Brake bleeding

[ericbushby]

Hi All,
This works but I do not know how.
I had a seized brake master cylinder, so I fitted a new cylinder and also replaced a front brake hose with no difficulties until I came to bleeding the brakes.
I used an Easybleed bottle with one way valve in the tube and did the usual procedure.
However I could not get all the air out and the brakes were spongy to the point that the pedal would go down to the floor and still was not firm. I went round the car bleeding each cylinder in turn until I ran out of fluid.
Remembering previous suggestions on the forum I wedged the brake pedal down under as much pressure as I could using a piece of wood to the seat mounting. It was left for 24 hours under pressure.
Today on the way home from a walk I bought another bottle of fluid and went to the garage to check.
The brake pedal is now firm and correct, nothing needs doing. !!
If it was air trapped in the pipework where has it gone and how does it get out. ? Any ideas?
Eric in Burnley
1967 S3SE DHC

[rgh0]

I have done the same with the clutch. I beleive it is air in the master or slave cylinders that migrates slowly past the seals under pressure and escapes where the larger brake fluids molecules will not go.

An alternate theory is that small amounts of air can dissolve in the brake fluid under pressure and does not come out again once the pressure is released. Possible but I prefer the first theory

cheers
Rohan

[h20hamelan]

& or, how much h2o is in the air?
How does hydroscopic brake fluid accept water unless your James Bond (not the real Bond James)?
if it truly enters from rubber flex lines
does the h2o disassociate from the air in brake fluid, and migrate or?

[englishmaninwales]

I have done the same with the clutch. I beleive it is air in the master or slave cylinders that migrates slowly past the seals under pressure and escapes where the larger brake fluids molecules will not go.

An alternate theory is that small amounts of air can dissolve in the brake fluid under pressure and does not come out again once the pressure is released. Possible but I prefer the first theory

cheers
Rohan

I think the former idea. In the second scenario, and please feel free to correct me if my schoolboy physics is wrong, Henry’s Law would dictate that the dissolved gas (with the pedal forced down) would come out of solution when the pressure is released and you’d have a long travel pedal again (brake system would suffer the “Bends”! )
Malcolm

[RichardHawkins]

I worked in the chemical industry, basically a sulphuric acid reaction. The plant was built around 1947/48, and used lots of lead for its resistance to sulphuric acid solutions. Lead being soft did not last long at higher temperature and pressure.

As technology advanced we were keen to try plastic lined steel pipes (PTFE), and were interested to find that all the pipes had a vent or weep hole. The manufacturers informed us that as the plastic is water vapour permeable, the liner would collapse due to pressure increase between the liner and the steel. We had thought that the permeability was too small to be of any consequence, but were quite wrong. Later in my working life I discovered that EPDM, the rubber that brake flexibles are made from, is also water vapour permeable.

A friend repiped the fuel line for his Elan inside the cabin using PTFE and could only get rid of the smell by using metal pipe.

I don’t pretend to understand the science of vapour permeability, but maybe it has something to do with keeping pressure on the brake pedal over night.

Richard Hawkins

Richard Hawkins

[ericbushby]

Hi All.
Thankyou for suggestions.
Richard,
can the air molecules be escaping through the caliper hoses as well as past or through the seals. Air is a mixture not a compound so each element could escape separately.
There would be very little water in the fluid as a lot had run out whilst I was working on it. I did not bother as I was changing all the fluid. I put half a litre in from a new bottle. so it would be quite clean.
I am just fascinated to understand why it works so well.
Eric in Burnley
1967 S3SE DHC

[Hawksfield]

Hi
I read about this some years ago and have used this method to complete a brake fluid change or when I have worked on the system, I even now have bamboo stick with a pad at one end to pressure the brake pedal from the seat back,
Works every time don't know why but happy it does.

[patrics]

Hi
All that happens is the air is absorbed into the fluid which then turns in to an acid – basically you are prematurely ageing the fluid. Putting compressed air on to the fluid in the reservoir is also a bad idea.

Regards
Steve

[pharriso]

Putting compressed air on to the fluid in the reservoir is also a bad idea.

Regards
Steve

Like all Power Bleeders? Sorry I disagree....

[Andy8421]

Putting compressed air on to the fluid in the reservoir is also a bad idea.

Regards
Steve

Like all Power Bleeders? Sorry I disagree....

Now my offspring have left home, its tough to bleed the brakes as a one man band. I have become a convert to compressed air power bleeders, which allow a quick and easy way to bleed the brakes.

As far as I am aware manufacturers use a combination of system evacuation and power bleeding in conjunction with cycling the ABS system and ABS pump on the production line.

I have one of these:

https://www.eurocarparts.com/p/gunson-g4062-eezibleed-kit-lasg4062?gad_source=1&gclid=EAIaIQobChMIj5Xsgv2ahAMVuJxQBh2g1ghbEAQYASABEgKmcPD_BwE

I have adapted it to run off my airline, but the pressure has to be very low to avoid problems. It was a faff to get the cap to fit and seal on the master cylinder, but that was a one off. It takes no time now to fit and bleed the brakes. I discard the fulid remaining in the bottle after I have used it, and always start with fresh fluid.

[patrics]

Hi
Professional power bleeders do not but air pressure directly on to the fluid. The old type used air pressure on a diaphragm which then applied the necessary pressure to the fluid. These days it is by electric pump.
In production, the air in the system is evacuated through the reservoir, the vacuum source is disconnected via a tap and the vacuum is held to check for leaks. Then fluid under pressure is introduced to the system – the pressure is generated by electric pump.
Some of the reasons that people find bleeding vehicles difficult is that the calipers are only designed for evac-fill but you can get around that.

Regards
Steve

[USA64]

Perhaps the overnight pressure just -beds- the seals so to speak?

[h20hamelan]

Mine

https://www.kincrome.com.au/automotive- ... 00ml-k8161