Brake servo failure

Fortunately brake failures because of servo problems are rare but unlike difficulties with slave cylinders where early warning signs can usually be detected, early warning signs of potential servo failure are few. The consequences of a sudden servo failure are spine chilling! This note from Victor Smith (Harvest Gold V8 1089) with contributions from Roger Parker, Geoff Allen and Jim Gibson, examines a couple of cases of servo failure and considers the options. (Jun 01)

"Never start anything you cannot stop - so think about the brakes before modifying other parts"
was the advice given to me by some of the Club members who raced MGBs when I began racing an MGB Roadster back in the mid seventies. So the braking system hydraulics were thoroughly checked, DS11 pads fitted, Mini Cooper slave cylinders fitted at the back to restore the balance and the dust shields removed up front to improve disc cooling. With the introduction of the MGBGTV8, the luxury of servo braking arrived providing reliable and powerful braking for the high performance MGB model. As the V8 model approaches its 30th year, a number of the components are clearly ageing and preventative maintenance needs to be considered.

One area for consideration is the servo because there have been reports of a few failures, fortunately a few, but the consequence of a failure of the rubber seals or other key components in the servo can lead to the servo vacuum swallowing the brake fluid resulting in a rapid loss of braking power! A spine chilling thought and for those of us who have experienced a servo failure, a very seriously unpleasant shock that causes not only considerable shock and but reflection on life generally!!

Three cases of servo failure have been reported over the years which have involved the following problems:

o Cracked servo vacuum piston
On stripping down my servo following a rapid brake failure a few years ago (fortunately at 25 mph which the handbrake could control), Geoff Allen found the cause was a cracked vacuum piston. He felt that the component may well have been cracked from manufacture but it was the only one he had seen in that condition in over 30 years working on MGBs. Usually servos with problems had the plastic seal on the piston leaking although Geoff recalled that one restoration of a very late car which had stood for many years, had corrosion on the hydraulic cylinder wall of the servo.

Seal failure in the servo
Jim Gibson (Flamenco Red V8 2435) reported he had recently had a brake failure but fortunately it occurred as he was reversing out of his garage. He was able to grab his handbrake when the foot pedal went straight to the floor - the beech tree behind the car survived unscathed! Jim commented "when my heart rate returned to normal, I realised that the more than usual quantity of "steam" from the exhaust on starting up should have told me something was not quite right - it was of course the brake fluid from the servo going straight through to the inlet manifold."

o Hydraulic fluid leaks around air valve piston
Peter Berry (Teal Blue V8 0201) also reports a brake servo failure due to hydraulic fluid leaking around a tiny air valve piston. This occurred slowly over a period of time, (more frequent checks of fluid levels have been performed ever since!) quietly depositing the contents of the master cylinder into the servo shell. When the level of the fluid reached a point where air entered the system, the results were obvious. In this instance repeated pumping of the pedal produced sufficient pressure to stop the car and no damage was done. On removal of the servo, the contents of the master cylinder were drained from the shell. No fluid actually left the car, it was simply redistributed.

Roger Parker (Green V8 4092) comments that "seal failure in the servo is about as probable as leakage from a master cylinder. Over time there is a more than reasonable likelihood that the seals will fail and result in a sudden failure, unless you happen to be the Lottery winning type who just happens to check the brake Hydraulic fluid level when only half the reservoir has emptied as the seal begins to go! With a weak seal between the end of the servo piston chamber and the hydraulic fluid system, there is a natural draw on the fluid from the vacuum created by the engine. I can see that with a failure it would follow that you could see a complete drain of the reservoir - very quickly!"

"The very big issue here is not the failure, which is something to expect over longer periods (but still a serious shock when it occurs), but two specific points. Firstly the service schedule states that all
brake hoses and seals should be replaced
routinely every 36,000 miles or 36 months. This is a recognition that these parts degrade and so the routine replacement is a point that has to be stressed as an essential safety service item. The other point to mention is that with a servo seal failure you do not get the same warning of an impending problem as you do with a master or wheel cylinder failing - the tell-tale wetness that you get running down the pedal or the dampness on the drum inside the tyre. Often you also get an indication of a problem through a reduction in or a clear change in brake efficiency."

"Usually it is the end seal that attaches to the piston inside the servo that leaks. The leakage is much the same as that with the end seal of the master cylinder. However unlike the master cylinder, where fluid leaks visibly, the fluid is contained within the servo reservoir body and ultimately is drawn into the inlet manifold and is burnt in the engine. Although there is an increase in visible "smoke" from the exhaust, this is usually so small a difference from the normal water vapour cloud from a cold engine. Most people will not notice the slightly increased "smoke" in the exhaust from the fluid leaks. So with no visible clues, the sudden onset of the consequences of a servo failure are obvious and understandable. Hence the need to emphasise the need and value of the brake hydraulic inspections and maintenance at the recommended service intervals. So whatever you do, don't scrimp on brake and servo maintenance." A replacement servo costs around £170 or alternatively have a good unit refurbished with new seals. A small price to pay for reliable brakes!

Now a final caution about silicone brake fluid
This comes from Ron Gammons (Black V81931) who says silicone fluid attacks rubber seals and causes swelling. Girling, a leading brake component manufacturer does not recommend silicone brake fluid. So make sure you use the correct hydraulic brake fluid.

Reproduction of a warning note from AP Lockheed on problems with silicone fluids in braking systems

Silicone brake fluids - beware

Our technical service department is receiving an alarming number of calls from motorists reporting problems with silicone fluids. AP Lockheed neither markets such fluids nor recommends their use with our own or any other braking system.

Virtually all the problems relate to long/spongy pedal, sudden loss of brakes and hanging on of brakes. They reflect certain properties of silicone fluids identified by us over many years and recently ratified in SAE publications, namely high ambient viscosity, high air absorption, high compressibility, low lubricity and immiscibility with water. Research has shown that the relationships between problems reported and properties identified may be expressed as follows:

Long/spongy pedal
o Compressibility, up to three times that of glycol based fluids.
o High viscosity, twice that of glycol based fluids, leading to slow rates of fill and retention of free air entrapped during filling and hence bleeding difficulties.

Sudden loss of brakes
o Air absorption - gasification of absorbed air at relatively low temperature produces vapour lock effect.
o Immiscibility (failure to mix) with water - whilst the presence of dissolved water will reduce the boiling point of glycol based fluids, any free water entrapped in silicone filled systems will boil and produce vapour lock at much lower temperatures (100C or thereabouts).

Hanging-on of brakes
o Low lubricity - in disc brake systems the sole mechanism for normalisation if system pressure upon release of pedal pressure is a designed-in tendency of seals to recover to their "at rest" attitude. Low lubricity works against this tendency.
o High viscocity, exacerbating the above effect.
It should not be assumed, therefore, that the high price of silicone fluids implies higher performance in hard driving or even normal road use.
AP Lockheed glycol based fluids do not contain the adverse properties described above. The recently introduced Supreme DOT 5.1, which exceeds the performance criteria of DOT5, is suitable for all conditions likely to be encountered in modern driving conditions.

See comprehensive article on brake fluids. More
V8NOTE228 - servo failure & brake fluid.

V8NOTE228A - additional notes on the debate - silicone or mineral brake fluid for V8s?

V8NOTE228B - extra note with the views of Georg Muller on silicone fluid.

V8NOTE398 - comprehensive review of brake fluids by Bob Owen
V8 Register - MG Car Club with support and services for MGV8 enthusiasts