No brakes

[AndyC]

I have recently resprayed the engine bay which involved removal of the master cylinder and servo. On replacing these and bleeding the brakes using the old fashioned method of an assistant ended up with a spongy brake. Further bleeding of brakes had no change, spongy then become stiffer after several pumps but then return to spongy unless pumped. Replaced master cylinder as this seemed to have a leak from the piston. No improvement despite using an EZi bleed system air still coming through and no brakes. I have used a vacuum to try and bleed the brakes to no avail and this seemed to increase the air bubbles. Replaced the wheel cylinders as theses seemed to be more oily than they should be. No improvement. There is no sign of a leak anywhere having inspected the pipes to each brake. So is it a reasonable conclusion to now assume there is an issue with the servo. which has been disturbed during work. Is this likely to mean a new servo or is there something else to do. Many thanks.

[Ian F]

I haven't had problems with my servo (the remote type fitted to 1972 B) but there are a couple of things you could try.

Try bleeding the brakes with the engine running and the servo working.

Take a look inside the servo pipe which leads to the inlet manifold - is it wet with brake fluid? I am assuming that the servo still appears to be working ( makes a noise when the brake pedal is pressed with the engine running)?

Ian F

[Bumpa]

I would say it is a servo problem, especially since you have checked just about everything else. There have been a few instances reported of sudden and total loss of brakes because the fluid has all leaked into the servo can. Looking at the servo there is nothing to see until you press the pedal and there is nothing. Scary.

Honestly, an MGB's brakes are perfectly alright without a servo. The cars never had a servo for the first several years and when a servo was fitted there was no change to the rest of the brakes. So to get your brakes working again you could simply take the servo out of the circuit by connecting the two pipes together. Here's some more info from Paul Hunt's website mgb-stuff.org.uk

Other problems can be:
The most serious is when the seal round the push-rod fails, allowing fluid from the cylinder to be sucked into the engine via the vacuum chamber and vacuum pipe. This will empty the master cylinder, the first thing you know about it being when you suddenly don't have any brakes! A fluid level warning system is available from TE Electronics, however I made my own for a fraction of that cost.
A leak in the vacuum hose or main diaphragm, which will cause a gross vacuum leak into the manifold with the consequent effect on mixture and running, as well as both tests failing. Note the hose is special vacuum hose to resist collapsing under vacuum, not standard hose as for, say, cooling and heater systems.
A sticking air-valve piston causing the brakes to stick on. This happens on Bee in very warm weather when we are following a route on a run i.e. slower speeds, but not more normal speeds i.e. 50-70mph when presumably the engine compartment is cooler. It happened on a pal's V8 touring Ireland, and got so bad we opted to disconnect the hose from the servo and seal it with the pointy end of a spark plug clamped with a hose clip (that's when I had the opportunity to try one without the servo). One recommendation has been to put a dab of silicone grease on the air-valve piston. Mine had brake fluid on it, so I didn't think that was it. Someone else has said that the servo should be turned so that the air-valve assembly points downwards rather than upwards to keep fluid round the piston, but again as mine had fluid round out I didn't think that was it either. Yet another person reckoned he had tried both those without effect, so removed the piston and polished the bore with fine wet-and-dry and that did the trick. It does seem more likely i.e. jamming due to differential expansion in higher temperatures.
The servo can also be the cause of difficult bleeding. The 'correct' mounting position on an MGB is with the cylinder feeding the outlet horizontal and the air-valve assembly facing upwards and slightly forwards. However this can allow a small amount of air to get trapped under the air-valve piston, as well as a larger amount in the cylinder. This latter is because the outlet is drilled concentric with the cylinder and not at its upper edge like a bleed port is in the calipers and clutch slave. The effect would be much worse if there was any tendency for the cylinder to be tilted downwards, either due to the mounting brackets used or the attitude of the car. This is where mounting the servo with the air-valve assembly pointing downwards may help, and angling the cylinder upwards towards the outlet. I have seen fitting instructions for an after-market Powertune/Lockheed servo which is outwardly identical to the MGB unit, but shows a totally different mounting arrangement.

[Dave Wheatley]

Before you go the "no servo" route, check if there were no other factory alterations to the braking system e.g. different master cylinder, different calipers or diferent pads. 50 years ago I had a Mk2 Jag and the servo vacuum tank, strategicallyplaced under the front mudgaurd, rusted through - result absolutely no brakes. Just recently I had a similar experience with my Cortina which originally wasn't fitted with a servo which was an option at the time. Previous owner said he had fitted new calipers and that the brakes needed bedding in. The brakes continued to be hopeless. I eventually found that the calipers he had fitted were for a servo car. I eventually found a servo type master cylinder and servo, which when fitted cured the brakes.
We are lucky with MGBs that we can obtain spares for virtually everything on the car. Mk3 Cortinas, despite nearly 1,500.000 being made, are very difficult to find spares for.

[AndyC]

I haven't had problems with my servo (the remote type fitted to 1972 B) but there are a couple of things you could try.

Try bleeding the brakes with the engine running and the servo working.

Take a look inside the servo pipe which leads to the inlet manifold - is it wet with brake fluid? I am assuming that the servo still appears to be working ( makes a noise when the brake pedal is pressed with the engine running)?

Ian F

Thanks Ian, before I proceed with yet another attempt at bleeding, I have had the a look at the manifold pipe and a listen to the servo. The inside of the pipe is bone dry and the servo makes a sound like a bicycle pump when the brake is depressed. What does this potentially mean?

[Ian F]

That all sounds as if the servo is workung as it should. A majpor proviso though - diagnosis at this distance can be unreliable posting.php?f=4&mode=reply&t=1162&sid=1 ... d2c436a4ba#

Ian F

[Bumpa]

Regarding what Dave said about possible changes to brake components (which is a very sensible comment), I have double checked in two parts books and as far as I can see the front calipers and brake pads for the 1800cc MGB are the same from 1962 to 1980. As far as the brake master cylinder goes, assuming Andy's car has single circuit brakes, the master cylinder was not changed from 1962 until the adoption of the dual circuit brakes. Paul Hunt, who has compiled the remarkable resource mgb-stuff.org.uk, says that he has tried cars both with and without servos. In his opinion the difference in the foot pressure needed to operate the brake is hardly noticeable but that the non-servo car had less free travel on the pedal so felt as though it braked better (which it doesn't of course, but feel is important). I have driven many thousands of miles in a non-servoed MGB and have always thought the brakes were good, even when compared to a modern car.

[Dave Wheatley]

Strange, but true, my Standard 10 has the best brakes of my 4 cars. No servo. From 30/35 mph it will stop on a sixpence!

[Bumpa]

The reason I have brought this up is because of this article taken from the V8 Register website. When the article was written there had been 14 reported instances of near instant loss of brakes due to servo failure. The remote servo on the V8 is the same type as that on the 1800 car.
https://www.v8register.net/subpages/ser ... aution.htm

[AndyC]

Thankyou for all the replies, This weekend will involve another go at bleeding but this time with the engine running. Failing this if I remove the servo is there a marvellous piece of kit for joining the two pipes together or is it just a (simple)! case of taking the existing one from the master cylinder and connecting this to the brake union.