Braking my spirit

[twoqu]

Not sure if this is of any use.
I have a BMW which also uses a similiar hydraulic brake servo system. It failed an MOT due to the brake pedal sinking. Consensus of opinion was that it was M/C. Changed that, the problem was still there. In the end I made up a brake pressure gauge. By plumbing that in at various points in the braking system I discovered that the ABS unit was at fault. Its unusual but the internal valvework can leak. In my case it was delivering only half of the M/C pressure to one side of the vehicle.
This car also has a bomb and it was found that if you had the brake pedal depressed when the hydraulic brake system was recharging up to its highest level, the brake pedal would sink.
So one possibility is that there is an ABS block internal leak or that repeated hard application of the brake pedal when the vehicle is stationary, causes a situation where the recharging of the servo system effectively adds more pressure on the master cylinder than you can apply manually. Hence the pedal sinks.
You really need to use some gauges to find out what is going on where rather than trying to guess. By the way, do you have an 3B or ABY?

[bertman]

Not sure if this is of any use.
I have a BMW which also uses a similiar hydraulic brake servo system. It failed an MOT due to the brake pedal sinking. Consensus of opinion was that it was M/C. Changed that, the problem was still there. In the end I made up a brake pressure gauge. By plumbing that in at various points in the braking system I discovered that the ABS unit was at fault. Its unusual but the internal valvework can leak. In my case it was delivering only half of the M/C pressure to one side of the vehicle.
This car also has a bomb and it was found that if you had the brake pedal depressed when the hydraulic brake system was recharging up to its highest level, the brake pedal would sink.
So one possibility is that there is an ABS block internal leak or that repeated hard application of the brake pedal when the vehicle is stationary, causes a situation where the recharging of the servo system effectively adds more pressure on the master cylinder than you can apply manually. Hence the pedal sinks.
You really need to use some gauges to find out what is going on where rather than trying to guess. By the way, do you have an 3B or ABY?

sounds plausible,and quite possible.The car is an 94 ABY avant

[twoqu]

One way of eliminating potential sources of the problem is to get some brake pipe joiners and some bleed nipples. Lets say you want to eliminate the calipers. Rather than remove everthing, just screw a joiner on an available male end (existing male union) and the bleed nipple in the other end. Then bleed. I have found that unless the nipples are new they are often quite loose. If you get brake fluid escaping up the threads it might be policy to put a small amount of thread sealent on them.
My adapting this principle with perhaps a length of brake pipe with the appropriate fittings, you can eliminate most of the hydraulic components. For example you can use this on one port or both of the M/C to eliminate those components on those circuits. Should be able to get kitted out from local accessory shop relatively cheaply. Get some clean containers to recycle the brake fluid, assuming its fresh. Whilst normally I wouldn't re-use fluid, its another expense!
I understand its a bit demoralising, but you need to test not guess!

[twoqu]

What brake hoses have you got? If they are originals then its possible they are balloning under pressure. That means instead of the all the pressure operating the hydraulic components, some of it is being used to expand the flexible hoses.
Might be policy to treat the car to some braided hoses!

[s2driveruk]

Rob did you sort this out in the end?

[bertman]

Jeez,this is an old thread.Yes I did in the end David,I will try and explain later what I did to cure it.But the anchors are working well and consistently

[s2driveruk]

lol, was searching for something and it came back and i remembered you struggling with it but as in lots of threads, there wasn't the happy ending post

Glad they are working as Mr brembo intended anyway...

Cheers
David

[TDi90]

Guess whos attempting this soon! And seeing as you are my local buddies, and of course Al, i will be in touch!

[bertman]

Guess whos attempting this soon! And seeing as you are my local buddies, and of course Al, i will be in touch!

Jeez,I wish David hadn't ressurected this threadGive me a headache just thinking what I tried to cure the problem.
servo change
MC change
Bomb change
Brake lines
caliper swap(cheers mark)
Bled out the system more than your local slaughter house
Etc
Etc

So Rob I am well experienced in this department,so as I said my help is there if you need it mate.

[s2driveruk]

sorry Rob...

However...what actually cured it. was reading it thinking had you tried everything and somehow bled and rebled etc with the handbrake on or something.

Spill the beans

[bertman]

sorry Rob...

However...what actually cured it. was reading it thinking had you tried everything and somehow bled and rebled etc with the handbrake on or something.

Spill the beans

Tbh David,it was a whole load of things.Firstly I think the brakes took alot longer to bed in in than expected,despite doing the bedding in procedure.Then I bled the system(again)using a sealey vacuum bleeder rather than a easybleed.The sealey bleeder uses air from a compresser to create a vacumm inside the bleeder.(rather than pushing the brake fluid out using spare tyre pressure as used in the easybleed),it actually sucks the fluid out of the caliper.The thing I do not like about the easybleed is that when you pressurize the system it does create a load of micro bubbles in the brake fluid resevoir,which the get drawn in the lines when bleeding.The sealey set up doesn't.
Most importantantly the carriers were ever so slightly off centre,only by half a mm,but in a fixed caliper set up the caliper has to be precisely centred over the disc.I had this measurement machined off to align the caliper to the disc.
Lastly after speaking to a shed load of people,a mate of mine said that using something like the DS2500 pad which is a semi racing pad(I think)you will always have a slightly different pedal feel dependant on what temperature the brakes are operating at,which adds to the confusion.He knows his stuff,he runs his own Bmw race preparation company.(had a ride in his 3.0csl batmobile,truly awesome)
That's it really,probably missing something,bust just glad they are ok.

[TDi90]

Yup - that's v kind of you Rob I will be in touch , I need to pick up the callipers and spend some £££