Brake Bleeding- How much fluid to flush and fill system

[virden]

I have just replaced the front calipers and the rear cylinders on my live axle X/F, a new master cylinder having been fitted last year. In refilling the system I have thus far  bled more than 600ml of fluid and am still getting some small air bubbles appearing.  The pedal has gone from no resistance to rock solid, but I cannot get rid of a soft half inch of initial  travel.  Am I doing something wrong/ have another problem, or is this as good as it going to get?    Cannot road test it yet as the engine is still in bits.

[Jonathan Kay]

Don't know the nominal volume: my Guide says "As required"!

Is it definitely worse than before the change?

Have you checked for play between the pedal and the master cylinder?

Are all the nipples in the right place for the air to go upwards into them? That came up recently with one configuration, but I can't remember which.

Jonathan

[martyn]

If you're still getting bubbles appearing, then you need to keep bleeding until they don't.  Either that or your technique is somehow introducing new air.  Dont open the bleed nipple any further than necessary, if you do then it can appear like new bubbles when in fact it is actually air getting sucked in around the loose bleed nipple screw thread.

A firm pedal with lots of free travel can be caused by the rear calipers not being correctly self-adjusted. The free travel you then feel is actually the caliper moving before the pad contacts the disc. To correct this (as much as is possible given the caliper design)..  

1. Disengage the handbrake and loosen off the handbrake adjuster.
2. Wind back both rear calipers, then refit the pads.
3. Firmly press the brake pedal several times to center the caliper on the disc (there will be lots of free travel the first few presses).
4. Readjust the handbrake.

Keep an eye on the master cylinder reservoir as you wind back the calipers because the volume of fluid may cause it to overflow.

If you are having trouble with air in the rears, then it can help to bleed while the caliper is wound back.  Like this there is less internal volume for air to get stuck.  you'll need to bleed with the windback tool in place to prevent the piston from moving.

[elie boone]

Adjust your rear drums and see if it's any better, it takes about 750 ml to fill and bleed the whole system. 

[SM25T]

Martyn .... rears appear to be drums, not discs. No windback of piston.

[OliverSedlacek]

I have to say that no one ever seems to come up with a definitive procedure for bleeding the brakes. For standard brakes 500mL should be enough. I've taken to collecting the fluid in a clean container and returning it to the reservoir if it's coming out clean but still with bubbles.

[martyn]

Sorry all - really not paying attention.  Ignore my post  

[Terry Field]

I think I have cracked this problem. Same live axle set up, and I always used lots of fluid, took ages, never really got rid of all the bubbles, but somehow got a firm pedal. I always felt that air was leaking back through the nipple once the downwards pressure was released.

The I discovered  Eezibleed! Did the whole job single handed in no time at all, with no trouble getting rid of bubbles. I reckon that, because Eezibleed keeps a positive pressure on the system, air cannot the sucked back in, the result being an air free system very quickly. Wish I had bought one years ago.

I also used Automec Dot 5 silicone brake fluid. More expensive than ordinary stuff, but allegedly never needs changing, absorbing virtually no moisture.

Give it a try

Cheers

Terry

[7 wonders of the world]

DOT 5 Silicone fluid should be avoided at all costs!

Yes it will not harm paintwork and is not hygroscopic as is normal glycol fluid but is had miserable lubricating properties and responds very poorly to heat suffering from aeration leading to vapour lock and a lousy pedal at remarkably low temperatures - far from ideal for a performance car.

Conventional DOT 5.1 is much cheaper and its characteristics meet with a performance cars requirements.

[virden]

Thanks All, I think Martyn has a strong point in  that my technique may not be up to scratch and I have not been ensuring the bleed grommet has not allowed external air to be sucked in.

By the way i am using DOT 4 and Mintex 1144 pads, road use only, is this a potential fade problem?

[SM25T]

That's fine.

[DJ.]

With regard to Eesibleed, I recently had and idea when changing my brake fluid, as I didn't have helper available.

I never liked my Eesibleed kit, because it was very difficult not to get drips of fluid on the car when removing the cap from the brake reservoir. Also, I was always concerned that any leak between the Eesibleed fluid container and the brake reservoir would result in large quantities of brake fluid spraying everywhere.

What I decided to do, was to fill the brake reservoir up as usual, but then to attach the Eesibleed kit as normal, but with no fluid in the bottle. The air pressure allowed one man bleeding of the system with no chance of air being sucked back in, and as the Eesibleed pipes were all empty there was no risk of getting fluid everywhere

I will definitely use this technique again. the only drawback I could see, is that the tyre valve has to be disconnected and the reservoir cap removed to top up the brake reservoir.

Duncan

[SM25T]

Definitely. No fluid in Ezibleed bottle. Been there .... had the high pressure shower. Works fine dry.

[OliverSedlacek]

I wonder if all eezibleed users come to the same conclusion, don't bother with fluid in the bottle. I have to say I'm still struggling to get a good feel on my brake pedal and I'm starting to wonder if something isn't working properly. I'm really starting to regret converting to disks.

