Bleeding brakes a warning

[T-Cut]

Originally Posted by wuzerk

My theory is that since the vacuum generated for the brake system is done with a pump on the diesels and any pump should have a relief valve in it somewhere to prevent damage so, perhaps the sinking is when this valve is doing its job? It will not occur when the engine is not on since the vacuum pump is not working then?

That would mean SPP in petrol models is due to a completely different cause. Since all models exhibit exactly the same phenomenon (as you correctly describe) a common cause seems most likely. A vacuum pump doesn't require a relief system because the absolute maximum pressure differential possible is 15psi (1 atmosphere). Unlike the aircon compressor or the power steering pump, vacuum pumps only pump air at or below atmospheric pressure. In spite of the reports of SPP elimination after an ABS bleed, the evidence that it's caused by air in the module isn't very convincing (IMO). There would simply be no brakes if that were so. Since the Government department responsible for controlling the MOT test has reported that SPP is not a fail item (providing it only happens with engine running) then IMO, it's not an air issue and the effect has yet to be explained.

TC

[T-Cut]

Originally Posted by thebiglad

Surely you can actuate the abs pump by proving a supply to the abs relay.

Logically, that's all the T4 can be doing.

TC

[T-Cut]

For anyone interested in this, here's the original discussion. It doesn't seem to have got any closer to an explanation (IMO) than it was at the time.

That Sinking Feeling: http://www.the75andztclub.co.uk/foru...ad.php?t=24916

[COLVERT]

When you operate the brakes there is a sudden pressure drop in the system generated by the then vacuum in the pump. After the first drop doesn't the pump carry on increasing the vacuum and cause the pedal to slowly drop further??




Colvert.

[geofftl1000r]

Originally Posted by COLVERT

When you operate the brakes there is a sudden pressure drop in the system generated by the then vacuum in the pump. After the first drop doesn't the pump carry on increasing the vacuum and cause the pedal to slowly drop further??




Colvert.

if the peddle is dropping, where is all that fluid going? is there a piston in the abs pump that is slowly moving?

My peddle was much firmer with an abs change, and master cylinder change. Last time I checked SPP was gone or very greatly reduced. It's been a few months so need to confirm once the snow goes...

[COLVERT]

Originally Posted by geofftl1000r

if the peddle is dropping, where is all that fluid going? is there a piston in the abs pump that is slowly moving?

My peddle was much firmer with an abs change, and master cylinder change. Last time I checked SPP was gone or very greatly reduced. It's been a few months so need to confirm once the snow goes...

I think that the amount of pedal drop we are talking about doesn't take much fluid. I think that it is that some components give slightly because the pressure is quite high.( ie. The casing on the servo for instance. ) And that gives the slight increase in pedal movement. Even though steel is very hard it still can flex under certain circumstances.

[Jules]

I use my Sealy sump pump regularly to bleed brakes and clutches never had a problem!!
Sucking it out from the bleed nipple end gets all the air out in one go WITHOUT pressing any pedals. One man job too.

Replaced an ABS modulator recently (so a full drain down)
Tried it without T4 (just as an experiment) and pedal rock solid afterwards

Jules motto
KIS Keep it simple

[HarryM1BYT]

Originally Posted by COLVERT

I think that the amount of pedal drop we are talking about doesn't take much fluid. I think that it is that some components give slightly because the pressure is quite high.( ie. The casing on the servo for instance. ) And that gives the slight increase in pedal movement. Even though steel is very hard it still can flex under certain circumstances.

I very much agree!

With engine off, vacuum exhausted your foot on the pedal generates very little hydraulic pressure compared to engine running - so the pedal has little movement and will feel very hard. That is alkways the test I use for air in the system following any work on the brakes.

Run the engine and the pressure rises dramatically, with the vehical stood, you will put much more pressure on the pedal than you would ever be likely to do on the road. On the road you barely tickle the brake pedal in normal use. Under extreme pressure things do give, like the flexible hoses the rigid pipes and etc.. Which is why the tuners replace the flexibles with the reinforced ones.

If the pedal goes down and doesn't stop before it hits the floor, then you have an issue.

[COLVERT]

Originally Posted by HarryM1BYT

I very much agree!

With engine off, vacuum exhausted your foot on the pedal generates very little hydraulic pressure compared to engine running - so the pedal has little movement and will feel very hard. That is alkways the test I use for air in the system following any work on the brakes.

Run the engine and the pressure rises dramatically, with the vehical stood, you will put much more pressure on the pedal than you would ever be likely to do on the road. On the road you barely tickle the brake pedal in normal use. Under extreme pressure things do give, like the flexible hoses the rigid pipes and etc.. Which is why the tuners replace the flexibles with the reinforced ones.

If the pedal goes down and doesn't stop before it hits the floor, then you have an issue.

Harry, I think we've finally cracked it. SPS finally resolved.

[T-Cut]

I think I'd need some hard evidence to explain it. A theory that can be tested and demonstrated. It requires measurement of the actual pedal force, changes in pipe diameters, etc. When I first observed it, the pedal didn't require extreme force to sink. I still believe it's an electro-mechanical effect of road speed (zero), engine speed, hydraulic pressure, the ABS system, the ECUs and such things. SPP is observed across a wide spectrum of cars and is known to MOT testers. I can't see hose expansion being an acceptable effect of pressing too hard on the brakes. IMO, no individual could generate enough hydraulic pressure to expand the pipework to any significant extent. And significantly, SPP doesn't happen when the car's being driven. It would be a very hairy experience if it did. I think the jury's still out on this one.

TC