"Please Note: These are general guidelines only and assume you have some mechanical knowledge. If you are not 100% confident in what you are doing, please seek advice or let a professional do the job".
Early Mokes had a single circuit brake system, and a round limiting valve mounted on the rear subframe. With the introduction of ADR's, Leyland moved to a tandem master cylinder with a 5-way pressure reducing valve mounted on the firewall/bulkhead and a T-piece at the rear. Later Mokes did away with this valve and used an inertia valve mounted on the rear subframe.
Here's the 5-way valve in it's natural habitat.......
Inside are two pistons - the top one for the rear brakes and the bottom one for the front. The front circuit piston has a slighly larger bore than the rear one, so wants to move down when braking. Under light braking, this movement is balanced by the spring under the cap at the bottom. Under heavy braking the spring compresses, the bottom piston moves down and pulls the top piston down with it. This closes a small ball valve in the top piston, reducing pressure to the rear brakes.
Common symptoms of a faulty valve are excessive brake pedal travel, even though the brakes are adjusted and bled correctly, and no rear brakes or trouble bleeding the rear brakes. While there are a lot of parts inside, it's not really wear that is the killer - more likely it is moisture and dirty brake fluid.
New valves are over $300, and only have 4 ports so wont bolt straight on. The cheapest reconditioned ones I found were $135 exchange. Total cost to rebuild this one was $6 for the seals - so it's well worth having a go yourself!
Pulling the valve off is pretty straight forward - but be aware they are metric fittings so you will need a 10mm metric flare spanner. The master cylinder will drain through the outlet pipes, so either have some bleed nipples ready to block the ports, or turn the lines up so they are higher that the fluid level.
Once it is off, the only way to get it apart is to remove the tin cap covering the spring. I don't think there is an easy way to do this - but if there is, please let me know! Use a socket to hold it in a vice, and gently lever the tabs up with a screwdriver.
There is no seal under the cap, so you can do the vice up quite tight. Also means you won't get any free play on the tabs to make it easier. This one was quite rusted and put up a good fight!
Unfortunately the cap looks a bit secondhand afterwards, but heating it up and tapping it straight on something the right size gets it back into shape.
Under the cap is a spring which should fall out, and under that a plunger that fits into the front circuit piston. If it doesn't fall out, it should come out with some pliers.
Looks like this....
Under the plunger is the piston for the front brake circuit. In the middle of this is an 8mm nut, that holds the front and rear pistons together.
To undo the nut, you need take the cap off the top (32mm) and hold the rear circuit piston.
Note the little square bar - it may fall out, so watch for it. The top of the rear circuit piston has a slot in it like this...
Hold that with a big screwdriver and undo the 8mm nut on the other end.
Now if the pistons aren't seized, they should come out either end by slamming the whole valve down on a block of wood. But normally they will be stuck - which is probably why you are pulling it apart! So we need to get creative, and force them out.
The best way is to use a grease gun to push them. You will need to block both outlet ports for the front brakes - that's the two ports side by side. Bleed nipples are the best, but if you don't have any a couple of bolts will do. The thread is 10mm x 1, but you can use the easier to find 3/8 UNF. Screw a grease nipple in the inlet port and give it a few pumps.
This is the front circuit piston, plunger and spring.
Once that is out, you should see a long pin in the centre that had the 8mm nut on it. Sit the valve on a socket and use a punch to push the pin and the rear circuit piston out.
Inside the rear circuit piston is a ball and spring valve. This is the part that does all the work, and shuts off fluid to the rear brakes under heavy braking. It can be a bit tricky to get out - but now you are this far it's worth giving it a clean.
Grip the piston in a vice, but not on the larger radius where it slides in the bore. I used a 1/16 roll pin to stop it turning, and had to use two big screwdrivers to undo the cap.
Here it is all apart.
Remove the seals and give all the parts a good clean with metho or brake cleaner (don't use petrol or the like - it ruins the seals and leaves an oily residue). Use a stiff toothbrush or even better a brass wire brush or Scotch Brite pad. Don't use a steel brush - it can spark and that's not good with metho around!
Here's all the parts.
The bores are a bit hard to hone, not only because they are small but also very shallow. A normal hone doesn't really go in far enough to work properly, so I cut some old stones in half and made a mini hone.
Give the bores a light hone with a cordless drill or a pedestal drill and use plenty of lubricant. I prefer kerosene because it's easier to clean up - but you can use brake fluid.
You should end up with something like this.
Don't forget the little bore in the middle - it's about 8mm, so the hone won't fit. I used an old pencil with a slot cut in the end and a strip of 600 grit wet & dry held in place with an o-ring.
Easiest to start from the threaded end where there is a taper.
Also a good idea to re-anneal the two copper washers (if you're fussy like me).
The original seals in my valve were one o-ring and three special ones. The brake specialist where I got the new seals assured me o-rings are fine and that is all they use for rebuilding them, so I will trust their greater knowledge. You will need to take the pistons in for them to measure, as I don't think there is a kit available.
If you want to get the seals first so your Moke is not off the road too long, you will need one each of these standard O-rings...
008 3/16 x 5/16 x 1/16
112 1/2 x 11/16 x 3/32
113 9/16 x 3/4 x 3/32
115 11/16 x 7/8 x 3/32
Start by putting the rear circuit valve together. Check it's correct by putting the square bar back in - it should stick out about 1/8" and pop back up if you push it down.
Give all the metal parts one final clean to make sure they are spotless with no grease or dirt and blow dry - especially the bores if you have honed them.
Using plenty of rubber grease slip the O-rings onto the pistons.
The two larger O-rings go on the lower piston, and the smaller two on the top piston.
Lube the bores with more rubber grease and slide the pistons in. Put the little 8mm nut and washer back on the pin and tighten. You can screw the cap back on the top now - but don't forget the little square bar.
Under the bottom piston where the tin cap fits never sees any brake fluid and can rust. I painted mine with the thick red rubber grease to keep the moisture out.
Install the plunger over the nut and then the large spring. Sit the tin cap on and clamp it all up in a vice using a socket.
Tap the tabs down gently with a screwdriver (I ground a slight curve on the end to make it look neat).
If you haven't got any parts left over - then you're all done!
When re-installing the valve there is one thing to be aware of. Where the two lines exit the valve for the front brakes, make sure to install the one closest to the firewall first and make sure it is good and tight. Once the second line is on you won't be able to get a spanner in there to tighten it again.
Bleed the brakes, check for leaks and you're ready for another 30 years!