Invicta Military-Vehicle Preservation Society


Jeep Rear Axle Disk Conversion Project

By Nigel Hudson – Weald Group

I ended my second article on upgrading the jeeps brakes with my intention to convert the rear axle over to discs as well but at the time hadn’t then managed to source any of the Lucas/Girling calipers. Late last year I did manage to find a second hand pair of Lucas calipers along with their associated carriers and mounting bolts. Unfortunately I was not so lucky with the condition of the calipers as with those I sourced for the front axle project. They were too far gone to be of use. However the carriers which allow the calipers to ‘float’ on the disc were in fine shape and I was able to find some brand new Lucas calipers manufactured by an aftermarket parts company. Soon after whilst browsing through Milweb, I noticed an ad by Rodney Rushton of the MVT for some laser cut disc brake brackets to allow for the use of Suzuki calipers on war time jeeps. That would certainly save time on getting a pair cut myself. Looks like I now had everything I needed to proceed with my rear axle project. However, after converting the front axle on my Hotchkiss jeep  back in 2016 to disc brakes, I noticed that although they provided a much greater stopping power, the effort required at the pedal remained the same. After a long drive in the jeep my right knee would begin to ache. I figured there were 2 ways to overcome this. Join a gym and build up my leg muscles or fit a brake booster (or remote servo as they’re otherwise known). The original Master Cylinder (MC) I was using was designed to  function with a drum not  a disc brake set up and discs do seem to prefer the added boost of a servo to get the best out of them.

I bought an aftermarket Lockheed 6″ booster  designed to fit an old MGB. The aftermarket boosters more commonly fitted to the MGB’s have a servo ratio of 1.9:1 but I managed to find one which gave an increase of 2.3:1 for even less effort at the pedal. My original plan was to fit it up on the firewall where the horn would normally sit. Try as I may I could not get the unit to fit in this space. Apart from it snagging on the carb, the fitment instructions advise it to be at least 6″ away from the hot exhaust and that location brought it into too close a contact with the exhaust manifold. Those of you with wartime jeeps have the option to fit a booster behind the driver side headlamp but with a 24V Hotchkiss, the air cleaner occupies this space. The only other logical space was up on the firewall the other side of the engine block. Unfortunately that is taken up by the voltage regulator (VR) unit. I got to thinking how I could get rid of the VR unit. My Hotchkiss was running the original 24V generator/VR set up so I had a couple of options. Remove the bulky VR unit with its old relays and fit an electronic unit in a much smaller relocated housing or get rid of the whole generator/VR set up altogether and fit a modern 24V alternator. Watching the ammeter needle ‘bounce’ back and forth whilst driving the jeep, it seemed to me that the original charging system could probably do with an upgrade too so I plumped for the 24V alternator option. 

My engine compartment before the project.
Booster bracket made from some thick alloy plate

When I mentioned all these proposed modifications to Gareth Wear, he said that I was going to end up with a modern jeep! I got back in touch with Rodney Rushton and got him to send me one of his 24V alternator kits. I won’t dwell here on swapping out the charging system other than to say I must have saved a considerable weight by removing the original set up. Including all the redundant bracketry, it all weighed a ton!

The fitting instructions for the booster are very specific in that it needs to sit angled upwards between 25 and 45 degrees with its air control valve assembly at the half past four position. I fabricated some suitable bracketry and braced the slave cylinder outlet port with a headlamp bracket off my wife’s now long gone Triumph 500 chop (the chrome finish now hidden beneath some OD paint).

I then had to extend the brake lines up from the MC to the booster and then back down via an inline 2lb residual pressure valve (RPV) to the T junction where the lines split above the axle to each caliper. As before I used cupro-nickel tubing as it’s so easy to work with and routed it up out of the way under the rainwater guttering fitted under the Hotchkiss’ hood.

 Alongside the residual pressure valve on the firewall isis an inline coupling between the MC and the booster. It seemed the obvious place to create a junction as it was too tortuous a run to connect with one section of tubing.

2lb RPV on the firewall.
Booster securely in place

The booster also needed a vacuum feed running off the intake manifold. The manifold has a square section plug which I removed and screwed in a 1/4 NPT male 3/8″ brass hosetail fitting. It was then a matter of routing a thick-walled rubber vacuum hose above the engine block back to the booster.

I formed a ‘U’ trap in this hose and also fitted a one way inline valve to prevent fuel contamination of the booster.

I figured that as I was increasing the hydraulic fluid pressure to the calipers, it may be wiser to swap my older calipers already mounted to the front axle and replace them with the newer units that I had bought for the rear. The newer seals would stand a better chance with the increased fluid pressure. I wasn’t planning to ‘boost’ the line to the rear axle so the ‘older’ calipers could move to the rear axle.

