Bleeding brakes a warning

[T-Cut]

Originally Posted by RodgerD

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 believe most people will find this happens if they do the test. I can't think that they all have air in the modulator. I first noticed it when I did the three year fluid change from buying the car new. I vacuum bled the brakes ensuring the reservoir was always full. There was no possibility of air entrainment, but I noticed the pedal sinking when the engine was running. If the pedal was displacing air, I'd expect a more rapid descent leading to a spongy feel. That's not what happens in my experience. The pedal feels hard enough, but descends slowly to about a half way though the arc, then it stops hard. There's no spongy feel of an air lock. It feels more like the fluid is moving to somewhere out of the brakes circuit. Had I tested it before servicing the brakes or even when the car was new, I think the sinking would have been present, but obviously that's supposition.

EDIT: As a matter of interest, Google for 'brake pedal sinks' and read some of the links. It seems to be so widespread as to be considered normal. Even some brand new unused cars do it. Even MOT testers seem flummuxed. Check out their forums as well.

TC

[alanaslan]

Hi I see many people have found problems when trying to bleed their braking systems. I have always found that soaking the bleed Nipples in a quality penetrating oil " beware the front nipples are prone to sheering, soak the nipples well before opening them."
The magic tool required is a "Sealy pressure bleeder" this looks like a garden weed sprayer. The unit comes with two different kinds of caps one screw on and one clamp on.
Attach the correct top on to the master cylinder, fill the pressure unit with fluid pump bleed the unit then connect the units hose to the socket on the cylinder top. The unit holds 2.5 litres of fluid. So no problems of running out of fluid when bleeding the system or flushing the system. The unit can be pressurised up to 28psi much more than is needed for any car on the road today.
Bleed the brakes in the order described in the manual. Always put a bleed tube on the nipple and in to a container to collect the fluid pumped through, brake fluid is nasty stuff, do not pore down drains dispose of old fluid as you would with old engine oil. Pump the bleed unit up to about 20 to 22 psi this is sufficient pressure to bleed the full system. You will need just over 2 litres to flush the full system.
Once the callipers are all running clean you should have a nice firm pedal which does not sink even with the engine running. Remember you will need to pump up the bleed unit from time to time during the process to keep the pressure up.
The unit has a pressure gauge on it so it is easy to see what pressure the system is working at. When you have finished and all the nipples are closed you depressurise the unit by releasing the pressure release valve, disconnect the unit from the master cylinder unit, this is easy as it has a quick release connector, this is quite like Plc connectors found on air lines. Then remove the units cylinder top from the master cylinder. Adjust the level in the master cylinder to the correct level as described in the manual then replace the cars own master cylinders own lid.
The instructions that come with the bleeding kit are comprehensive and explain each stage of the job.
I have used this unit on over a hundred different cars and on more than twenty rover 75s. The unit is not cheep but gives great results and makes bleeding brakes and clutches a one man job, with great results every time whither the car has ABS, tandem or single line systems. I have never had a Rover with a soft pedal or one that sinks slowly over time, as many of the people whom I have sorted their sinking pedal problems have been amazed at how firm their pedals can be.
You should be able to buy one of these units for under £65 with both the master cylinder tops.
Hope anyone with problems finds this helpful. These cars are destined to become a classic, please look after your car we need to keep them going as a tribute to the last volume British car manufacturer.

[klarzy]

I recently changed my front calipers and flushed all of the old fluid through the main circuts and the abs system.... Very firm pedal now...
also checked vac system for leaks to help servo...

k

[clf]

Just been reading through this, and I dont know how relevant this 'solution' I watched on youtube. It actually makes sense, and perhaps over time, the leakage (air) could build up and ultimately cause the sinking pedal. With the difference between air tight and fluid tight fittings, it could explain why you introduce air, dont lose fluid, and dont totally lose braking.

The 'solution' starts around the 11.30 point, which this links directly to it.

Basically corrosion around the bleed nipple is allowing a tiny amount of air into the system, especially when bleeding the system. So when bled, it looks like he is constantly is getting air. In the video around the 7 or 8 minute mark, the wobbling of the screw seems to reinforce his theory. Replacing the nipples and adding PTFE tape to the threads, seems to cure his issue.

