Issues of changing Drums to Disc

[JErskine]

Hi All,

I realise there are a few different threads about Disc brake conversions however I'm struggling to find out the answer to a fairly simple question...

What is the issue of a straight part swap? And what machining is required to make them fit?

For some context, I have a 69 SIIa LR

Cheers

[xychix]

JErskine, there is no series III with disc's so a 'straight part swap' is impossible as those parts don't exist

Unless you wanted to fit coilsprings and defenser axles, but that is a totally different conversion with it's own issues.

[daslandroverman]

I do believe the old 'Chris Perfect' kits used Santana bits from IIIA and IV models, and as such were an easy bolt on swop.

Not as common now, but there are other options.

YakYak Classics supply a kit which includes a bolt on calliper carrier and a machined hub to carry a standard Land Rover disc and works with a 90 front calliper, pads etc.

Heystee Automotive do a kit that includes a new swivel and a machined hub for each side which uses standard Series steering components, with 90 discs, callipers etc.

Both neat conversions, and both work out similarly priced once fitted.

[xychix]

I've opted for a zeus kit. As far as I could see the cheapest option. Custom disks I believe which might changing those costly... but with ~1000 km a year.....

Zeus promises to deliver a compete kit while with Heystee you still need a load of defender parts.

[missingsid]

I'm inthe middle of ac custom build as Zeus had stopped selling their kit.

Sods law it is available again! That would be my choice.

[JErskine]

The big issue for me with the conversion kits would have to be shipping (A lot of companies don't ship to New Zealand and if they do you're paying an arm and a leg).

If I know what machining is required I have a few engineering contacts who should be able to help. Mainly need a starting point for it

[xychix]

I'm inthe middle of ac custom build as Zeus had stopped selling their kit.

Sods law it is available again! That would be my choice.

Since when did zeus stop selling their kit ?

At the moment they seem out lf stock.

[missingsid]

They seemed to stop for a year or two? But reappered on the web site again.

[xychix]

They seemed to stop for a year or two? But reappered on the web site again.

mine have just been delivered to France box is still to be opened. (front kit)

[De Ranged]

The big issue for me with the conversion kits would have to be shipping (A lot of companies don't ship to New Zealand and if they do you're paying an arm and a leg).

If I know what machining is required I have a few engineering contacts who should be able to help. Mainly need a starting point for it

I'm doing a conversion for a mates SII, just got it at a stage where I'm waiting on him to get the blanks for the caliper mounts so I can machine them.... Mazda RX7 s5 front four pot calipers and machined BMW X5 disks

Will post up the details when I'm done

[Team Idris]

Those Zeus kits look pretty swish! I wonder if you need power assisted brakes or if they work fine on the standard master cylinder?

[De Ranged]

you will need to change the master, the drum masters will as the disks wear not creep out to compensate for wear so your pedal travel will get further and further as the disks wear

In the system I'm planing on for my mate I'm going to a hilux double diaphragm booster and a toyota master I'll start with a 80 series cruiser (disk front drum rear)

Could some one who has one of these Zeus kits post up the piston sizes on the caliper..... I hunted long and hard to find something with resumble piston size and still able to fit inside the rim.... why I ended up with the Mazda calipers they are a resumble piston size and the mounting posts are way out at the ends of the calipers so they fit closer to the swivel housing

[Tim2809]

The master cylinder does not creep at all. It applies pressure and then releases when pedal released. The pistons in the caliper does not fully retract only pulled back slightly by the seal.

[De Ranged]

I think my explanation might be a bit bad, its not creep its something to do with the valving in a disk master.... a drum brake self adjusts using a tension bolt setup at the brake slave end so the shoe stays around the same distance from the drum as it wears so the master cylinder doesn't have to compensate for this.... on a disk system the master adjusts to keep the pedal at the same place as the pads wear and the pistons stay out (why you have to top up your disk brake master cylinder as the pads wear)

So if you use a drum master on a disk brake calipers system your foot will get further and further away as the pads wear and the caliper pistons extend

Now don't ask me how a disk master does this lol I don't know and since I'm not allowed to modify this (legally, and they are way to cheap to bother playing with) I've never bothered to find out.... but after yrs of selling disk conversions for hilux's I can tell you the results if you don't swop the masters

Run a search on Pirate... I've seen a couple of threads on there about this you'll find the reason why

Oh and I have had one muppet who didn't replace the master when I sold him a conversion with a large piston disk conversion... he came back later because his brake pedal slowly got to the stage where it would hit the firewall!

[David Sparkes]

I agree with Tim2809, in that the master cylinders of drum braked Land Rovers do not have any sort of compensation mechanism built into them, nor do the master cylinders of LR disc / drum systems.

As the shoes wear fluid is transferred from the reservoir, via the master cylinder to the wheel cylinders.

The same happens to the 'simple' disk braked systems that LR used to supersede the all drum system.

You seem to to basing your views on your understanding of what happens in a Hilux braking system.

I, and I suspect most others on this LR forum, know sweet FA about the Hilux brake system, so I'm not about to challenge your explanation about what happens there.

What I will reiterate is that with a LR drum system, or LR non-pressurised disc system, all the compensation for wear in the friction material takes place in the wheel cylinders or calipers; no compensation takes place in the master cylinder.

