Hi all,
I drive a 1981 BMW 635csi which has an unusual brake circuit. As the car is RHD there was no room for BMW to fit a servo behind the brake pedal on this model so BMW opted to use twin remote servos instead. Each servo has a "line in" from the master cylinder and then one servo powers one (of two) pair of pistons on the front callipers and the other servo powers the other pair of pistons on the front callipers and the single pair of pistons on the rear callipers. So the brakes overall are four pot at the front and two pot at the rear.
the issue I have is that in slow moving traffic the rear brakes bind and the pedal gets very firm indeed, I have not noticed this problem happening at cruising speeds. Another potentially related problem is that the first press of the pedal after start-up seems to take a while to take up the slack, i.e. the pedal seems to move a fair way before the brakes start to operate.
So far I have rebuilt all 4 callipers (the rear ones twice in one year) and checked the adjustment of the handbrake (this operates like a brake drum inside the hub of the brake disc with cables operating a lever which pushes apart the shoes) and have replaced all the flexible hoses except the rear ones which I could not get undone without damaging the brake line.
I now suspect a faulty seal on the brake servo powering the rear brakes, i.e. under high manifold vacuum (engine idling) the remote servo is leaking slightly and is therefore the servo is operating slightly. Is there any way I can test this?
Does anyone have an exploded piccie of how these things are put together?
Any help or advice gratefully received
|
I would think you could test this with a vacuum guage first by testing the manifold vacuum on it's own then using a T connector with the suspect servo in the line. Gunsons used to sell a cheap one.
Steve.
---
Xantia HDi.
Buy a Citroen and get to know the local GSF staff better...
|
Reading over that again, it couldn't be a leak to the manifold vacuum could it or the tick over would suffer?
Steve.
---
Xantia HDi.
Buy a Citroen and get to know the local GSF staff better...
|
|
|
|
I would be much more inclined to look at the master cylinder first.
I would check for a sticking piston, and I would also check that there was enough clearance between the pedal / rod and the fully retracted master cylinder piston. (If the piston doesn't go back far enough, you trap a volume of fluid in the circuit, and as everything heats up, pressure builds - just as if you had a foot on the brake!)
Sometimes, where there is an easily accessible pedal / pushrod adjustor, this fault is noticed just after someone has "helpfully" adjusted the free play out of the pedal assembly! Land Rover and Morris Minor owners DIYers were among the more common culprits.
One way to diagnose this fault is while the brake are binding, loosen the fluid inlet pipe to the servo that feeds the rear brakes. If this releases the brakes, the fault is upstream of that servo. If however, you have to slacken the pipe downstream of the servo to release the brakes, then the operation of the servo should be investigated.
Obviously, after slackening these pipes, the brakes will need to be bled before going onto the road again.
Number_Cruncher
|
Some info about how servos work can be found here;
www.honestjohn.co.uk/forum/post/index.htm?v=e&t=35...4
Although the post describes direct acting suspended vacuum servos, remote servos are fundamentally the same, except that the function of the brake pushrod is performed by the inlet fluid from the master cylinder, acting on a piston.
As the servos are of the suspended vacuum type, (i.e. they work by allowing atmospheric air into the pressure side of the diaphragm), measuring the vacuum levels at the servo actually tells you very little about what the servo is doing at any point in time.
Number_Cruncher
|
It's a servo fault BMW & early Alfa's do this & it's damned expensive to fix as the servos will need replacing, 99% sure that these can't be rebuilt. Talk to experts on the Marque like Munich Legends & not the guessers on here.
I've had this on an Alfa Guilia which uses the same system & we disconnected the servos both vac & brake line & ran without, but this wasn't for road use.
|
>>not the guessers on here.
How can providing the original poster with an essentially free and remarkably simple test method which will help clear or condemn an expensive component be called guessing?
Or would you prefer the OP to just fit the new servo because you say so?
Number_Cruncher
|
"and have replaced all the flexible hoses except the rear ones which I could not get undone without damaging the brake line".
