Brake bleeding advice

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Kiwisholland
Posts: 12
Joined: Sat Nov 03, 2018 7:34 pm
Location: Lehigh Valley, PA

Brake bleeding advice

Post by Kiwisholland »

I recently bled the brakes on my ‘83 633, using a Motive power bleeder. The fronts worked fine (done after rears in recommended order). I could never get the rears to stop having small air bubbles. I went through two liters of fluid, and the bubbles never stopped.

I’m going to replace all hoses with stainless ones, so I’ll be doing the bleed again.

Do I need to pump the pedal 12 times even with Motive bleeder?

Thanks
‘83 633csi
GripGreg
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Re: Brake bleeding advice

Post by GripGreg »

WOW!
It's been a month since this was posted; and no one has responded with any info? :shock:
You dudes use to respond a lot sooner; what happened? I know someone has the answer. #-o
I know I don't! Maybe you were hoping someone else would chime in? Yup, that's it. :roll:
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Re: Brake bleeding advice

Post by ron »

Kiwisholland wrote: Mon Nov 05, 2018 12:48 amDo I need to pump the pedal 12 times even with Motive bleeder?
Yes to dump the pressure from the "bomb". Take the cap off the power steering reservoir and pump the pedal. The fluid level should rise. When the pedal goes hard, you have dumped all the pressure, and you can change your brake hoses and
re-bleed the system.

There are numerous posts on the brake bleeding, do a search.
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NigelC
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Re: Brake bleeding advice

Post by NigelC »

Sometimes when bleeding brake calipers -even with a pressure bleeder kit, you can see a stream of small air bubbles coming out
which does not seem to stop after a lot of fluid has been used. It can be due to air being drawn up the brake nipple thread loosened
for bleeding. Some brake nipples are quite loose in their threads when not 'nipped up', you can overcome this by carefully wrapping
PTFE tape around the nipple and refitting to stop air getting in when bleeding.
When using a power bleeder this can be misleading as the caliper may be full of clear fluid, but air gets picked up from the loose
thread as it goes into the nipple and bleed-pipe ( a bit like a spray gun picking up paint by pressure difference)
hope this helps, Nigel.
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JCS
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Re: Brake bleeding advice

Post by JCS »

Hi,

Here is a link on YouTube on how to bleed a clutch M/C & Slave, this principle can be also used on brakes

LINK : https://www.youtube.com/watch?v=wdf--suwqw0&t=313s

Start at the RR,LR RF,LF

A lot of people do not like this system, as you can push old dirty Fluid back into the braking components,

I use it only after I have put in new brake fluid, thus flushing the system.

Best,
Last edited by JCS on Mon Dec 03, 2018 2:46 pm, edited 1 time in total.
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Brucey
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Re: Brake bleeding advice

Post by Brucey »

note that on standard (post 5/82, non-M635CSi) E24s there are brake pressure proportioning valves in the lines to the rear brakes (not far away from the ABS pump). [BTW in M635CSi there are pipe unions in the same place; since the standard valves are little bigger than unions are, it isn't that obvious whether you have valves or unions fitted in fact.]

These proportioning valves are not at all likely to leak directly to air but they can interfere with the bleeding process; they won't open for 'forwards flow' (to the calipers) until a threshold pressure has been reached and they open very much more easily for 'return flow' i.e. back towards the MC.

This means that some 'normal' types of bleeding operation work less well than normal. A pressure bleeder (positively pressurising the system to ~+5psi at the MC reservoir) won't always open the proportioning valves properly, so getting fluid flow out of the rear calipers is difficult. It also makes it more likely that the calipers will suck air in via the bleed nipple when the pressure is reduced.

If the caliper always sees high positive pressure when the brakes are bled, this means that a trapped pocket of air will occupy a smaller volume during the bleed, and expand back again to full size once the system pressure is back to normal. This means that a given 'quality of bleed' (or bubble size at bleed pressure) results in a spongier pedal than normal.

It makes quite a lot of sense to bleed the rear calipers by

1) doing it in the normal way (i.e. top down) in the first instance and then
2) bleeding in reverse, i.e. forcing (clean) fluid in via the bleed nipple drilling.

The latter runs the risk of pushing old (contaminated) fluid into the rest of the system but hopefully 1) will prevent that.

If you don't fancy step 2 or don't have the right plumbing parts, there is an alternative, which is to use the caliper pistons as 'a pump' (call it CPP method why not).

In this method you

a) install suitable spacers (eg an old set of pad backings with no friction material on them) in the calipers and then
b) pump the brake until the brake pistons are fully extended and the spacers/backings are clamped to the disc, then
c) push the pistons back in fairly briskly.

During step c you must make sure that the brake union on the caliper is uppermost, i.e. so that any air bubbles near the caliper pistons are immediately pushed out down the brake lines. This method pushes ~100cc (per piston) up the brake lines, which given that the lines themselves hold about 20cc means that you should be able to displace any air trapped in the lines. Note that the 'brisk' part is important if the brake lines contain any sections that have an unfavourable slope; only fairly violent passage of fluid will push air bubbles out of such locations.

Note that the CPP method does not require that you even open the bleed valves on the caliper. It is not very good if your objective is to fill the system with fresh fluid though; probably if you want to do this you should pump the pistons out (a, b) then

d) hold the brake pedal depressed slightly (e.g. using a piece of wood) so that the MC compensation ports are closed and
e) open a union near the caliper then
f) push the brake pistons back (slowly) and catch the displaced fluid coming out of the open union as best you can.
g) close the union before letting the brake pedal back up and repeating the process

A few cycles of a,b, d-g will get rid of the manky old fluid from the calipers without it getting into the rest of the system or letting air into the workings.

BTW I didn't reply to this thread earlier because I didn't (and still don't) have the slightest idea of what a 'motive bleeder' is.... :wink:

HTH

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Kiwisholland
Posts: 12
Joined: Sat Nov 03, 2018 7:34 pm
Location: Lehigh Valley, PA

Re: Brake bleeding advice

Post by Kiwisholland »

https://www.bavauto.com/www.bavauto.com ... eeder-0100

Thanks for all the thoughtful advice, very helpful.

BTW, I spend most of time here “searching” the forum for answers. If I type in “Bilstein” for example, every car ever for sale with Bilstein is listed in the results. I did not post questions to waste anyone’s time, and I do truly appreciate the help.
‘83 633csi
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