Landseer

86 951. Replaced rotors and pads. Perfomring manual bleed with helper


Hard pedal, about half stroke, fronts work.
Can only get a tiny bit of air / fluid from rears.
No progression of rear pads at all. Tried 40- times or so, having helper slowly stroking pedal


With pedal down hard 1/2 way to floor, opening a rear bleer gets only a little air puff or a bit of fluid... and pedal doesn't drop further though. ( I expected it to drop to floor).


What is wrong?

Is it possible for a proportioning valve to stick and prevent flow to rear?

zerMATT951

You might try pulling the rear calipers and clamping/squeezing the pistons back into the housings. That will force some fluid back up the line and you should see the reservoir increase in volume. Start with the right-rear side (farthest from the master cylinder). This process may dislodge whatever is blocking the lines and you end up reverse-purging the line.

If you see increased fluid volume doing that at the right-rear, proceed to the left-rear and do the same thing. Suck excess fluid out of the reservoir with a turkey baster (or other implement), and get as much of the old fluid as you can out of there. Next, top off the reservoir with new fluid, bolt your calipers back on, and go back to pumping the brakes and resume your normal bleeding process. Make sure to check the fluid level frequently when you first start pumping... the level will drop very quickly as the calipers are re-filled.

Hopefully this process of moving fluid in the reverse direction, then back out to the calipers will clear up whatever is in there causing a problem.

Landseer

Pistons are pretty much bottomed into the calipers, so I'm afraid no easy way to back up the flow.
Fluid was very clean when I had the calipers off. Will find a way to clear lines .

Master went essentially dry when I had calipers off.
Is there a way to bleed the master cylinder, particularly the section dedicated to the rear brakes?
Am wondering about the piston seals for rear brake actuation.

Don't understand this.

Fronts are bled and activating, holding pedal high.
Opening any rear bleeder doesn't cause pedal to fall.
I don't think there is a line obstruction.
Is it possible to have/keep front brakes, but no rear brakes by design?

zerMATT951

If you had the calipers off and fluid drained out, than maybe there is air in the line and it just needs help for some initial fluid draw into the master cylinder so that your pedal pumps fluid not air. Do you have a vacuum pump (Mighty-Vac or similar)? With that, you could try "pulling" fluid to the rear calipers rather than "pushing".

Van

It's a long line to the rear... They can be a pain in the ***. I'd focus on the caliper farthest from the master cylinder (rear left) and try to cycle about a quart of fluid through one bleeder (that will be a lot of pedal pumping for your helper). Then do the other three bleeders. Hopefully that will get all the air out of the lines.

This is where a pressure bleeder can come in handy - it can move that volume through quickly and without stopping.

Landseer

OK, thanks, will put additional effort to the rears.

As fronts raised the pedal, less stroke was available for the rears.

I have a acton vac tester that I've used a bit this morning on the rears, trying to pull air through.
These multi piston chambers do have a lot of volume.
Have an A/C evac pump, but with my luck I'd destroy it.

Have put an awful lot of fluid into the master this morning, getting little out. Keeps sending up air bubbles. Very interesting. Almost seems like I'm filling the power booster with fluid.

Will report back. Could be multiple problems on this one.

Some days I wish my kids were still bringing home stray cats and dogs.
Its shifted to neglected cars. Damn thing is fast, though.

JohnKoaWood

Chris,

I have a motive pressure bleeder, and would be more than happy to assist sometime!

John

Landseer

Thanks John, that might help. (Thanks Matt + Van as well)
Have a pretty good manual bleed method, but this one is challenging.

So far have put 1/2 liter fluid into the master this morning.
Car is sitting high on jackstands, no leaks, no pooling in the engine bay.
I've got to be filling the booster at this point.
Add some... bubble every 2 seconds, same interval and size bubble all morning, as fluid drops.


Can't pump-up the rears. Bottomless pit for fluid.
If the rearmost chamber of the master is for rear brakes, then I conjecture that the seal(s) are bad.

I pull air with the hand vac pump, each bleeder, opening only one at a time --- but like its pulling from an empty chamber, very little fluid.

Van

I always thought the rear part of the MC (closest to the booster) was for the front and the front was for the rear brakes. Since the front brakes are larger and require more force, you need a larger piston in the master cylinder for those. Due to the limitations of machining the master cylinder, you can't make the small diameter first, and then a larger diameter deeper in (unless you have something like an expanding drill bit!).

Keep putting that fluid through. The good thing is, as long as it's coming out clean, re-use it and put it back in. Cycle that whole quarter or liter of fluid through about 3 times, if you need to.

Scott H

Take John up on his offer then buy your own Motive bleeder. You will wonder how you ever got by without one once you see it in action.

Landseer

OK. Understood.

Theory nixed. Back to work. Have PM'd John.

zerMATT951

+1 on Motive. I've used a Mighty Vac on brakes for years and finally bought a Motive just before I sold the 924S. Both methods are 1-man, but Motive cuts your total effort to a fraction of the time needed for 2-man method or the manual vac pump. I just wish that Motive had a better way to adapt to american cars with flat tops than the chain-and-plate method.

mwc951

The Motive is nice...but you can build the same thing with $20 and a trip to Lowes!
http://www.bmw-m.net/TechProc/bleeder.htm

Landseer...good luck solving your problem.

Tom M'Guinn

Check the carpet under the pedals just to make sure it's not leaking inside the cabin. Then get a motive bleeder! You might also take the bleeder screw all the way out just to make sure it's not clogged.

Landseer

(got the message about the Motive; thanks for the Lowe's alternative, too!! John lives a couple hours away, but will collaborate with me at some point / needing a 951 guru like him or one of you to help me priortize other items on this one.)



Update. Works but brakes aren't great yet...
(Bleeders were confirmed working when loading pads. Cleaned up seats with 600 paper and blew through each)
These are ATE rotors w/ PBR ceramic-containing pads

I guess we tried to bleed with vastly too little fluid.
Added a lot this morning.

Manually bled around the car 3 times, got high pedal.
Good fluid flow from rear calipers. Clearly actuating on rear discs.

Wiped down rotors with mineral spirits.
Drove car to bed pads. Made 7 or 8 stops from 55mph.
These are ATE plain smooth rotors w/ PBR ceramic-containing pads.

How should they feel? Braking under moderate pressure is ok. With more pressure, not much different. Can't possibly lock it up (but not an abs car). Should 951 with new brakes and stock distribution valve be able to lock-up wheels?

Probably still some air --- will bleed again after getting more blue fluid.
Don't want to reuse the flushings.