Losing my mind on bleeding the brakes

[jaredmac11]

After removing the rear end and disconnecting the brake lines; Ive been having issues bleeding the rear brakes. My wife isnt much help on the two-man system so I've been using a gravity pull which has been working well. Tons of air came from the rear left but I believe both RR and RL are good now. I have great brake feel until I start the car and roll her around in drive. Brake feel shortly after dissipates and I have to double pump to stop. When I take the wheels off and crack the lines again, there's no air bubbles.

This makes me think I have some air in the master cylinder. I really dont wanna remove the MC to bench bleed, so do I have any other options here?

[320iAman]

I use Motive Power brake bleeder-0100, pump it up to 10 psi and bleed the system,, pump the brakes a few times and it removes air bubbles from the Master Brake Cylinder as well. Its good, set up a receiver to brake bleed valve with hose ,loosen the valve a little, pump the brakes a few times and then watch the air bubbles come out , then tightened it and do the next one.

These cars are not just step on the brake to stop--not power brakes nor air pressure vacuum assist, they are vacuum assist brakes--I have read to stop these cars you pump the brakes is whats recommended.

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https://www.ebay.com/itm/Motive-Prod...frcectupt=true

Randy

[jaredmac11]

I hear you Randy it just doesn't feel like it did before I parked it. I have a Central Pneumatic pump so I'll give that a try.

[Somjuan]

Could always park her on a hill. My friend and I had a similar issue bleeding the clutch master/slave after his metal lines had to be replaced. A few days later and it was back to normal just from good ‘ol gravity. You could bench prime the master and power bleed the rest, but I wouldn’t be surprised if you just had to wait it out.

[dreiermann]

Because your brakes are fine until you start the car, I wonder if you are leaking fluid into the booster.

[pofo]

I broke down and got a power bleeder and it works awesome.

- - - Updated - - -

Because your brakes are fine until you start the car, I wonder if you are leaking fluid into the booster.

I had the same issue and there is air somewhere in the system

[dreiermann]

When I could not get the clutch on my son's E34 to bleed after hours and hours of trying, I connected a reservoir of clean fluid to the slave bleed-screw and then pulled a vacuum on the reservoir. That worked.

[Drew320]

When I could not get the clutch on my son's E34 to bleed after hours and hours of trying, I connected a reservoir of clean fluid to the slave bleed-screw and then pulled a vacuum on the reservoir. That worked.

Honestly, from a non-automotive, scientific point of view, this makes sense to me. Air is less dense than brake fluid so it would make sense to bleed it from the reservoir since it is technically higher (in altitude) than the bleed screws. It interests me how automotive tech has evolved to push the more dense fluid to expel air out from the bottom. Essentially syphoning the fluid in to ensure absolutely no air enters the system- as it could only escape.

[pofo]

Honestly, from a non-automotive, scientific point of view, this makes sense to me. Air is less dense than brake fluid so it would make sense to bleed it from the reservoir since it is technically higher (in altitude) than the bleed screws. It interests me how automotive tech has evolved to push the more dense fluid to expel air out from the bottom. Essentially syphoning the fluid in to ensure absolutely no air enters the system- as it could only escape.

I have used reverse bleeders before and they work awesome, I dont have one myself unfortunately.

[jaredmac11]

I must have had the wrong idea but I get it now, regarding reverse bleeders. Ill look to pick one up but the wife always eyeballs my growing collection of tools...

I really hope its not the MC leaking in to the booster.. Brakes worked 100% OK before so I think its just the air. Possiblythe brake pedal is collecting more air, but it must be closer to the MC than the bleed screws.

[epmedia]

Remember to bleed the one furthest from the master, first.

Also, if the rear shoes are not adjusted properly (too loose), it will feel like air in the lines too. And don't adjust the rear brakes too tight, or they'll skid dangerously when least expected.

