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MG MGB Technical - mgb brakes

Hi
i got problems with my brakes, its a rubber bumper 1980 model, with servo. Fail on the MOT rear brakes, more than 30% between left and right rear wheels effencie.
replace the rear brake cylrs, no good, trace back the fault, the copper pipes were 90 % blocked, due to been smashed against the rear shaft by suspention vertical rods.
now replace flexible copper pipes, still no pressure at rear wheels.
remove the master brack pump from servo, strip it and replace new seals,put it back, new dot4 oil, still no pressure at the rear wheels...what am I doing wrong??
i know this is a trick brake master pump..but..
thanks
amf rodrigues

So you say no pressure at rear wheels? I would now trace back for blockages. The rear cylinders are new so this should be ok, the copper lines presumably from the 'T'piece at the axle to the rear cylinders should be ok. Was the 'T piece itself blocked? and what about the flexible line from the main brake line fixed to the floor of the car to the 'T' piece? Did the rear cylinders flow fluid well during the brake bleeding sequence? I have an Ezee bleed system which is very good as you can handle it single-handed and you are using pressure from the spare tyre to force fluid through the system. But you are seeing now what I mean, work back to the master cylinder itself checking where the pressure fails. The front is ok I presume? On my GT the master line leads to another 'T' piece where it plits front and rear. In fact in mine I have a remote servo, so the line from the master, goes to input of servo, then output of servo to the 'T' pice for the split front rear. hope this helps. Mike
J.M. Doust

A 1980 has the split system so the rear brakes are separate to the fronts from inside the master cylinder onwards. With a 30% difference in pressure between the two rear wheels it could have been something as simple as contaminated drums and shoes one side, or a crushed pipe running along the axle to one side, which can be caused by strapping or chaining down to a flat-bed truck.

If you now have 'no pressure' then something else is wrong, as this seems to be worse than the original fault. Air in the rear pipes as a result of having opened the hydraulics and replaced components will reduce rear braking effort to zero even though the front brakes are working normally, and the car appears to be stopping normally (the rears add very little braking effort). But if that were the case the braking balance warning would be activated - *if* it is working.

As Mike asks did you get a flow of fluid out of the rear bleeders after all the work? The problem with a split system is that one circuit working normally will stop the pedal going to the floor which makes bleeding the other circuit difficult, unless you use a continuous flow system like the EeziBleed. The pedal will also trip the balance switch (unless slackened beforehand) and have to be reset. If there is no flow now then there is something wrong with your parts or work, you will need to slacken each connector between the master and the slaves to see just where you have fluid and where you don't.
PaulH Solihull

I have a 77 b with the bendix servo and the dual brake system. bleeding the brakes is very different from earlier brake systems. The haynes manual refers to the procedure. The basics is that bleeding has to start on the wheel closest to the master cylinder the last brake to bleed is the on the wheel furthest away. Before doing that one has to remove wiring from the pressure switch and then screw out the switch far enough so as to not touch the piston in the master cylinder. The piston balances the distrubtion of the fluid and thereby pressure in the system. But as Paul indicates the warning light should come on if pressure distribution is wrong due lack of fluid.
/Moss
Moss

Thanks, i have replaced the master seals, i still get no pressure on the line, both front and rear, and as Moss reckons bleding is diferent from the early ones, thats where i think was wrong, start from longest bleed, so i will try from the closest one.
regs
amf rodrigues

..something else, when i stripped the the master i noticed that the first rubber seal near the servo was facing backwords, but i assembled it according to the manual sketch facing foward...i though i was confused...which is the right way??
Also now when the brake pedal is depressed, Servo tube push air into the manifold..does not seems right..
regs
amf rodrigues

For each circuit, each piston should have two seals. On the piston nearest the pedal, i.e. the primary piston for the front circuit both seals should face forwards. On the secondary piston i.e. for the rear circuit the rear-most seal faces backwards, and the front-most forwards. The seals should face the direction where they are going to face the greater pressure. The rear seal on the primary piston only prevents leakage from the bypass port when the pedal is operated, the pressure is negligible to zero, which is why it may have got away with being round the wrong way. The front seal on the primary piston generates the pressure for the front circuit of course, but this pressure also presses on the back of the secondary piston, so both those seals must face each other. The front seal on the secondary piston is pressurising the rear circuit of course, hence must face forwards. These other three seals must face the correct way or the system will not work correctly.

How are you breeding i.e. pedal or continuous flow such as EeziBleed? Are you getting flow without pressure? Or no flow? Pedal bleeding is trickier on split systems. It makes sense to start with the front circuit as otherwise half the pedal movement in trying to bleed the rears first is lost inside the master. However I don't see why one should start with calliper closest to the master when with a single circuit one does the furthest calliper before the nearest. With an EeziBleed it won't make any difference which end you start with, but it will make the same difference as to which side you do first as with a single circuit, i.e. I would still do the longer run before the shorter.

Are you sure the servo is pushing air into the manifold? How are you determining this? Or is the engine running and so it is the manifold sucking air from the servo? This can be from any number of mechanical causes inside the servo such as split diaphragam etc.
PaulH Solihull

Paul
the front piston (rear brakes) has both seals facing foward, the back piston (front brakes)also facing foward, so i assume they right on as per sketch
the servo is pushing air into then manifold, i determined it by disconecting the pipe from there, when engine is off.
iam trying to bleed it by pedal, no easybleed, so tomorrow i will get the master off, onto to the vice, and operate it there until i got pressure, them move to position tight the pipes a so on.
even with a fault servo shouldnt i be able to bleed them or at least get some pressure?
regs

amf rodrigues

Didn't Paul just say that the rear brake seals don't face the same way?

<On the secondary piston i.e. for the rear circuit the rear-most seal faces backwards, and the front-most forwards.>

Charley

C R Huff

Amf Rodriues, Is there someone close to where you live that can help you? This is all sounding like a 'pigs ear'!And I do not mean this unkindly, but when I get into a complete 'pickle' I get someone in. Mike
J.M. Doust

Thanks for your replys i most appreciate, remove master cylinder again check everything, use Paul advice, but i put the rear seal facing backwards as found, make sure on the bench that the cylinders were oil fil, assembly on position and there you are pressure is coming back...
iam too stubborn to give up...
bst regards
amf rodrigues

If the front piston i.e. rear circuit seals both face forwards then all the pressure developed by the rear piston for the front circuit will go past the incorrect seal and up into the reservoir via the rear circuit bypass hole.

I've not got an integral servo only the remote, but it does seem to me that there could be some air displaced by the servo when it's manifold hose is disconnected and the brake pedal operated. However until you get the correct pressure and pedal travel at all four corners there is no point looking at the servo.
PaulH Solihull

Man made fault...i got it on the road, but i asked for an expert to come along, the problem was that there is a nonreturn valve missing, that is suppose to be on the first filler (for front wheels), without it the oil returns to tank when relise the pedal...i think the previuos ower lost, but manage to bleed like we did...
the servo appears to work ok.
thanks
Alfredo
amf rodrigues

Well, yes, the residual pressure valve that is on one MGB master has been mentioned before as a source of long pedal travel, but it shouldn't have caused total lack of pressure.
PaulH Solihull

Have you taken the bendix servo apart (not the master cylinder) - two halves ? After reading all the threads the diaphram condition has not been metioned only the master cylinder! My problem was a split diaphram which caused idle speed to change and lack of pressure in the system...
/Moss
Moss

This thread was discussed between 27/01/2011 and 09/02/2011

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