Brake Servo and ABS

As the garage is too cold at present, and I had found a company who said that they could recondition the brake servo, I made a trip to see Past Parts in Bury St Edmunds today. They listened to the story of the car, took a look at the servo, and apart from saying they couldn't help with the electronic part confirmed that they had the parts required for the servo and master cylinder. Apparently these have been used on some Jaguars and Jensens, and they saw no problem in a rebuild to as new - they are going to fit a stainless steel liner to the master cylinder - that will stop it corroding in the future.

That left me with the strange parts on the side of the servo.

A gentle strip down commenced. Interestingly neither AF nor Metric spanners fit the nuts and bolts which is very strange, so I resorted to an adjustable spanner (sorry). Must be whitworth or bsf.

First off was this cover, which revealed a mesh filter, with a decomposed fine plastic (sponge like) filter sat in front of it.

 

Then I removed what I had thought to be a motor from the top of it, leaving this

 

The silver rod is a plunger, connected inside to a valve block, here is the "motor" which is actually a solenoid

 

This has some contacts (a bit like an SU fuel pump) which I cleaned

 

At the other end (which I didn't remove) is a cover over a rubber diaphragm. The plunger moved freely, so I did not want to risk damage to the diaphragm.

 

How it Operates.

After sucking and blowing on the various pipes and emulating movement of the valves via the plunger and solenoid, this is how I think it works.

In normal brake operation the plunger is kept pulled in, by vacuum applied to the rear of the diaphragm, by the clear yellow tube. This allows full vacuum in the servo.

However when an ABS operation is signalled by the speed differential sensor and Mullard black box, power is applied to the solenoid. This pulls on the plunger, which moves out and then allows normal air via the filter to reduce pressure in the servo, either turning the brakes off, or at least significantly reducing pressure allowing the wheels to rotate again.

As the solenoid stays active, the plunger travels further up, and via an insulated rod breaks the supply to the solenoid, via the contacts. Vacuum on the rubber diaphragm then pulls the plunger back, which cuts the air supply from outside and restores full vacuum, applying the brakes fully again.

If the abs is still active, the cycle repeats, again and again, until the speed sensor does not indicate a lock condition (actually speed differential between front and rear wheels)

I imagine, and hope to find out that this gives a pulsing pedal like modern systems.

I am now going to content myself with cleaning and repainting this unit and replacing the hoses and clips, if it aint broke dont mend it, is the motto here.

Today we take ABS for granted, but if you consider the great British companies (Mullard and Ferguson Formula) that designed and made this work in 1972 it was one huge achievement, and so far ahead of its time.

2 hours work, 5 hours driving!