Front Brake Calipers - Strip Down and Overhaul

The front brake calipers are now serviced and ready to go back on the engine.

• The procedure for removing the disk brake units is covered in Operation DJ.451-1 in the first section of Volume 2 of Manual 814 (or Op. Dx.451-1 in 518). 
• Overhaul is included as Operation 453-1 in the second section of the same volume 2 of the 814 manual (or DX.451-3 of 518).
• Adjusting the alignment of the brake units (parallel to the discs) is covered in Op. D. 451-0 in Volume 1 of manual 814.
• Bleeding the brakes is covered in Op. D. 453-0 in Volume 1 of manual 814.

The brake units bolt on to the differential housings - which in turn are bolted to the gearbox/ bell housing combo.

The brake units bolt onto the differential shaft housings

So you first separate the drive shafts from the engine. I had removed my brake units from the bell housing several years previously when the engine was stripped. The two big black bolts on each side were very tight but eventually yielded with a satisfying crack.

Brake units removed - September 2015

I’ve heard of various techniques for removing the pistons from the units – including pumping grease in and pumping LHM in. I just needed a squirt of compressed air and the pistons popped out.

Brake piston

The hydraulic pipes that run between the two halves/ cylinders were removed. The rubber anti-vibration 'diablos' were also easy to get off. Clamping each unit carefully in my vice, I drifted out the metal tube in the rubber 'diablos'. This then made the diablos much easier to remove.

While the units were in the vice, the bolts holding the halves together were undone . Again, they were quite tight and I used a breaker bar to get purchase.

Splitting the caliper halves

 I separated the caliper halves and took stock of there bits and pieces.

Each caliper half has a rubber seal - to provide the main seal against hydraulic fluid leakage - and a felt dust seal.

The old felt seals were winkled out.......

......as were the rubber seals.

The grooves and general cylinder area were covered in black oily goo. Although I considered sending the housings for vapour blasting, the pairs of brake unit halves were first given a bath in my ultrasonic cleaner. In particular I used a toothbrush to clean out the grooves of the seals as I planned to cover up the cylinder apertures before the units were vapour blasted. I wanted to keep the cylinder bores as clean and smooth as possible.

I managed to get a lot of dirt off.......

..........but ultimately wasn't happy with the results on the alloy calipers and decided to have them vapour-blasted. After I got them back, I gave them a good rinse to make sure any stray glass beads were flushed out. All of that was more than four years ago!

Picking up the task again in 2023, I soaked new felt seals in LHM for a couple of hours before fitting and gave the rubber seals a brush with LHM before fitting those. 

Parts organised. Felt seals soaking

They went in very easily.

New seals fitted

With new seals fitted, some people have reported struggling to get the pistons to go back in the calipers but I didn't have any problems. I brushed the piston with LHM an carefully inserted it in the body - being careful to make sure that it didnt snag on the felt seal. The piston stops when it reaches the rubber seal. 

 

There is a very slight bevel on the lip of the piston which I think is to help it get past the rubber seal.

I just pushed down evenly on the rim of the piston with the palm of my hands and the piston suddenly went past the seal with a 'pop'. 

A piston refitted

I reunited the caliper halves and torqued them up (49 Nms).

The rubber anti-vibration 'diablos' that are part of the front engine mount set-up had been easy to remove, but were a pain to fit. The manual says to use a little water to help ease them in but they were stubborn buggers. All four of them.

 

My car is a bvh so the arrangement of the hydraulic pipes is different. There are no bleed nipples on the calipers. 

Working out the pipe connections

Instead the right hand brake unit is bled from the centrifugal regulator sitting over the hydraulic pump, and the left hand unit is bled from the accelerated idling device connected to the carburetor!

 

I found one of the bolts and the clamps for the brake pipes.........but then spent AGES looking for the one shouldered bolt that serves to hold a brake pipe clamp and also some other clamp that I simply can't remember. I'd definitely had it, as it was there in one of my disassembly photos.

I'd even put it aside for zinc-plating at some point.

But couldn't find it in 'box 14'.........I looked through all my other boxes before I found it. In Box 14 the whole time. Grrrrr!  With the bolt found, I refitted the hydraulic pipes that join the caliper halves. 

Refitting the hydraulic pipes

That's that for the moment. I'm not fitting the brake units to the car just yet. However when I’d removed the drive shafts, half of the studs had come out with their nuts. These studs needed to be reset in the shaft flanges so i finished that job too.

I double-nutted them on the opposite ends to the stubborn nuts to give me some purchase. The studs were cleaned up on my wire wheel and degreased. I ran a small wire brush over the female threads and followed up with a nylon brush dipped in isopropanol alcohol.

With the studs and threads de-greased, I applied threadlock to each and double-nutted them back on the flanges.

Studs cleaned and refitted

All ready to go!