I may be classed as a guesser but it is a documented fact that binding as described can be caused by elderly rubber hoses partially collapsing inside and forming a one-way valve, allowing pressure to remain in the system beyond the hose. Before spending £100's on servo(s) I would change the rear hoses as even if you have to change the attendant pipework you will still have ruled out a potential cause.
|
cheers guys, that's a whole loada info for me to digest. I will be changing the rear brake hoses now as they need to be eliminated from the investigation; this is tonights job. Say goodbye to my knuckles!!
for the servo/ master-cylinder debate, if I disconnected the vac to the suspect servo and left the other connected would that be a useful diagnostic, ie if the problem goes when the servo is disconnected and comes back when I reconnect the vac could I say for sure that it is the servo at fault?? or could it still be the master-cylinder?? I don't really understand how the remote servo does its stuff (but will read the thread linked above next).
rebuilding the servo is not an option I gather (well BMW doesn't sell a rebuild kit) but I can get hold of 2nd hand servos. Master cylinders can be rebuilt, not something I have done before though. How hard can it be!!!?
|
Can you swap the two servos over and see if the fault goes to the other axle?
659.
|
>>I would change the rear hoses
>>Can you swap the two servos over and see if the fault goes to the other axle?
Are also, IMO, excellent suggestions because they are cheap, quick, and simple.
Disconnecting the vacuum pipe to the servo could help find the fault, but it is also possible that removing the pressure amplification provided by the servo could just be hiding a fault elsewhere - so, I would say that the test isn't 100% certain.
If you have a locking wheel situation, and can jack the car up a bit so the wheel would turn if the brake were free, with an assistant trying to rotate the wheel, you can quickly begin to loosen connections, starting from the master cylinder, working towards the wheel, undo connections. You should find at some point, some fluid spurts out, and the wheel becomes free.
This tells you where the fault is. All the connections you loosen upstream of the fault will make no difference - the wheel stays locked. The first connection you loosen downstream of the fault will release the wheel. Try the procedure a time or two - to make sure it isn't a fluke, and you then have a diagnosis.
Number_Cruncher
|
|
|
swapping the servos around is possible but would be a large faff with great potential to do expensive damage. The real reason I have not taken this route (I have considered it) is that both servos power the front calipers and one servo has an additional line out which heads to a pressure bias valve, then a splitter and then the rear servos and I haven't eliminated the front calipers from being part of the issue.
ie if the fault is in the servo supplying the rear brakes I'd also expect to have pressure at both front calipers (on one of the two pairs of pistons). I have not noticed this as I have not managed to reproduce the fault with wheels off the ground and the only symptoms I have are brake drag (somewhere on the car) that is very noticeable in traffic coupled with a very firm brake pedal and squeal from the rear brakes. All of these faults are intermitent but simultaneous. So I only know for sure that there is an issue with the rear, there is also likely to be a problem with the front (if its the servo or master cylinder at fault)
could also be the hoses (to be done)
the rear brake bias valve??
plus I found last night that the offside handbrake cable was almost resting on the exhaust, could this cause the symptoms?? maybe
I have the engine in bits at the mo too (major service time) so I can do all this stuff but I can't do any daignostic stuff at the mo.
|
|
|
|
|
Agree with you 100% Number cruncher.
|
I had a thought last night, when the brakes drag the brake pedal goes hard at the same time. Would that not indicate that their is pressure in the primary brake line (mastercylinder to servo)? ie per the diagnosis technique given above, releasing the union at the mastercylinder would give a spurt of brake fluid and release the brakes???
this would seem to indicate that the problem lies with the master-cylinder then.
or is there a way that a faulty remote servo could pressurise the primary brake line (and therefore the mastercylinder) as well as the output lines. Again I have a problem as I don't understand how the servo operates (ie how does it provide additional pressure to the output lines without also applying the same pressure to the line in?)
I have an offer of a pair of remote servos (2nd hand) for £60, so I'm inclined to go for them
thanks for help so far
|
Yes, the brake pedal going hard is what first made me think of a problem at the master cylinder side of the servo - but, once your car is running again, you can check it out.
The upstream / downstream diagnosis logic I quoted above is how I would track down the fault if the car were in front of me - a properly working master cylinder should vent the brake lines to the reservoir when the pedal is released.
But, your pedal going hard could also simply be that the servo isn't working properly, and isn't assisting your foot, thus, the pedal feels harder than normal.
I suspect £60 is a drop in the ocean compared with the costs for new or reconditioned units - but it may be worth making sure why these second hand units were removed from the seller's car before buying them though!
Number_Cruncher
|
|
|
|