[builtinbetter]

Sounds to me like the master cylinder is failing

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[dreiermann]

Relative to reverse-bleeding: I didn't use any special tool to pull a vacuum from the reservoir, I just cut an old bicycle inner-tube with the valve part (with the core pulled out - this is easier with a Presta valve) included, hose-clamped it to the top of the reservoir, and connected the little vacuum hand-pump (with an in-line receiver so I didn't accidentally suck fluid into the pump). Even with the stretchiness of the inner-tube, you can still pull enough vacuum to draw new fluid in from the slave cylinder.

[320iAman]

Its quite normal for the brakes to be really firm when the car is not running and then softer when the car is started-thats what vacuum assist does it uses the vacuum created by the motor to reduce the force on the brake pedal to engage the brakes, fully or less, by creating a vacuum in the brake booster,assisted by the one way check valve to keep the vacuum in tact.

If your leaking fluid into the Brake Booster or Vacuum Assist Brake Booster your level of fluid will go down, if the clutch slave is leaking a little this too will cause the brake fluid to go down with spongy brakes.

As Robert said--The back brakes need to be adjusted right as well.



Randy

[goinstrong]

+2 for adjusting your rear brake shoes to be tighter. A simple way to tell is by how far you have to pull up the parking brake. If it goes more than 3-5 clicks, your shoes could be adjusted tighter. If fine, then it could be the master cylinder...
Tim

[pofo]

Remember to bleed the one furthest from the master, first.

Also, if the rear shoes are not adjusted properly (too loose), it will feel like air in the lines too. And don't adjust the rear brakes too tight, or they'll skid dangerously when least expected.

I also ran into this issue as my self adjusters where not working properly.

[jaredmac11]

I think my rear brake shoes were always sorta out of whack. It took 6 clicks to get it locked but now Im closer to 8+. I had to struggle to get the parking brake line back on so its possible something is off kilter. I'll adjust it and see if it does it!

I got even more air out of the RL brake line so maybe its a series of complications. I really hope its not the master. I had a hard time getting those lines on the first time. Would the booster have to be replaced as well?

[builtinbetter]

No, the master cylinder and booster are separate parts...
I do still have a booster for sale though!


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[jaredmac11]

No I get they are different just asking if they should be replaced together. Is your booster rebuilt or original?

[okieflats]

It's been said that if the M/C is leaking then the booster is probably damaged and is best replaced. That's because a worn M/C would have fluid leaking past the piston, this fluid getting to the booster and damaging seals so the booster may not hold vacuum like it should.

[builtinbetter]

The booster I have is from the red 320i I parted out a few months back. Pretty sure it's original.

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[320iAman]

If you need a Brake master cylinder--rebuild it--- Saves dollars--It does not take very much time to rebuild-remanufacture the BMC.

As to bleeding it--you'll notice the BMC slightly at an up angle attached to the Brake Booster when the car is on all 4 tires. Chock the front tires -front and back and jack up the rear end until a small level placed on the BMC shows its downward slightly-the air bubbles in the BMC will be right under the openings to the Brake Fluid Reservoir, ready to come out by themselves mostly,still need to pump the brake pedal to rid the BMC of small air pockets, your all welcome... Make sure the jacked up car is secured from back and forth and side to side movements. Be safe.

Brake Booster when leaking will hiss as air rushes past the seals, No hiss when applying the brakes most likely its good ! Test your one-way check valve also.

Last picture is seal on BMC that seals BMC to Brake Booster air tight so vacuum assist works properly, all new in kit below, No grommets though.

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https://www.bimmerforums.com/forum/s...aster-Cylinder

Randy

[pofo]

I might have a rebuild kit for cheap I never used

[jaredmac11]

Thanks Randy. Good info. I'll get the ass end higher than I have been (just enough to get tires off). I found a relatively large air bubble in the clear piping to the brake pressure regulator. I'm not sure how it functions but if it's anything like it's namesake I'd gather it's this. I'll try out the vehicle tomorrow.

I don't think it's the MC or booster. No leaking, no hissing, all new rubber hoses. The pedal feels like the opposite from when I had a vacuum leak. Plus the pedal seems to hold a vacuum well.

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[jaredmac11]

No dice. I was able to take the car up and down the driveway and I could better hear a pretty distinct hissing noise when I was pressing the brakes; so I guess all signs indicate a failing brake booster. On to a rebuild...