I have actually cleaned the threads of my 190 calipers and got new nipples for them, before reading this thread and seeing the video, fearing that this could be an issue when bleeding. I didnt think that it could create an issue in use though, but it looks like that may happen. To me, it is such a simple and anonymous issue, I reckon this could be overlooked and dismissed very easily. Yet over time, with the small volumes involved and the number of times the brake pedal is pressed on each journey we make, could actually be the cause of a soft pedal without a complete pedal to floor situation.

If this is the case, then there could well be a tiny amount of air in the ABS system, which wont become apparent until the ABS is triggered, and the air released, which may or may not require the module to be bled after.

[klarzy]

ok, so he is getting air drawn in by venturi and putting bubbles in the bleed line, but this is after the nipple and would not draw air back into the caliper chamber....

[HarryM1BYT]

When I first read through that, I dismissed it as being very unlikely, but after giving it a bit more thought....

Doing the above, you are triggering the ABS ports to open up releasing any trapped air in the ABS modulator. You are supposed to use a T4 to bleed the brakes, the idea being to open up those ABS ports.

[Jules]

Most sinking pedal faults can be diagnosed quite easily.

Most cases we've seen are nothing to do with hydraulics but are to do with the vacuum/servo side.

Method;

With engine off press the pedal 6 times to exhaust all the vacuum from servo diaghram.

Next press hard and hold the pedal.
If the pedal holds with no further sinking, you have proved the hydraulics circuits are ok so definitely something to do with the servo or vacuum.

Common faults in the vacuum line are:

Diesels
The short rubber coupling pipe on the vacuum pump goes brittle and develops a leak

The small rubber vacuum pipe which tees off to the EGR gets chaffed through after the one way valve.
Either the run from the EGR solenoid or from solenoid to EGR
Even with an EGR bypass these pipes still form part of the vacuum circuit.
Block off from one way valve to prove the circuit.

Finally the vacuum pump itself could be weak.
The sinking pedal is simply an increasing vacuum building up in the servo slowly assisting you to press the pedal while the weak pump "catches up"

Revving the engine while pressing brake with left foot may sink the pedal to it's desired point quicker indicating something amiss in the vacuum circuit.


Now go and do some more tests and report back with your findings folks

[clf]

Originally Posted by klarzy

ok, so he is getting air drawn in by venturi and putting bubbles in the bleed line, but this is after the nipple and would not draw air back into the caliper chamber....

that is unlikely, the tube on the nipple is probably a snug fit - ie air tight. but when he moves the nipple it can be assumed that the whole bleed bolt moves and therefore the air is entering via the threads. if that was the case, then it would be reasonable to assume that air is being drawn in through the threads as the pedal is released. due to corrosion.

in fact corrosion anywhere on near to a sealed area could introduce a leak. thinking of this, my rear pistons are quite badly corroded and air could leak by I am sure the rubber seal

[klarzy]

Originally Posted by clf

that is unlikely, the tube on the nipple is probably a snug fit - ie air tight. but when he moves the nipple it can be assumed that the whole bleed bolt moves and therefore the air is entering via the threads. if that was the case, then it would be reasonable to assume that air is being drawn in through the threads as the pedal is released. due to corrosion.

in fact corrosion anywhere on near to a sealed area could introduce a leak. thinking of this, my rear pistons are quite badly corroded and air could leak by I am sure the rubber seal

true, but he is not using the pedal but a pressure bottle, so the flow is always outwards.. hence the air is drawn in and pushed out of the line...

also why if you use the pedal you should close the nipple on the end of the down stroke and never let it up with the nipple open..

[clf]

Originally Posted by klarzy

also why if you use the pedal you should close the nipple on the end of the down stroke and never let it up with the nipple open..

and this would explain how the air is drawn in via any corroded components sealed with a rubber seal or bleed nipple. perhaps it draws in 0.0001 cubic cms , or less , every 10000 strokes of the pedal will draw 1 cubic cm of air. this issue is one that is only noticed over time (years). so then when it is bled, there is a likelihood that some of that air has made its way into the abs system to present itself after the abs has been activated.

those that have experienced an improvement after, was any corrosion cleaned up or the calipers refurbished?