News:

The ORG - 100% FREE advice!

Main Menu

Brake talk again.

Started by craigb, 06 September 2009, 09:21 PM

Big_Richard

Quote from: TJ 450 on 23 September 2009, 08:46 AM
I'm not sure why the different offset. In fact, PB discovered this difference when he went to fit the W126 calipers to his 6.9.

But yes, the rears are all the same.

Tim

Ahh yes, that's right, TJ swore black and blue that i could fit w126 calipers on my new front 116 rotors. I then purchased the rebuild kits for them and after all the effort, they couldn't be installed. Coincidentally TJ more than happily took the calipers off my hands at no cost ;)

I'm glad i ended up going with the new ate's anyway, its all good.


oscar

Who's up for another theory for spongy brakes?  ::)

During bleeding via "pump the pedal" method, I was wondering about the effects of releasing the brake pedal too quickly, recuperating ability of the MC and the effect of vacuum on brake fluid.  Last time I bled my brakes I continued to get miniscule bubbles from the rear calipers in particular.  Could not work out where they were coming from. 

Maybe as the pistons withdraw in the MC a vacuum is created within the MC and the lines before the MC gets a chance to refill.  This might be exacerbated if it's true that the MC's seals could be damaged by pushing the pedal to the floor during previous bleeds.  I had a hunch that maybe as pressure drops in the lines, some gases would devolop and remain out of solution when the fluid returns to atm pressure. 

So this morning I've cracked open a new bottle of brake fluid, grabbed 10ml syringe, half filled it with brake fluid and pulled the plunger.  The fluid appears to boil and when the plunger returns there's bubbles left behind inside the syringe.  There's one obvious big one but many tiny ones around the syringe not readily visible in the vid. 
http://www.youtube.com/v/S4y3BbkFxTU&hl=en&fs=1

I'm sure the method could be criticised by accusing the plunger's seal of not being air tight or the tip of the syringe not being air tight, but bubbles develop and stay in a gaseous state in parts of the syringe indepedantly of the two ends of the syringe.  It occured with numerous tests on fresh fluid. 

Assembling the syringe and submerging the entire test in brake fluid would exclude air leaks as a variable but I'm convinced it's not necessary to go that far to prove that air pockets can develop in brake fluid under vacuum.  Thing is, do you think it's plausible that this might be happening during bleeding attempts?
1973 350SE, my first & fave

koan

Hmmm... not sure.

I can certainly see bubbles form in the fluid but I can also see what seems to be a stream of bubbles from the needle end.

The bubbles that form in the fluid are going to be one of two things, either some volatile component in the fluid that boils under reduced pressure and becomes a gas or they are bubbles of vacuum, voids, not gas.

Either way they should disappear when pressure returns to normal.

How did you fill the syringe oscar, can you be sure that there are no microscopic air bubbles trapped on the inside surface of the syringe?

What would happen if you did the same test with water with a bit of detergent added to reduce surface tension so the water "wets" the inside surfaces. Maybe even boil the water to drive off dissolved air.

I don't think a vacuum or voids would form during bleeding. If something like you describe was happening the pedal wouldn't return or the caliper pistons would be "sucked" back.

Vigorous pumping is something I avoid because fluid spurts about in the m/c and tank just inviting air bubbles to get trapped in the fluid.

Good on you for testing oscar and interesting to watch but not sure this avenue of research.

koan
Boogity, Boogity, Boogity, Amen!

oscar

Glad to receive some feedback koan.  It looks like I'm going to have to work a bit harder to convince you.   ;D  I'll try the submersive test to clear doubts over where the bubbles come from in the syringe but regardless about what can be done to brake fluid under vacuum, I suppose all that matters is if it applies to our brake systems.   If I'm wong I'm wrong, no probs.  My bleed methods have probably contributed to the problem.  I reckon my pressure bleed rig caused bubbles in the reservoir and I'm yet to try smaller downward pedal movements with a slow return.  To answer a couple of your queries and/or doubts -

Quote from: koan on 03 October 2009, 05:12 AM
I can certainly see bubbles form in the fluid but I can also see what seems to be a stream of bubbles from the needle end
I'll admit, that does look dodgy, but I swear no air enters that end.  If I uploaded one of the other tests from when I held the syringe at different angles, it doesn't appear the same and the left over bubbles develop elsewhere.  A submersive test should prove this.

QuoteThe bubbles that form in the fluid are going to be one of two things, either some volatile component in the fluid that boils under reduced pressure and becomes a gas or they are bubbles of vacuum, voids, not gas.

