1991 E34 Brake Nightmare (2 weeks and counting)

[dennoz]

Hi,

Before someone says I haven't researched enough, I've literally combed through Google, www.bimmeforums.com and www.bimmerfest.com for solutions and I can't find any pertaining my issue.

Well the story is this.


Its a 1991 E34 520i (BC05400) belongs to a pal. After having a brake pipe (metal) one at the rear right wheel break and replaced, the brake booster and master cylinder were replaced and "supposedly" bled.


Ever since, all calipers would lock and not retract thus locking all wheels. To unlock them, we had to rebleed every moment and not go above gear 3 or above 2500rpm.


Now we rebuilt the calipers and the pistons with new seals, another brake booster (2 others) and another master cylinder (4 others) but now the brakes don't work at all.


Replaced the ABS pump as well but now the abs pump, only sends fluid to the front left and rear right tires and at less than 10kph, the brakes work but the pedal travels all the way down to the plate.


When driven above 10 kph, only God can help you brake, nothing works at all, even the two tires that are receiving brake fluid don't do shit. It's been 2 weeks now and there's no solution at hand apart from fully bypassing the ABS system and having it non-ABS by changing the brake booster and master cylinder plus all the brake pipes as well.


So I'm curious as to how I should confirm/check if the ABS control unit is functional or fried. No abs error has popped up in the cluster and when scanning the vehicle, no error code appears. Self test the car and switching it on and off, the ABS light comes on and off as expected meaning all's fine.


Any help will be highly appreciated.

[Avec]

I had a metal line burst on my GMC Denali, and it caused a number of problems, some of which you describe. The issue came down to the ABS block needed to be properly bled. I am not sure that is your problem, but a lot of what you describe would tie back to that.

[The Chauffeur]

It still sounds like there might be some trapped air... same thing happened to me when I replaced the reservoir and master's grommets and seal. How was the system bled, vacuum or push? I had to bleed and re-bleed it a couple of times to get them functioning properly and, oddly enough, the rear ones were easier than the front ones.

[dennoz]

It still sounds like there might be some trapped air... same thing happened to me when I replaced the reservoir and master's grommets and seal. How was the system bled, vacuum or push? I had to bleed and re-bleed it a couple of times to get them functioning properly and, oddly enough, the rear ones were easier than the front ones.

By push you mean by use of the pedal or what exactly?

I've re-bled the system with over 5l of brake fluid, how much more can I use?

I suspect air as well

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[dennoz]

I had a metal line burst on my GMC Denali, and it caused a number of problems, some of which you describe. The issue came down to the ABS block needed to be properly bled. I am not sure that is your problem, but a lot of what you describe would tie back to that.

I'm thinking the ABS pump, the DSC unit as I have a DSC unit

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[The Chauffeur]

By push you mean by use of the pedal or what exactly?

I've re-bled the system with over 5l of brake fluid, how much more can I use?

I suspect air as well

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You're right, I didn't describe it that well... By "push" I meant reverse bleeding; it works ok on some systems, but not on this one, according to the Bentley... What worked pretty darn well for me was a "one man bleeder", the type that's basically just a tube with a check valve.

[dennoz]

You're right, I didn't describe it that well... By "push" I meant reverse bleeding; it works ok on some systems, but not on this one, according to the Bentley... What worked pretty darn well for me was a "one man bleeder", the type that's basically just a tube with a check valve.

Re' the Bentley manual, I also read that if an E34 was equipped with the DSC option, bleeding such a system is not pretty much the same with a system without.

This E34 has the DSC option.

Now with the car being grounded for a long time, the worst possible problem you can ever encounter is what we call "a DSC hydraulic block".

Now this is the mother of all troubles in the E34s that came with this option.

Now a fluid change is just straight forward, same as most cars but if anything from the master cylinder down to the and including the ABS unit is disconnected, you need to activate the valves and pump in the ABS unit during the bleeding process to fully get the air out, unclamp the abs pump valves and with the clean brake fluid pushing the valves, they actually in turn force the DSC hydraulic block to open up and push all the fluid and air in the pipes out of the system.

Bloody Germans

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[ross1]

"all calipers would lock and not retract thus locking all wheels"
I suspect the pushrod into the booster or master is in too far not allowing fluid to return to the reservoir. When the now dragging brake warms up the fluid expands and with nowhere to go presses the piston out of the caliper further dragging the brakes.
Are your brakes getting hot?
NONE of this scenario concurs with the low pedal you further describe unless you have moisture in the system which is boiling, unlikely after 5 lt of fluid.

[dennoz]

"all calipers would lock and not retract thus locking all wheels"
I suspect the pushrod into the booster or master is in too far not allowing fluid to return to the reservoir. When the now dragging brake warms up the fluid expands and with nowhere to go presses the piston out of the caliper further dragging the brakes.
Are your brakes getting hot?
NONE of this scenario concurs with the low pedal you further describe unless you have moisture in the system which is boiling, unlikely after 5 lt of fluid.

Yes they would get seriously hot. As for the moisture, I'm not sure about that.

But the pushrod has been fixed and is working accordingly just that the car isn't stopping at all.

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[ross1]

Yes they would get seriously hot. As for the moisture, I'm not sure about that.

But the pushrod has been fixed and is working accordingly just that the car isn't stopping at all.

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Sounds like air trapped in the ABS

[moroza]

It would have ASC, not DSC. On a 1991 car there were (in the US, anyway) two separate hydraulic manifolds - one for the ABS, and another for the ASC with a pressurized gas cylinder bolted to it. It would also have a larger steering fluid tank and a dual-stage steering pump. Later cars (from 9/93) integrated these functions into one manifold, and I think they used a normal pump and tank.

