME ?? air trapped somewhere.

Still not too sure if the servo or the master cylinder was at fault as the master cylinder seals were all in good condition, the bore had been sleeved some 10 years ago and was perfect.

Anyway job done for under BP 200.00
t burnett

If the pedal is a bit long and spongy, but goes shorter and harder with a couple of quick pumps, then goes long and spongy again if released for a few seconds, then it's air.

If it's just long then could be rear brake/handbrake cable adjustment.

I could understand (up to a point) remote servo studs being a bit off, but on the integral servo that is very poor and I'd be concerned about other issues.
paulh4

This thread was discussed between 21/06/2021 and 25/07/2021

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