Bleeding brakes a warning

[Arctic]

Noted and book marked thank you Russ appreciated Arctic

[Jules]

I'm not a fan of pipe clamps myself they cannot do the rubber any good long term by crushing them to nothing ! (never owned a set)

The easiest way to shut off the master cylinder is to use a suitable item like a piece of wood (our spirit level is just the right length) to depress the brake pedel a few inches.

This closes the reservoir valve in the master preventing fluid loss.
Full pedal depression is not necessary and not possible unless both circuits are open.

The last sinking pedal we had was vacuum pipe related !

[HarryM1BYT]

Correct me if I am wrong...

The need for T4 is when the system is dry and the only way to get fluid properly into the ABS units galleries, is to open the valves with the T4, so the air is pushed out.

When doing a thorough flushing of the system, I would guess old fluid might also remain trapped in these galleries unless T4 is used.

Might the sinking pedal be due to air trapped in the galleries, being compressed, as a valve in the ABS leaks slightly?

When working on the pipework/ cylinders or what ever, my trick to stop all the fluid draining out of the open pipes, is to simply seal the top of the master cylinder with a plastic bag and replace the cap on top. Once a small quantity has escaped, a vacuum is formed under the bag, which is enough to prevent further loss of fluid until the bag is removed and air admitted.

[kaiser]

I wonder how people were able to bleed brakes before the arival of computers???

[Number 6]

Originally Posted by kaiser

I wonder how people were able to bleed brakes before the arival of computers???

I dont know about bleeding the brakes with a computer,I have just been told that on new Citroen,s you need a computer to pull the piston back in to fit new padsThis. is instead of using a wind back tool

Where will it end??will you need a computer to put air in the tyres??

[RodgerD]

Originally Posted by HarryM1BYT

Correct me if I am wrong...

The need for T4 is when the system is dry and the only way to get fluid properly into the ABS units galleries, is to open the valves with the T4, so the air is pushed out.

When doing a thorough flushing of the system, I would guess old fluid might also remain trapped in these galleries unless T4 is used.

Might the sinking pedal be due to air trapped in the galleries, being compressed, as a valve in the ABS leaks slightly?

When working on the pipework/ cylinders or what ever, my trick to stop all the fluid draining out of the open pipes, is to simply seal the top of the master cylinder with a plastic bag and replace the cap on top. Once a small quantity has escaped, a vacuum is formed under the bag, which is enough to prevent further loss of fluid until the bag is removed and air admitted.

No a T4 is needed if there's air trapped in the ABS unit. I think I read somewhere even new ABS units were primed in that brake fluid has been passed through the unit and that air shouldn't be trapped in the various modulator valves.

[RodgerD]

Originally Posted by kaiser

I wonder how people were able to bleed brakes before the arival of computers???

They didn't have ABS units to complicate things. Our car's ABS isn't just an ABS unit but a brake stability unit as well. Info from the 75 ABS technical manual:

The ABS ECU also controls electronic brake-force distribution (EBD) via the modulator. EBD
modulates the brake pressure applied to the rear of the vehicle relative to that applied to the front,
and takes the place of a conventional pressure conscious reducing valve. The amount of pressure
applied to the rear brakes relative to the amount applied to the front brakes is critical to ensure
vehicle stability is maintained when braking. The state of the vehicle, with regard to load and
distribution of load, will affect the distribution of brake pressure. A vehicle fully loaded without EBD
(and without ABS) will remain stable, with equal pressure applied to the front and rear axles.
However, if equal pressure is applied to the front and rear axles of a lightly loaded vehicle the rear
axle would be excessively braked and the vehicle could become unstable with possible rear wheel
lock (on a vehicle without a pressure conscious reducing valve (PCRV)). Using the modulator
allows the braking force to be appropriately distributed to the front and rear axles under varying
vehicle loading conditions. The ABS ECU adjusts the brake pressure using the solenoid valves
within the modulator and compensates automatically for different vehicle load. The amount of
brake pressure adjustment required to meet a given load is calculated by the ECU. The calculation
is based on slip measurement it receives from the wheel speed sensors. EBD brake apportioning
starts with a 1:1 ratio and is then adjusted by the ABS ECU based on the signals received from
the wheel speed sensors. Unlike the anti-lock braking system, EBD is active under normal braking
conditions. The ABS ECU will only adjust the rear brake pressure within a percentage of the
pressure applied to the front. Whilst EBD is active, the driver may feel a slight brake pedal drop
and the slight lowering of brake pressure. This is caused by the rapid switching of the solenoid
valves in the modulator.

As you see the ABS unit is working 100% of the time, not just under wheel locking conditions, as it is constantly calculating the brake balance under ALL braking conditions. It is possibly the activation of the EBD that is giving rise to the sinking pedal phenomena.

[COLVERT]

If the ABS unit is working ALL the time then surely any trapped air would soon be released. ?---

[RodgerD]

Originally Posted by COLVERT

If the ABS unit is working ALL the time then surely any trapped air would soon be released. ?---

Not all valves within the ABS unit are operating all the time the brakes are being applied, brake balance control valves will be operating but the anti-lock modulator valves will only operate in a wheel lock up situation. The ABS unit is a combined units having two functions. In order to purge air from the anti-lock modulator valves, it is necessary to operate those specific valves and not the entire ABS unit. As I understand it T4 just opens the anti-lock modulator valves to the four wheels.

It is highly unlikely air will be trapped in the anti-lock modulator valves as the ABS unit when fitted is pre-bled and as I see it air can only get into these valves whilst the anti-lock system is functioning with air being pumped back from say from the calipers.

T4 software manual states (page 13) the following: The Bleed System and the Bleed Status are only required when fitting a new modulator that has not been pre-bled. As all service modulators are supplied pre-bled, this facility should only be used when directed to do so by Rover Service.

The problem is that if air is trapped in one of the four anti-lock modulator valves, it won't show until the anti-lock operation is required, then when one or more of the valves open air could be released back into the service line. By bleeding without operating the valves, you could achieve a hard pedal as the brake balance valves will have opened, but the anti-lock valve will remain shut with air trapped behind them. I suspect that is what Russ found out when he used his T4 to assist in bleeding the brakes, air trapped in the anti-lock side was released back into the service line. Trapped air behind the anti-lock modulator valves could explain pedal sinking as you could have a hard pedal then one of the anti-lock modulator valves opened, releasing air into the service line with a resultant soft pedal.

I remember reading in one of the workbooks that it is recommended that 75 brakes are bled with the T4, possibly to ensure ABS/brake balance units are completely purged of all air in the four anti-lock modulator valves.

[spyder]

Originally Posted by Number 6

Where will it end??will you need a computer to put air in the tyres??

Already standard on some SUVs.