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Brake Caliper - Primary & Secondary Connections


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Hi Not wanting to hi jack your post.

I am about to clean my calipers and fit new seals mine are L/heed also. I was going to post as to which of the three bleed nipples to do first?????

With the pistons out you can check which hole supplies which pistons.

I was just going to stick a brake line in each hole on the caliper and bleed it.

Or does it matter ?????

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On my rebuild I am fitting Lockheed brake calipers that have both a primary and secondary

brake line supply's. The problem is which is which on the caliper itself?

All very interesting and very helpful but any idea's which is primary and which is secondary??

If anyone come's across the answer in Haynes etc etc please post.... cheers Rich


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  • 2 weeks later...

From Rave - Describes which calipers pistons do what...

Brake calipers - front - (5) rear - (6)

NOTE: To identify separate hydraulic

circuits, they are referred to as


Power circuit - Consists of rear calipers and upper

pistons in the front calipers, supplied by direct

hydraulic power from power valve.

Hydrostatic circuit - Lower pistons in the front

calipers form this circuit, supplied with hydraulic

energy from servo cylinders comprising a combination

of master

this is the bleed order for an ABS equiped truck...

5. Bleed four front caliper lower (hydrostatic) bleed

screws in the conventional manner.

In the order: outer bleed screw driver’s side,

opposite caliper outer bleed screw, inner bleed

screw, inner bleed screw driver’s side. Depress

brake pedal slowly and progressively, locking

bleed screw at bottom of each stroke.

6. Bleed hydraulic pump - open bleed screw on

pump and allow fluid to flow until clear of air

bubbles. Do not use bleed bottle, use a clean

absorbant cloth to prevent fluid spillage.

And for an non abs truck...

The hydraulic system comprises two completely

independent circuits. The rear calipers and lower

pistons in the front calipers form the secondary circuit,

while upper pistons in the front calipers form the

primary circuit. The following procedure covers

bleeding complete system, but it is permissible to

bleed one circuit only if disconnection is limited to that


Bleeding will be assisted if engine is run or a vacuum

supply is connected to servo.

WARNING: If engine is running during

brake bleeding process ensure that neutral

or park is selected in main gearbox and

that park brake is applied.

When bleeding any part of secondary circuit, almost

full brake pedal travel is available. When bleeding

primary circuit only, brake pedal travel will be

restricted to approximately half.

NOTE: When bleeding system commence

with caliper furthest from master cylinder.

Finally tighten bleed screws to 10 Nm.


1. Fill fluid reservoir with correct grade of fluid. See


Information, Recommended Lubricants and


NOTE: Maintain correct fluid level

throughout procedure of bleeding.

2. Connect bleed hose to bleed screw on rear

caliper furthest from master cylinder.

3. Submerge free end of bleed hose in a container

of clean brake fluid.

4. Loosen bleed screw 1/2-3/4 of a turn.

5. Operate brake pedal fully and allow to return.

NOTE: Allow at least five seconds to

elapse with foot right off pedal to ensure

pistons fully return before operating pedal


6. Repeat 5 until fluid clear of air bubbles appears

in container. Keeping pedal fully depressed,

tighten bleed screw.

7. Remove bleed hose, replace dust cap on bleed


8. Repeat 1 to 7 for other rear caliper.

9. Remove front wheel furthest from master


10. Connect a bleed hose to primary bleed screw on

front caliper.

11. Connect a bleed hose to secondary bleed screw

on same side of caliper as primary screw.

12. Repeat 3 to 7 for front caliper, bleeding two

screws simultaneously.

13. Connect a bleed hose to other screw on front

caliper furthest from master cylinder.

14. Repeat 3 to 7 for second secondary screw on

front caliper.

15. Refit front wheel.

16. Repeat 9 to 15 for front caliper nearest master


It seems to illustrate the top outboard screw is the primary circuit (10) and the two 'front' mid posn screws as the secondary (11 & 13)

Sorry it doesn't copy and past the illustrations.

Now i'll erturn to my disco ABS search - if anybody knows how to fix my disco ABS I'll be gratefull lol. It's quite different. :-)


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  • 6 months later...

Do they have primary and secondary?

I always thought that they were dual circuit, meaning that it doesn't matter which is connected to which, when one fails then the other does the job.

There are two separate chambers in the caliper and each supplies two of the pistons (one either side) so it wont matter which port the pipes are connected to, both will be the same capacity and operate in case (or in spite of) of a failure in the other

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Landowner I believe what you saying is correct.

I was able to put mig wire through each hole and see where they came out.

So I just connected the 2 fluid lines one to each hole, bled the three nipples and the brakes work as they should.

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  • 8 years later...

You do need to get the pipes in the right holes.  If you don’t then you won’t be able to bleed the system.  To check after you have installed the pipes open the nipple on the outside of the calliper and ignition off press the brake pedal.  If you don’t get fluid or can’t get pedal down you have them the wrong way round.

if you open to top nipple and ignition off press the pedal and get fluid that also shows they are the wrong way round.


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  • 1 month later...

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