ABS releases brake pressure as needed to prevent lock. ASC does the opposite: it provides pressure (via a combination of the gas cylinder, and hydraulic pressure from the steering pump) to the rear brakes as needed. It also closes a second throttle plate (do you have this second plate?) to cut engine power. Not sure, but I think DSC does the same to all wheels, not just rears.

My experience has been that an ABS-only E34 can be bled from scratch (not just a flush but after a lot of stuff's been disconnected) like a normal car, without needing to cycle the valves. I've never attempted to bleed an ASC car from scratch; Bentley manual says that to do so, the ASC valves need to be cycled by some BMW special computer tool.

Brake masters usually fail by leaking externally or internally (failing to build up full pressure, or releasing it on their own). I once experienced a master failing to release; after a few applications of the brakes, I'd have to crack one of the lines loose to release the pressure. That got old quick! It was an old Hilux and a new master cured it. But if you've been through several masters and boosters, that is not likely your problem. I would still check the pedal assembly for binding.

Another thing: in ~8 years of professional wrenching I have never seen a machine or technique that can bleed as well as one person on the pedal and another at the bleeder screws. A car-clueless person working the pedal is better than a $$$ power bleeder or a vacuum pump. First cycle the system (brakes or clutch) with the screws open, then with the screw closed, pump hard and fast 5 times and hold it at the fifth, then release the screw. If you do the latter three times with no air, it's good.

[ross1]

IF you are able to brake hard enough to cycle the ABS PERHAPS you could purge some of the air that way. ABS stop, bleed, ABS stop, bleed, etc.

[dennoz]

It would have ASC, not DSC. On a 1991 car there were (in the US, anyway) two separate hydraulic manifolds - one for the ABS, and another for the ASC with a pressurized gas cylinder bolted to it. It would also have a larger steering fluid tank and a dual-stage steering pump. Later cars (from 9/93) integrated these functions into one manifold, and I think they used a normal pump and tank.

ABS releases brake pressure as needed to prevent lock. ASC does the opposite: it provides pressure (via a combination of the gas cylinder, and hydraulic pressure from the steering pump) to the rear brakes as needed. It also closes a second throttle plate (do you have this second plate?) to cut engine power. Not sure, but I think DSC does the same to all wheels, not just rears.

My experience has been that an ABS-only E34 can be bled from scratch (not just a flush but after a lot of stuff's been disconnected) like a normal car, without needing to cycle the valves. I've never attempted to bleed an ASC car from scratch; Bentley manual says that to do so, the ASC valves need to be cycled by some BMW special computer tool.

Brake masters usually fail by leaking externally or internally (failing to build up full pressure, or releasing it on their own). I once experienced a master failing to release; after a few applications of the brakes, I'd have to crack one of the lines loose to release the pressure. That got old quick! It was an old Hilux and a new master cured it. But if you've been through several masters and boosters, that is not likely your problem. I would still check the pedal assembly for binding.

Another thing: in ~8 years of professional wrenching I have never seen a machine or technique that can bleed as well as one person on the pedal and another at the bleeder screws. A car-clueless person working the pedal is better than a $$$ power bleeder or a vacuum pump. First cycle the system (brakes or clutch) with the screws open, then with the screw closed, pump hard and fast 5 times and hold it at the fifth, then release the screw. If you do the latter three times with no air, it's good.

This is the option list for the E34 and as you can see that it only came with the DSC and not ASC so I'm not sure how both are related.

As for the bleeding, I'm also thinking of doing the hard braking a few times to see if I can activate the ABS into cycling the valves on its own and get to see if they'll work.

The power steering reservoir is the same size as the one in a non abs car.

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[dennoz]

Sounds like air trapped in the ABS

My thinking as well

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[dennoz]

I would still check the pedal assembly for binding. Regarding this, I will have to check this.

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[moroza]

Huh, you're right. It's a different option code than ASC (S214A, at least for my 1/94 525iA). I've never heard of an E34 with DSC before. Has anyone else?

[dennoz]

Well, this is tiring. My front left caliper is locking up, AGAIN. I'm gonna go nuts. I can't go beyond 2nd gear and the car is smoking too much at that said caliper.

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[moroza]

There are reasonable arguments for fighting to keep the system intact, but if I were in your situation, I would throw that ABS-DSC-WTF alphabet soup as far into a swamp as I can*, replace with a manual proportioning valve, and call it a day.

*Just kidding. I don't condone waste or pollution. Sell it instead.

[jehu]

When I first got my car I was told due to the ASC+T I required the dealership to replace brake fluid because they had the machine to cycle the ABS block and purge that properly otherwise just doing each caliper by hand you still have old fluid... can you bring it to a BMW dealership and have them do it right?

[dennoz]

When I first got my car I was told due to the ASC+T I required the dealership to replace brake fluid because they had the machine to cycle the ABS block and purge that properly otherwise just doing each caliper by hand you still have old fluid... can you bring it to a BMW dealership and have them do it right?

Well, our local dealership at the moment doesn't support any bmw Pre 1996, so that was a downside for me.....I'd love to know which system that is and how easily it is to acquire one.

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[zubbie]

There are reasonable arguments for fighting to keep the system intact, but if I were in your situation, I would throw that ABS-DSC-WTF alphabet soup as far into a swamp as I can*, replace with a manual proportioning valve, and call it a day.

This is my exact thoughts also. Simplify your problem. ASC, DSC offers little advantage over plain ABS 95% of the time. Just another example of BMW engineering run amuck.