[TomB]

Ive never not bled brakes without an Easibleed. I think they are great and make the whole job a simple one man operation.  I usually fill the container, but with plenty of rags and newspaper down its fine.  Ive found a bit of PTFE tape around the thread helps a bit and not over tightening the cap causing the plastic to distort.  Did the 7 last autumn before a track day, and last week did my new-to-me-but-old MINI Cooper, used it on current Golf and previous A3.  Something no home garage should be without IMHO.

[aerobod - near CYYC]

I use a Motive pressure bleeder which has it's own pressurisation hand pump and gauge: https://www.amazon.co.uk/Motive-Products-Brake-Bleeder-System/dp/B0002KM5L0

I find that using over 10 PSI, but less than 15 PSI works the best and never causes a leak issue. On the Caterham with the 4-pot front brakes, I fill the  motive container with 1 litre of fluid, remove the old fluid from the reservoir with a syringe, then bleed the 6 brake and 1 clutch bleed points in one operation (my car uses the same reservoir for brakes and clutch, with a divider inside between the circuits). To make sure all air bubbles are released, I gently tap the calipers with a dead blow mallet while the brake fluid flows into the catch bottle.

Perfect pedal firmness and a quick and easy bleed every time.

[Jonathan Kay]

I use an Eezibleed without fluid in the reservoir and a valved tube on the bleed nipple.

Jonathan

[elie boone]

i use an old brake reservoir cap with a 8mm hole in it and a blow gun at 0.8 bar, never had a problem bleeding brakes even on the Sierra rear brakes with a system with only 1 bleed nipple.

[OliverSedlacek]

In the struggle to get a firm pedal I borrowed a vacuum bleeder, and it's done a better job than the Eezibleed. I discussed this with my my professional mechanic chum and who felt that the disadvantage of any pressure (as opposed to vacuum) system is that it squeezes air bubbles, making it easier for them to stick in some niche. Whether his explanation is right or not, the proof of the pudding is that it succeeded where the Eezibleed ddn't.

[Steve_K]

Intuitively this seems sensible. While the solubility of air in oil is small, if you double the pressure to 1 bar say, you'll double the amount of air dissolved in the oil.

Old jet aircraft used to use a deaeration rig to place hydraulic oil under a low pressure to release dissolved gas. Worked amazingly well but a waste of time really as the gas was reabsorbed as soon as the Hyds were repressurised.

Steve

[martyn]

I've never considered that benefit of vacuum bleeding, but it is absolutely true.

When bleeding the brakes on my mountain bike, you use a syringe to create negative pressure.  As you do this, you can see the previously invisible bubbles grow in size to the point that they are dislodged.  If you pull hard enough on the syringe, you can force bubbles out of seemingly clear fluid. That only works a couple of times though until all air is removed.

That process is actually performed before adding the fluid to the system to ensure it isn't aerated.

Then interestingly, you attach a syringe to both the master cylinder and caliper, and flush the fluid back and forth. So effectively bleeding in both directions using both negative and positive pressure.  It seems like a pretty comprehensive way to ensure no trapped bubbles.  I wonder why nobody has developed something similar for cars - a couple of syringes is so much simpler than faffing about with compressed air in a spare wheel.

[OliverSedlacek]

I don't know how the commercial brake bleeders work, but they can cost £1400.

[Jonathan Kay]

Not convinced by the smaller bubbles but the solubility argument is interesting. What's the relationship between dissolved gas content and compressibility?

I was wondering why professionals use negative pressure, and we use positive pressure. But that could be a different trade-off between capital cost and labour time rather than improved outcome. (Crossed with Oliver's costing.)

Jonathan

PS: I'm cautious about attributing superior performance to the last used method. See also finding keys in the last place you look.

PPS: I worked with bubbles in two very different ways in my career. Bubble management was the technological breakthrough that led to the Technicon AutoAnalyzer. And that's what allowed cheap, widespread multichannel clinical chemistry analyses that most of you will have used as patients. It was eventually beaten by pre-existing conventional liquid-only systems that replicate manual techniques using pipettes and cuvetttes.

[martyn]

Yeah you're probably right about it being dissolved gas rather than small bubbles.  However exposing the fluid to negative pressure definitely releases it (whatever it is) and also helps to shift any bubbles stuck to the side much easier. From what I've seen though, it takes a greater vacuum than you're likely to achieve with any bleed system or syringe. However through extrapolation I suppose we can (unscientifically) assume that some negative pressure is better than none.

[Steve_K]

FWIW gas solubility in liquid is directly proportional to pressure and that's pretty much any gas or liquid. Any reduced pressure will help. In the jet example I mentioned, normal system pressure was around 3000 psi so the amount of gas dissolved was huge. Mostly it actually doesn't matter until the fluid passes through a restriction (such as a shuttle valve) at high flow rates when the gas comes out of solution due to the local lower pressure. The response of the valve can be fairly strange when the working fluid changes...

Steve