Oh No - Not more on rear brakes

[oldtimer]

Hi Mike, Jeff and All,
My interest in this subject has been prompted by the various problems members have experienced with Electric Handbrakes and rear brakes generally, so, to that end my aim is to gain a better understanding and share my findings with our members.

I now have both the electric motor assembly and a scrap rear Brake Calliper for a 03 year S type with manual transmission and electric handbrake, both of which are dismantled and laid out on the bench.

Just as a reminder the calliper is a single piston unit comprising of two parts. Part one, the anchor plate bolts to the hub carrier and accommodates the brake pads which in turn takes all the torsional loads imposed during the braking process. Part two, the body of the assembly houses the hydraulically and mechanically operated piston. This part is attached to the anchor plate via two slider pins. By pressing the foot brake hydraulic pressure pushes the piston and brake pad onto the inner face of the disc. The reaction to this, is for the body to slide on the slide pins and draw the outer brake pad onto the outer face of the disc. Both pads are now in balance and equal pressure is applied to both sides of the disc.

Having established how hydraulic pressure is applied the next bit is to understand how the same is achieved by mechanical means. The body of the unit is about three inches long into which a two inch long operating piston fits. The inside of the chromium plated piston is hollow. If we strike an imaginary centre line running from the centre of the piston through to the exit hole where the hand brake operating arm attaches, it is along this line that the mechanical elements run. About half way along this centre line is a cylindrical pressed steel cage or cassette which contains two thick discs separated by three 1/8 inch dia ball bearings. One disc is secured to the cassette by protruding lugs whilst the other is attached to the spindle that protrudes through the body onto which the hand brake operating arm attaches. The three balls are located into elongated troughs on one disc so that when the hand brake operating arm is rotated the balls run up the ramp in which the balls sit and force the two plates apart by a maximum of 1/8 of an inch.
Protruding from the other end of the cassette is a course threaded bolt (perhaps more of a spiral). This bolt also has a thick washer attached to one end and is secured in the cassette by protruding lugs but allows for some fore and aft movement only, this being equal to the amount the balls separate the two discs. The aforementioned parts are kept in place by an extraordinary strong spring located inside the cassette. So the order of assembly of the cassette is to drop the spring into the open end of the cassette followed by the spiral threaded bolt which will face towards the inside of the piston. Now insert the two thick discs separated by three ball bearings , compress the spring and turn over the claws to keep it all together. That assembly can now be installed into the body by passing the hand brake operating spindle through the hole and securing it from the inside with a circlip.

In reality, if the cassette were ever opened up the chance of successful re assembly is pretty remote.

Summary. So we have established that by rotating the hand brake lever on the calliper through 20 degrees,(about the range the external operating arm stops allow, and also the length of the slots the ball bearings sit in) separates two discs by a maximum of 1/8 inch. And also that when the two discs separate the spiral threaded bolt has a linear movement of a maximum of 1/8 inch. Apart from the hand brake operating spindle and lever nothing else rotates it's all locked in position.

The next and last piece is in this jigsaw is a 1.5 inch long barrel which fits inside the chrome plated piston. This barrel has a flange halfway along drilled with six holes. and is also internally machined with spiral thread which matches the spiral thread on the bolt protruding from the cassette. When the brake wind back tool is used this barrel is wound along the spiral bolt protruding from the cassette. This barrel is fitted with a rubber seal and is secured into the piston with a circlip.

Gosh, This has been hard work and longwinded but we now come to the point.
I can now understand why Jeff said, with the engine running press the brake pedal five times.
The reason for this is that the brake application encourages the barrel to unscrew until it makes contact with the piston. Remember earlier I wrote that the ball bearings separate the two discs to a max of 1/8 of an inch. If the barrel is not making contact with the inside of the piston one can waggle the park brake lever for ever and a day and nothing will happen so it is imperative to ensure as far a possible that the barrel is in contact with the piston

In the last couple of days I have had the rear callipers apart and after re assembly I ran the engine and applied the brakes many many times just to make sure. As I have no electronic equipment I was fearful of creating a problem. Both the foot brakes and the park brake work fine.

I'll let you know what I discovered with electric hand brake next and its relationship with the calliper.

Time for bed
Regards Oldtimer

[oldtimer]

Just a bit more,
Whilst dismantling the unit and writing the above I was still not fully convinced as to how the barrel followed the piston as the pads wore down. Having a fresh look at it I realised that I had not taken into account that the circlip retaining the barrel in the back of the piston actually drags the barrel with it, simultaneously winding down the spiral thread. Between the flange on the barrel and the circlip are two stainless steel washers with a pressed steel ring with eight ball bearings of about a 1/16 inch in diameter between them. In addition there is also a thin wavy spring washer lightly tensioning the barrel onto the inner face of the piston. I believe this arrangement is designed to reduce resistance of the barrel turning.

I still think that my original focus in that hydraulic pressure plays a part in the barrel turning process is correct. The surface area either side of the flange on the barrel differs. I believe that when the brake pedal is pressed, for the briefest of moments the barrel lifts and compresses the wavy spring washer which again assists with the rotation of the barrel. If this is not the case I fail to see the purpose of the myriad of holes drilled in the barrel and flange. The above turning process is of course very slow and proportionate to the rate at which the brake pad wears

On a lighter point for a moment, if all the above was the text for an examination. And the question was, from the text make a drawing, I wonder what the result would be!

General condition. Externally the calliper looked as though it had been dredged out of a pond but the inside was in remarkable condition considering. The chrome plated piston was a bit gunged up but cleaned up OK revealing minor areas of corrosion. The hole body is full of brake fluid so the parts referred to are therefore immersed in oil and have no sign of corrosion.

The protective boot over the piston is vulcanised to a steel ring, this steel ring was nearly rotted through at the bottom edge. The piston is sealed by a single square section O ring with another seal around the spindle of the hand brake lever. Finally, a seal around the nose of the barrel.

This has been an interesting little project for me and I now have a good understanding of how things work. I hope others can make sense of the text and benefit from it

Good night All
Oldtimer

[J44EAG]

Well done, Phil.

This would make a good story for the magazine. Photographs would be needed. Any chance please?
Happy to help with editing.

Mike