MK 3 Brake mods

[Acco 4x4]

Hi All, apologise if this has already been covered or I have posted in the wrong place.........

I am renewing the whole brake system on my MK3 ready for Rego. I am renewing the whole system. I have sourced new wheel cylinders, made new flexible hoses and had the air side serviced too. LAst on my list is the master cylinder. Originally the truck was fitted with a single cylinder / piston master cylinder and I am converting this system to a conventional dual system as fitted to all new vehicles with a hydraulic brake system. Now the question is, Has anyone done this mod before? If so what is the master cylinder off or part numbers?? Looking for a 3/4 dual circuit master cylinder. This will keep the piston travel and brake ratio the same as standard.
Thanks.

[Diana Alan]

Wasn't there an EMEI for a dual circuit for the later Mk4 or was that the F1?

[Acco 4x4]

I know the did something with the 6x6 regarding dual circuit systems. I will have to find specs on the master cylinder used........ The master cylinder size needs to be 3/4 inch otherwise the piston travel etc will be all stuffed up.

[Diana Alan]

You could always put a restrictor in the line to the rear circuit.  That or fit a load sensing valve, so that when the trucks loaded you get all the pressure you can get to the rear.

[Minikeg]

You could always put a restrictor in the line to the rear circuit.  That or fit a load sensing valve, so that when the trucks loaded you get all the pressure you can get to the rear.

Pretty sure he is after a duel circuit for brake redundancy, ie if a brake hose blows (as my hiluxs did last month) you dont lose all the brakes - the two circuit means one will empty and one will still be good. Adding a restriction or load sensing valve does not help in that scenario?

[Diana Alan]

I know he wants to fit a dual circuit, but perhaps I didn't explain myself clearly.  The issue is that if the size and stroke of the new dual circuit master cylinder is incorrect, he'll likely lock up the rear brakes before the front, particularly when the truck is unloaded.

If he can't get the correct front-rear balance with the m/c options he can find, then I was suggesting fitting a pressure limiting or load sensing valve in the rear circuit.

.... . Originally the truck was fitted with a single cylinder / piston master cylinder and I am converting this system to a conventional dual system as fitted to all new vehicles with a hydraulic brake system. Now the question is, Has anyone done this mod before? If so what is the master cylinder off or part numbers?? Looking for a 3/4 dual circuit master cylinder. This will keep the piston travel and brake ratio the same as standard.
Thanks.

Wasn't there an EMEI for a dual circuit for the later Mk4 or was that the F1?

[Bluebell One-eight]

The dual circuit system fitted to the 6x6 trucks (in manufacture) utilized separate actuators and cylinders one for each circuit. If a combined cyl is substituted make sure it can handle the forces involved. Have a talk to someone that specializes in truck brakes, it might save a lot of grief. The m/c dia is 1 1/2" from memory and the important thing is the ratio of the area of each piston in a system. If the system is split in two then each half of the cylinder must have half the area to maintain parity. this does not mean half the diameter. If we halve the diameter then there will be only one quarter the area. The correct diameter should be about 1 1/16" . It's another reason to consult with a specialist. Apologies if the water has been muddied. John

[Diana Alan]

Just don't go to Burt Bros Automotive Services at Fairfield, they used to be very reliable for truck brakes, but recently have made a series of inexcusable errors in assembly of Inter brakes and clutches, including the seals in backwards.

Go to B & R Brakes Sunnyholt Road Blacktown 02 9621 6488

Was talking to an Inter owner and diesel mechanic just now (re: B&R B)

He suggests the F1/F2/F5 brakes use two of the same booster and m/c as the Mk3/Mk4 with a T piece in the air line (air over hydraulic so the treadle valve is on the air line).  They are plumbed (cir 1) FR/ML/RR and (cir 2) FL/MR/RL.  He goes further to suggest that the Brit Army also converted their trucks to dual circuits using the same booster and M/C, with cross overs in the plumbing FR/RL and FL/RR.

Diana

[Richard Farrant]

  He goes further to suggest that the Brit Army also converted their trucks to dual circuits using the same booster and M/C, with cross overs in the plumbing FR/RL and FL/RR.

Hi Diana,
An interesting comment on British Army vehicles, was he more specific in mentioning types? The only vehicle that I know of with cross over brake circuits is a Stalwart and that was not converted it was by design, front and rear left and centre right, the vice versa.

regards Richard

[Diana Alan]

Only going by what was told to me an hour ago, will ask next time I talk to him.

[Richard Farrant]

Thanks, will be interested to know which trucks were converted.

regards Richard

[Acco 4x4]

Hi All, thanks heaps for the suggestions etc. To clarify, the aim of the game is as Minikeg has stated. I want to set the brake system up to "fail safe" so if i blow a hydraulic hose or have a wheel cylinder failure etc, there will still be half the system to stop me plowing through a row of cars or off the side of a cliff.     The original master cylinder is 1 1/2 inch diameter so if i devide the system into 2  than i need a 3/4 inch diameter cylinder as this is half of the original one. As both cylinders act as one, the travel will remain the same.

[Minikeg]

The original master cylinder is 1 1/2 inch diameter so if i devide the system into 2  than i need a 3/4 inch diameter cylinder as this is half of the original one. As both cylinders act as one, the travel will remain the same.

Not sure this is going to go all peachy; As Bluebell One-eight mentioned, this dividing a diameter by 2 does not work

Have a talk to someone that specializes in truck brakes, it might save a lot of grief. The m/c dia is 1 1/2" from memory and the important thing is the ratio of the area of each piston in a system. If the system is split in two then each half of the cylinder must have half the area to maintain parity. this does not mean half the diameter. If we halve the diameter then there will be only one quarter the area. The correct diameter should be about 1 1/16" . It's another reason to consult with a specialist.

Ive photoshopped a pic to sort of demonstrate.. (didnt realise the size was so big)



hope this helps?

[Acco 4x4]

hahahahahah oooops!     my bad! after crunching numbers again i totaly agree with you. The correct master cylinder diameter is 1 1/16 close enough to 3 decimal points. I forgot good old Pie R Squared.......... how embarrassing! So, all i need is to find a master cylinder of the above value to fit.
oh dear.........

Oh and thanks for the cool pic! crystal clear.......

[Diana Alan]

Now my maths brain may be rusty, but if you halve the diameter don't you reduce the area by a factor of four?

Secondly we are talking about air over hydraulic braking where the brake master cylinder will push as far as needed under the effect of the air pressure from the air valve (treadle valve) on the pedal.  It is why you can duplicate the brake booster/master cylinder unit and merely add a T junction on the air line to operate the second master cylinder/booster unit.

Thirdly, dual circuit master cylinders are usually 2 pistons in a single channel. the front part of the cylinder being one circuit and rear the other circuit.  There is often a spring between the two pistons so that when one circuit has the shoes hard on the drums there is still pressure being applied to the other circuit.