Front axle showing new caliper and 5mm wheel spacer
Manifold vacuum take off

At this point, after checking all the lines and connections were ‘fluid tight’ I took the jeep out for a spin. I knew what effect the fitting of un-boosted disc brakes to the front axle on the stopping capability of the jeep was but how did it change once they were boosted? Wow, that 2.3:1 ratio makes a big difference to the effort required. On a dry road I could squeal the front tyres no problem without even trying. Don’t get me wrong, there’s still plenty of feel, you don’t have to squeal them but you can if you want to. This does now pose the issue of improving the braking at the rear axle to close the gap with that of the front. In my last update of the jeep’s brakes I advised anyone thinking of converting just the front axle to discs, to stick with the original 7/8″ brake cylinders on the rear drums. I would now say that if you then decide to boost the front disc brakes, it would be wise to change out the rear 7/8″ cylinders for the bigger 1″ ones from the front drums to give the rear more of a fighting chance to keep up. You may even have to play with the APV a bit too in order to dial in a bit more pressure so the rear ones kick in a bit stronger.

OK so now to put the jeep back up on the stands and sort out the rear axle. Off came the wheels, hubs and the drum brake backing plates.

Rear axle stripped clean

Just as with the front axle conversion, the drums needed removing from the hubs. Last time I was lucky enough to be able to remove the old wheel studs with the aid of some penetrating oil and a large hammer. This time I wasn’t quite so fortunate. The studs came out fine on one side but the others were stuck fast due to ‘swaging’.

 I had to drill out the heads of the studs and punch them out from the other direction. This time I knew better than to attempt to push the new longer studs in myself and took the hubs down to my local mechanic so that he could locate them with a hydraulic press.

Swaged wheel studs
Drilling off the heads of the old wheel studs

 The discs have a couple of threaded holes in them to allow for their easy removal by inserting two M8 bolts (the action of tightening them pushes the disc away from the hub). Just as with the front disc conversion, I thought I’d utilise these holes to hold the discs back against the hubs as they have a tendency to creep away from the hubs once the wheel is removed. Drilling 2 holes through the hub allows for 2 short bolts to secure the disc back against the hub.

 I bolted the new laser cut brake brackets to the axles at the half past ten position (on the passenger side) which put the caliper’s bleed nipple at its highest point. I also had to fabricate 2 right angled brackets to sit behind the axle flanges to create a secure junction for the union of the brake lines and the stainless steel brake hoses.

Hubs and disks
Mock up to determine length of stainless hose and design of its support bracket
Braided stainless hose kit

Back on went the hubs and I could then mount the caliper carrier assembly to the brake bracket. This needed to be centralised over the disc so I had to use some washers as shims to make up the difference. Because I didn’t know how long a brake hose to make up until I had everything else in place, it was now time to look at these. I went back to the company who made up my first set of braided stainless steel hoses and bought a DIY kit from them. This had to marry up an imperial 3/8″ 24 tpi female bulkhead (with convex seat) fitting from the hard brake line to a 10mm metric 20 degree-angled banjo fitting at the caliper.

I was quite confident in making these lines up myself having done them many times in the past when converting from standard manufacturers rubber hoses on my previous 2 wheeled steeds. This allowed me to get the length of the brake hoses just right. I waited until I was sure that the hoses were ‘fluid tight’ before moving the shrink-down tubing  into place over the fittings.

I made up some new cupro-nickel brake lines to run from the T junction union on top of the rear diff to each caliper making sure to keep as close to the original routing as possible.

New cupro-nickel brake lines connected up
Line runs from MC to APV then RPV (both shown) ending at rear calipers

As I was swapping the drums for discs I also needed to change the 10lb RPV mounted up along the chassis rail for a 2lb one. I’m not going to go over again of the necessity to use inline RPV’s and an adjustable pressure valve (APV) as I covered these issues in my first article which can be found on the IMPs website in the ‘Under the Hood’ section beneath the Membership tab in its dropdown list.

Time to try out the new set up. Remember earlier I said that I could squeal the front tyres with the booster fitted to the front discs yet still running the rear drums? What I didn’t mention was that it was also diving over into the kerb at the same time, a phenomenon well known to these jeeps upon heavy braking due to their steering geometry. This was all the more pronounced because all the braking was being done by the front axle. This time with the rear discs fitted, I had the APV dialled right out so that the valve was transmitting its least pressure to the calipers. The jeep still had a tendency to lock the front before the back had had a chance to come into play. With that in mind, I decided to start at the other end of the APV’s adjustment (dialled fully in for most pressure and maximum rear caliper braking effect).

The rear caliper mounted along with 5mm spacer
The completed project

Well that seemed to do the trick. No more diving into the kerb and now it gave the feeling that the rear discs were getting some purchase. Talking to Ryan at Jeffrey Engineering, he said that they had fitted quite a few disc set ups over the years including both axles with Suzuki calipers and a booster on only the front. He reckoned that seemed to produce the correct braking bias between the front and rear axles. No mention at all of an APV in the rear line. Who knows, perhaps I don’t even need an APV at all with my current set up. I’m probably going to leave it as it is for the time being as I’m very happy with my new braking set up. I drove all the way from SE London to Hastings and back the other day and no ache in my right knee at the end of it. The booster sure takes the effort out of hitting that pedal. For those of you who don’t want to move the jeep too far away from its original spec, fitting a brake booster to the existing drum set up on a wartime jeep behind the driver side headlamp apparently significantly improves the braking especially in conjunction with some of the new Mintex brake shoe linings now available. To anyone attempting a full or even partial disc brake conversion, I don’t think you will be disappointed with the results!


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