I have seen quite a few posts about this particular symptom set, but none indicated how they were resolved. The car is an 88' 535is with hydraulic booster.
With the engine off, if I pump the brakes a couple of times, the brakes build pressure and the pedal feels as it should, but the moment I start the engine, the pedal goes soft and to the floor when I apply my foot. Driving at slow speeds, there is some minimal braking power, but not much. If I stab the brakes hard, it will skid. A friend thinks it is an issue with the ABS. Bentley indicates it is probably the MC, but since the brakes will get "hard" (with the engine off), it seems to me the master cylinder must be ok and the issue is elsewhere.
I have been baselining the car and have just replaced and rebuilt all the power steering hoses, rebuilt the PS pump, and replaced the "bomb". I also replaced all of the brake lines and flushed/bled the brake fluid. The PS fluid is topped up and the steering feels good. I see no leaks of any sort and it does not feel like air in the brake lines - which brings me back to the master cylinder possibly being shot? I have another master cylinder I could swap in, but the one in the car was also working before the work I did, and a few of the posts I came across indicated that an MC swap did not resolve the issue.
The car is running great, and I am setting up to use it on vacation starting this weekend. Would love to resolve this. I promise to post how (and if) I resolve it.
Bad Master Cylinder?
Re: Bad Master Cylinder?
How did you bleed the brakes? Using a pressure/vacuum bleeder or manual with someone in the car pumping the pedal as you open/closed the bleeder? I've heard of damage happening to an old master cylinder with the manual method as it goes past it's "normal" stroke and seals can be damaged but honestly I doubt that has happened.
I'm also not convinced it is the ABS, as far as I know none of the solenoids etc cycle when the car is started and power is applied, if that were the issue I think it would show itself when the car is off as well.
I'm leaning to there is air in the system somewhere, with the power booster working this is evident, with the pressure in the accumulator relieved you are able to pump the brakes up. With all new rubber lines being replaced a lot of air would've gotten into the system and fluid drained from the system. I've never had to bleed the master on these cars even when I've replaced them but I do wonder about air in the ABS pump. In my E90 I struggled with getting a good pedal back after replacing the rubber lines and had to do a number of ABS/DSC pump cycles through ITSA to get all of the air (and old dirty fluid) out of it. Obviously we can't just plug in and do that on these old cars, do you have a gravel road/parking lot near by that you could stab the brakes and get the ABS to cycle followed by another bleed?
If you don't have a pressure bleeder they are cheap on amazon these days and make bleeding a very easy one person job. I am inclined to get a vacuum setup too as sucking the fluid will make any air coming out that much more apparent (bubbles get bigger with vacuum, smaller with pressure).
I'm also not convinced it is the ABS, as far as I know none of the solenoids etc cycle when the car is started and power is applied, if that were the issue I think it would show itself when the car is off as well.
I'm leaning to there is air in the system somewhere, with the power booster working this is evident, with the pressure in the accumulator relieved you are able to pump the brakes up. With all new rubber lines being replaced a lot of air would've gotten into the system and fluid drained from the system. I've never had to bleed the master on these cars even when I've replaced them but I do wonder about air in the ABS pump. In my E90 I struggled with getting a good pedal back after replacing the rubber lines and had to do a number of ABS/DSC pump cycles through ITSA to get all of the air (and old dirty fluid) out of it. Obviously we can't just plug in and do that on these old cars, do you have a gravel road/parking lot near by that you could stab the brakes and get the ABS to cycle followed by another bleed?
If you don't have a pressure bleeder they are cheap on amazon these days and make bleeding a very easy one person job. I am inclined to get a vacuum setup too as sucking the fluid will make any air coming out that much more apparent (bubbles get bigger with vacuum, smaller with pressure).
Re: Bad Master Cylinder?
Thanks for the thoughts. I started bleeding them using a vacuum at the right rear wheel, but could not get that too work (perhaps because there was no fluid to pull?) and so installed the Russel speed bleeders that I had purchased and did it using the pedal. I did it on my own the first time, and with a friend the second. There was initially a lot of air, but it eventually went clear as the fresh fluid showed up. Between the 4 calipers, I bled atleast a quart.