Either way they should disappear when pressure returns to normal.
I agree with the first bit of course but don't believe they can be pockets of vacuum or voids.  Atmospheric pressure is working to push the plunger in and it wins over when I release the plunger and pressure inside the syringe equals atmospheric pressure. Voids would collapse.  If I press the plunger in to increase the pressure inside, any bubbles get smaller.  There's of course some friction and resistance caused by the plunger's seal with the syringe wall that resist movement either way so the resting pressure inside the syringe wont be exactly the same as atmospheric pressure but any bubbles thought to be voids should collapse and stay collapsed after the plunger is released after being pressed in.

QuoteHow did you fill the syringe oscar, can you be sure that there are no microscopic air bubbles trapped on the inside surface of the syringe?
Basically I submersed the tip in fluid, withdrew a few mls, there's some dead space to begin with but I tapped the syringe whilst holding it upwards, squirt the fluid out (not into the bottle).  The dead space is taken up by fluid now, then I drew more fluid up to 10mls, did a few discards and drawbacks within the fluid as well then the final one was half filling it whilst the tip remained in the fluid.

QuoteWhat would happen if you did the same test with water with a bit of detergent added to reduce surface tension so the water "wets" the inside surfaces. Maybe even boil the water to drive off dissolved air.
I did this with water years ago, no detergent though.  It was during Uni days, submersed a disassembled syringe in water, tapping it to remove air, assembling it underwater and did the deed with blocking off the end whilst withdrawing the plunger.  It was that little experiment that made me think of trying it with brake fluid.

QuoteI don't think a vacuum or voids would form during bleeding. If something like you describe was happening the pedal wouldn't return or the caliper pistons would be "sucked" back.
Without some fancy test rig I don't know how I could demonstrate that a vacuum forms in the lines when bleeding.  My personal view is that my caliper pistons are tight enough that a vacuum wont draw them back and that the pedal would return due to the strength of the MC's springs.  The force required to compress them is so much more then what's required to subject the brake fluid to a vacuum.






1973 350SE, my first & fave

Big_Richard

could all of this trouble be cause by using poor quality brake fluid ?

what are you using Oscar ?

oscar

castrol superdot4.  I also tried a castrol dot 3 a couple months back thinking that that's the spec our systems were designed for but no joy there. 
1973 350SE, my first & fave

TJ 450

I'm still watching the brake threads with great interest. 8)

I just bled the brakes on my W126 today, which has the Bendix calipers that were previously on my 6.9. The pedal is absolutely rock hard without vacuum just like my 450. The front circuit has been empty for a few months, I might add and the brake fluid hasn't been changed for who knows how long. The car was languishing in a wrecking yard before coming into my possesion. :(

It was that easy, yet my 6.9 still has the lowish pedal that is spongy on the way down.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

craigb

If I haven't mentioned this before you are welcome to hit me next time you see me! You just made me think with the issue of pressure vs vacuum behaviour. Have you tried a pressure bleed? I got another master cap, and cut the valve assembly out of an old tube, drilled a hole that size in the cap and locked in place with the nut and removed the valve centre. Then a hose (1/4 I think) and then a fitting off a stuffed old foot pump. I think I let a tyre down to about 12 or so psi, fitted the hose to the tyre that puts pressure on the master. Then I just released each nipple starting from the furthest caliper and checking the level of the master in between. In my manual this caused a slight weep from the clutch master at the side, but you are only doing it for a few minutes so I just put a rag underneath. There is a 'proper' one - Gunson ezibleed, that fills the reservoir via a bottle but I just copied this set up from a forum post somewhere.

Given the hoops you have been jumping through, this might be worth a try and would solve your prob if the current theory is correct.
1980 280s

koan

I've tried a pressure bleeder, a DIY job made from an old reservoir cap and a tyre valve, still spongy pedal.

koan
Boogity, Boogity, Boogity, Amen!

oscar

Hey Craig, didn't mean to ignore your post but I didn't want to add to the thread until I did the bleeds I said I would which I've done now and still no improvement.  I did the half pedal depresses with a block of wood underneath and a slow returning pedal.

As for pressure bleeds, I used an air compressor and did what koan did.  Though I've forgotten now what peak PSI I was using (seems like so long ago I did it) I think it was too much and because I was using a trigger valve to regulate the pressurise to the reservoir, occasionally a jet of air would bomb the fluid and cause plenty of tiny bubbles to contaminate it.  Plus in hindsight, using compressed air that probably was contaminated with compressor oil, despite having filters, was not wise.  There's been many bleeds since then and still have the sponge.

I reckon if you could apply pressure to the reservoir via pressurised fluid instead of air that would be ideal.  Seems though the Pro Gunson thing does that.  Maybe I'll invest in something like that but I still have a hunch something else is happening too.  I'll take it to see a specialist eventually but not till my supply of stubborness is exhausted.  8)

1973 350SE, my first & fave