The one criteria that applies is that the master cylinder must always revert to the fully off position, as in that position the port to the reservoir is uncovered, allowing more fluid into the pipework, which essentially compensates for the pistons adopting their extended positions caused by the wear in the friction material.

The master cylinder specified for a LR wholly drum braked system may have to be changed if a disc braked system is installed, but that is due to the hydraulic pressure ratio required, and or the physical capacity of the master cylinder, wrt the new calipers. Again, I don't have the relevant experience to offer more definite advice on that point.

Regards.

[Bowie69]

I do know a lot of yank master cylinders hold a tiny bit of pressure to keep the shoes in place, and have to swap out to a 'disc' master when moving to disc brakes, to stop premature pad wear and dragging brakes.

My old moggy is the same, there's a valve you have to remove from the master if you fit discs up front.

So it does apply to some vehicles, but possibly not LR...

[De Ranged]

Done a bit of research turns out the bad pedal travel is due to an imbalance at the master due to the fact with the disc conversion the disc caliper requires a larger volume of fluid to activate the brakes so the master isnt delivering the correct amount of fluid for the size of the pistons

ie if a drum slave and a disc caliper have the same size pistons they still need different masters as the disk requires more fluid to work

This issue is made worse when the master still has a matched end, as in the case I described disk front that stayed disk and a drum rear that went to a large increased piston size disc caliper over the original drums

I still haven't found a resemble explanation as to why the pedal travel gets further on the original drum master as the new discs wear, this maybe something peculiar to the Toyota masters (I only use these as they have alot of different piston size and orientation options all in the same fitment) but I haven't yet found anything to say they are different

This does bring up the issue that if you convert to disk front and retain your drum rear you will have the same issue if you have the original drum/drum master.... but with the potential to under brake the front and lock the rear

Oh and the valving bit is in the Drum master, a residual pressure valve to hold pressure on the drums to stop the springs pulling them too far back in the early drum brakes..... I have found reference to modern drums not needing this due to ratcheting (self tightening) screws to adjust the return stops I think this will be a case by case basis as not all drum brakes I've pulled apart have these

My knowledge on this is based on alot of time literally skidding trucks down gravel and wet seal roads here in NZ we have hard to pass (with larger wheels) brake modification tests that require multiple road performance tests

In the case of my mates conversion I am doing.... I will complete the front calipers mount a hilux double diaphragm booster and take a best guess master (I have a selection here...) bleed it and test it... change the master based on pedal feel and performance eg which end locks first and how soon compared to the other... I have warned him there is a potential that the conversion I'm doing to the front will out brake the Sals rear drums due to both piston size and the disk dia, if that is the case I might have to convert the rear also... and change master lol

Please note my previous posts on this have inaccurate information in them and I am unable to edit this please refer to just this post

[Jon White]

Series single circuit master cylinder will not do. You need as a minimum late series dual circuit master cylinder with the servo.

However I found that with disks the series servo isn't really man enough, so changed it out to an early 110 servo and pedal box, which is larger. The servo needs to be the very early, small diameter one, as the later ones are larger diameter and the series bonnet won't shut!

Series master is biased 50:50 which is what I run. Defender ones are biased a bit more towards the front, but I've always found the series one satisfactory.

If making your own conversion you need to consider brake bias when selecting callipers. This is related to the diameter (and number) of the Pistons. If you get it right, your brake brias will be spot on. Get it wrong and you'll be pissing about with bias valves etc forever more.

I worked mine out look up the size and number of Pistons front and rear in standard Range Rover callipers, and worked from there......

If you're retaining drums on the back, I can tell you that the late swb rear brake set ups bias the same as Range Rover rear callipers if that helps.

Mine works well, and locks all four wheels simultaneously. I've run it for at least 12 years now.

Jon

[Snagger]

There is no master cylinder wear compensation system on either brake type - as the brakes wear, if they are kept adjusted, the slave piston displacement at rest will be greater and the reservoir fluid will need a top up. The differing comments are only valid where drum brakes are not adjusted as they wear, in other words, not maintained (so the shoe returns to the original position, with excess gap and excess pedal travel).

Disc brake master cylinders produce more pressure but less volume than drum as there is usually very little distance for the pads to move to apply pressure, unlike brake shoes on drum systems, contrary to some comments. That is why the master cylinder is so thin on a disc braked Land Rover than a drum braked one - the smaller diameter allows the same pedal force to create a greater hydraulic pressure as for the same force, reducing the square inches means a rise in pressure on each square inch.

You will find that for disc brakes, you will at the very least need the vacuum boosted master from a dual circuit SIII 109. That may be sufficient for the Zeus kit. However, it wasn't nearly good enough on my 109 when I fit Discovery/RRC brakes, and it needed a matching master cylinder. It is possible that the servo failed as the change was made, as the servo and master didn't work well on the Lightweight with its standard brakes. I did incorporate the biasing valve from the donor Discovery into the 109 rear line .

[xychix]

I've got a zeus kit waiting and a new master/slave cilinder. Additionally I've mounted a Hella UP28 electical brake vacuum instead of the engine smothering valve.