I do have a gravel lot nearby and will go try to cycle the ABS and bleed again. I'd like to try the vacuum again, but not sure it will work with the speed bleeders. I do get a bit of fluid weeping around the threads of the bleeders while bleeding in spite of applying some grease to the threads - as I was not seeing air in the expelled fluid, I am assuming the weeping fluid was not causing an issue, but perhaps that is wrong. Anyone try plumber's gas line pipe dope to seel those threads?
It seems you agree that the master cylinder is likely ok (since it does build up that pressure with engine off)?
I do have a gravel lot nearby and will go try to cycle the ABS and bleed again. I'd like to try the vacuum again, but not sure it will work with the speed bleeders. I do get a bit of fluid weeping around the threads of the bleeders while bleeding in spite of applying some grease to the threads - as I was not seeing air in the expelled fluid, I am assuming the weeping fluid was not causing an issue, but perhaps that is wrong. Anyone try plumber's gas line pipe dope to seel those threads?
It seems you agree that the master cylinder is likely ok (since it does build up that pressure with engine off)?
Re: Bad Master Cylinder?
Yeah, I'm not convinced yet that it is the master cylinder.
From memory with the hydraulic booster setup with the engine running and my foot pressing hard on the pedal it would very slowly sink, that booster setup can deliver a lot of force to the system. For my car I actually went to a vacuum booster from a 528e as I like the firmer, less boosted pedal for on track, even now I think about going to a manual, dual master setup for what I use the car for.
As a note, I usually run about a quart (litre up here) through the system when I do a brake fluid change, I do not have ABS in my car so not quite as much line to fill.
Based on what I'm reading, get that ABS to cycle a few times, then head home and bleed them again right away. If you can get a friend to help I'd just do the old school manual bleed/speed bleeders and be at the caliper so you can see what is coming out.
From memory with the hydraulic booster setup with the engine running and my foot pressing hard on the pedal it would very slowly sink, that booster setup can deliver a lot of force to the system. For my car I actually went to a vacuum booster from a 528e as I like the firmer, less boosted pedal for on track, even now I think about going to a manual, dual master setup for what I use the car for.
As a note, I usually run about a quart (litre up here) through the system when I do a brake fluid change, I do not have ABS in my car so not quite as much line to fill.
Based on what I'm reading, get that ABS to cycle a few times, then head home and bleed them again right away. If you can get a friend to help I'd just do the old school manual bleed/speed bleeders and be at the caliper so you can see what is coming out.
Re: Bad Master Cylinder?
My 535 always had something of a soft pedal until the accumulator bled off, after which it was rock hard. I had an E23 with similar hydroboost that was pretty much exactly like you're describing. I replaced the M/C and it was exactly the same. Never did figure it out.
New hoses, especially SS braided hoses might make a difference, I don't know. But they're cheaper than a M/C so might be worth a try. https://www.fcpeuro.com/BMW-parts/535i/ ... t=&b=Sedan
New hoses, especially SS braided hoses might make a difference, I don't know. But they're cheaper than a M/C so might be worth a try. https://www.fcpeuro.com/BMW-parts/535i/ ... t=&b=Sedan
Re: Bad Master Cylinder?
Thanks Mike. I did just intstall new stainless lines, front and back, as part of sorting the car. Do you agree since I can pump the pedal (with engine off) and get the brakes hard, that it is probably not a failed M/C?
What do you mean by "till the accumulator bled off"?
I continue to explore forum posts and see other references to the ABS causing similar problems. It seems there are two camps - one that have never had a problem with the ABS and bleeding and a smaller group that have. I see some posts that suggest one can stimulate/cycle the ABS with a jumper at the panel, but I am terrible at reading wiring diagrams. I'll try some skids on gravel and further bleeding.
What do you mean by "till the accumulator bled off"?
