The fitting of a servo is relatively simple, providing the unit is suitable. This means it must be independent of the master cylinder. The best model for our car is the Lockheed LE15741. This model was intended for the car by Lockheed themselves but was also fitted to the sixties Sunbeam Rapiers and Humber Sceptres. The “ratio” of a servo is important because it dictates the amount by which pedal force is boosted at the wheel cylinders. If you use too high a ratio, the least touch on the pedal will have you screeching to a halt! The LE15741 has a 2.3:1 ratio and this delivers a nice balance between pedal effort and response. If you are using another model, you need to be as close as possible to this. A rarer alternative is the Lockheed 4257 - 813 fitted to the Mini Cooper and Vauxhall VX90 which has a 1.9 :1 ratio. Suitable MGB servos are available new but various ratios are supplied so you need to be sure what you are getting.
In either case, you are strongly recommended to have the unit properly refurbished if it is second-hand. A member has recommended Norton Classic Servos on 01494 562235. A mail order service is available.
Suitable brackets will be needed to mount the unit in the engine bay. Much will depend on other engine modifications, but you will probably have to sacrifice the position of the original glass washer bottle, perhaps substituting something more compact in conjunction with an electric pump. When mounting the unit it is best to have the output end of the main cylinder tilted slightly upwards and the side pipe union above the horizontal. This way, any air in the unit will rise to the point where it can be successfully bled away and no air locks will occur.
Remember too that the servo will be more demanding of the other brake components, so everything needs to be in top condition, especially rubbers on wheel cylinders. Best to give the system a thorough overhaul while the pipe-work is off.
New drums and shoes can be had new from MGA parts specialists, but they are expensive!
Two new lengths of pipe need to be made up to connect the servo to the existing system. Aim to keep the pipe runs neat and unobtrusive. Secure them well to the body and make sure the flares are correct for the end unions.
The output from the master cylinder is diverted to the side union of the servo and the output from the servo is linked to a four-branch union with the stop-light switch mounted on top.
The vacuum connection is made to the inlet manifold. If you have already dispensed with the vacuum-operated windscreen washer system, then this provides the perfect connection. However, there is also another tapped hole that normally carries a blanking plug, so you need to obtain a suitably threaded connector for the vacuum pipe.
If using a second hand servo, ensure it is properly re-conditioned. Second-hand is probably a false economy. Better to buy a new unit and be safe. Re-fill the system with new fluid after flushing it until clean fluid emerges from all bleed nipples. There is a school of thought that suggests using silicon fluid, which does not absorb moisture from the air but this is essentially a matter of choice and means cleaning old fluid out very thoroughly or total pipework renewal..