75ZT Community

Today I've been fitting new calipers to improve the brake performance of the front brakes, one of them was sticking slightly.

Hoses clamped off and calipers changed with no problems

Time to bleed the brakes and thought I'd do it properly on the T4 (Testbook version 4.Computer Diagnostic System), the sequence is as follows:

Select brake to be bled on screen menu after fitting bleed tube to bleed nipple.
Open bleed nipple and depress pedal to expell air until it runs clear ensuring resevoir is topped up.
Next press continue to bleed the secondary circuit with the ABS (Antilock Braking System) pump until clear.
Finally tighten bleed nipple while pedal is depressed.

All was going well until it came to bleed the secondary circuit. absolutely full of air
Took a total of 4 bleed sequences on each side to get it free of air and 1/2 litre of fluid.
The pedal is now solid when depressed.

The problem for members is that normal bleeding or pressure bleed kits only bleed the primary circuit, in order to bleed the secondary circuit you will need to use suitable diagnostic equipment capable of using the ABS pump to bleed it properly.

I wonder if this is why some owners suffer from sinking pedal syndrome.

Russ

Not quite sure if I understand this.

Master cylinder runs to the brake servo, brake servo connects to the ABS modulator, ABS modulator connects to the calipers. If you bleed at the calipers, you bleed through the ABS modulator, so wondering what the 'secondary circuit' is and what T4 shows on where to bleed.

Also, bleeding at the calipers by the 'pedal' method is a two man job as you have to close the bleed nipple before pedal release/return to prevent air being sucked in.

Use of a vacuum bleeding tool is therefore the best option if by yourself. Make sure the vacuum is on before undoing the nipple.

The point is, without the testbook bleed procedure, you don't bleed anything on the closed side of the valves. In theory this shouldn't matter but sometimes it does.

In fairly basic terms without having to go to go into too much detail, in normal braking the abs modulator isn't used, the internal valves within the abs unit are closed and allow the fluid bypass the modulator to operate the calipers normally, however when the abs functions the valves open and operate the poppet valves of "secondary circuit" within the unit to modulate the braking.

It's the fluid trapped behind the valves etc. inside the modulator that you can't bleed by normal means and can have air trapped which can only normally be bled by operation of the ABS pump.

Any method is o.k. as long as you don't allow air back into the system, the advantage with T4 is that if you are on your own you can close the bleed nipple while the abs motor is running, once no more air bubbles are expelled.

Russ

Duncan wrote:The point is, without the testbook bleed procedure, you don't bleed anything on the closed side of the valves. In theory this shouldn't matter but sometimes it does.

True, but normally this only happens when the ABS unit has been replaced as no air should enter through the closed valves ..........Unless the unit is faulty and should be replaced anyway

On most ABS units there are screws for bleeding these without cycling the ABS..........(running outside to check mine )

VMax1000 wrote:
On most ABS units there are screws for bleeding these without cycling the ABS..........(running outside to check mine )

Never seen any myself, so tend to disagree.

VW based stuff has the same Tester based bleed procedure.

But I agree that in theory you should be OK unless the modulator gets drained. Replaced all the pipes on my V6 without needing the bleed procedure. However have also come across situations on other brands where just replacing the caliper ended up needing a proper tester bleed sequence, though even knowing how it all works, I can't explain why.

If at some point in it's past the car's braking system hasn't been bled properly then any air in the system can find it's way behind the valves when the ABS functions and become "trapped" behind the valves so subsequent normal bleeding won't remove it.

Russ

wow scary stuff