I continue to explore forum posts and see other references to the ABS causing similar problems. It seems there are two camps - one that have never had a problem with the ABS and bleeding and a smaller group that have. I see some posts that suggest one can stimulate/cycle the ABS with a jumper at the panel, but I am terrible at reading wiring diagrams. I'll try some skids on gravel and further bleeding.
Re: Bad Master Cylinder?
Not this system, but similar symptoms. Over the weekend i was helping a friend with his car, and it had the same sort of mushy pedal with poor brakes. Pedal felt fine when the engine wasn't running. His ABS didn't work because of a problem with the controller. To make a long story short, we fixed the problem with the ABS that was keeping the pump from running with the scan tool, then it bled out and worked normally. There was a very tiny amount of air in the system that only became obvious with the extra assist, and it wouldn't get out of there until we got the pump to operate.
Different system, but a bunch of years back I was involved in an ABS install on a car that never had ABS. It had the same thing, bled it like crazy and it had a lousy pedal. We didn't have the tools to force the ABS to engage, but we managed to lock the tires up on gravel and get the pump to kick in. Bled it after the pump ran, and the pedal was perfect.
so if your ABS works, see if you can get it to kick in and then try bleeding it again, just in case there is a bubble stuck somewhere in that ABS unit.
Whenever I've had master cylinder issues, either the pedal would slowly sink if foot pressure was held on it, or an initial application had nothing but it would pump up and stop decently.
The accumulator will give you several pedal pumps with assist pressure. Pump the pedal a few times and the pressure bleeds off, or just let it sit long enough. Its the difference in pedal feel from boosted brakes to non-boosted brakes.
Different system, but a bunch of years back I was involved in an ABS install on a car that never had ABS. It had the same thing, bled it like crazy and it had a lousy pedal. We didn't have the tools to force the ABS to engage, but we managed to lock the tires up on gravel and get the pump to kick in. Bled it after the pump ran, and the pedal was perfect.
so if your ABS works, see if you can get it to kick in and then try bleeding it again, just in case there is a bubble stuck somewhere in that ABS unit.
Whenever I've had master cylinder issues, either the pedal would slowly sink if foot pressure was held on it, or an initial application had nothing but it would pump up and stop decently.
The accumulator will give you several pedal pumps with assist pressure. Pump the pedal a few times and the pressure bleeds off, or just let it sit long enough. Its the difference in pedal feel from boosted brakes to non-boosted brakes.
Re: Bad Master Cylinder?
Happy update. Thankfully I had some nearby gravel to drive erratically on. I drove over and stabbed the breaks so that they would lock up for a moment before going soft again. I did this a couple of dozen times and then back to the garage to have a friend operate the pedal while I bled the lines again. I did it in pairs this time, right rear to front left and then left rear to front right. On the previous bleeds, I had been getting clear, bubble free fluid and it was the same for a few pumps this time too - until a big fart of air and dirty fluid popped out!
Anyhow, I cannot say for certain if it was a burp from the ABS, but after cycling the ABS and bleeding a 3rd time, I have great braking. Thanks for the support! I was losing it after working for a month to properly sort through this car and to have this little braking hiccup was pushing me over the edge.
Anyhow, I cannot say for certain if it was a burp from the ABS, but after cycling the ABS and bleeding a 3rd time, I have great braking. Thanks for the support! I was losing it after working for a month to properly sort through this car and to have this little braking hiccup was pushing me over the edge.
Re: Bad Master Cylinder?
I know you said you were averse to reading wire diagrams but it is definitely possible to bleed the ABS pump in the garage. I did it when I rebuilt the ABS pump and I don't recall it being that much of a struggle. A wire with a couple spade connectors plus a pressure bleeder should do it. I recall activating each of the 4 circuits manually for a little while. With pedal going to the ground I think it's always either air somewhere or seals on the MC. Boost of any kind just builds the pressure more but if the fluid is air free at a certain point it should just compress very slightly I think, not enough to sink the pedal.
Re: Bad Master Cylinder?
The old style ABS factory tools I know of are essentially just what you did but plugged into the harness connector for the controller. Its switches in a box rather than jumper wires but it does the same job.