QuestionQUESTION: Have just rebuilt the braking system and replaced both the m/cyl and servo with units from SNGB`s.Car hsas now done less than 100 miles and is suffering from brakes holding on for a fraction of a second(owners timing) before releasing pressure.I have all ready had to return the 1st m/cyl supplied as the piston was jamming in the bore.The repacement feels okay piston returns as should but??? Have been told that with these m/cyl if they have been left in stock for a period of time a small valve at the front end which contriols the action of the servo can,from lack of lubrication,be slow in action and can cause this problem? Any help on this problem gratefully recieved,as you no doubt know to get at any part of the brake syatem in the V 12 is not that easy!!!!
Paul.
ANSWER: Hi Paul,
What you say is one possibility, however other items need to be cleared as possible also. Any time we had a delay in release (on any car) we needed to isolate the problem by putting the car on stands or on a lift if one is available.
Have someone start the engine and pump up the brakes while you try to rotate a front wheel then have the person slide his/her foot off of the pedal so as to let the pedal hit the back stop. You should be able to hear that while you are trying to rotate the wheel. This gives you an exact moment the pedal is released and thus you can note the time it takes for the wheel to release. Do all four wheels to see if any are different.
This test establishes the difference between remote servo/master and caliper/pad problem. If you find any notable difference between any wheel/s you need to correct that wheel first before condemning the remote servo/master/reaction valve. If it were a slow release of either of the three units (master/reaction valve/remote servo all four wheels should be effected at the same time. Don't rule out something as simple as a reservoir cap so loosen the cap as one of the delay tests.
When you say "fraction of a second" you may find it difficult to locate the problem if there is one. In any of the dealerships I worked in if we had a delay release it was easy to locate because we could brake a line loose anywhere in the system to isolate a delay release. But when it is just a fraction of a second there is no time to do any action to isolate the problem into a smaller area.
There is also the possibility of just tight seals so you may need to note any change in the delay over a time period. If it improves with time it may just be tight seals.
As I remember the front calipers had retraction devices on each caliper piston to retract the pads. I don't remember if the Ser. III still had that but that too can add to the time. Plus you said you rebuilt the brake system. Caliper seals too? Remember that the caliper seals by design retract the pistons. If any one of the seals is deflected in the wrong direction this can cause a drag on that wheel.
I hope this helps, let me know,
Howard
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QUESTION: Many thanks for an excellant reply-I do not think that the problem is down to either one caliper or one end as the car pulls up straight with no pulling to either side and sits down evenly.On this system there is no retraction pin in the calipers(luckly) and relys on the deformation of the seal to pull the piston back.I always make sure that I have at least the given play in the wheel bearings to help push the pads out of contact.I must check on the reservoir caps when the car comes back in,I ahve asked the owner that if possible,bearing in mind safety factors etc,if he could complete the first 500 kms,because to retorque the heads the inlet manifolds and their carbs have to come off-so access to the brakes will be easier.Would you know if the servo reaction valve can be dismantled in situ without losing the fluid-as all the chassis tubes have been repainted and the bulkhead!!!!!! I just have a gut feeling that the answer lies there but always ready to learn.
Once again many thanks-Paul.
AnswerI think it would be less messy taking the cylinder and valve off as a unit as I remember the reaction valve had to be disassembled to get it off of the cylinder. It has been many years since I worked on an E-12 and the only book I have is not much help in that area. I use to keep many of the rubber dust covers for brake bleeder valves used a lot on German cars as they made good brake line stoppers so fluid didn't drip on paint. Sorry I am not much help in this area but I think there is a good chance the problem will just about be gone by the 500 Klm return for retorque. That type of problem tends to free up with use and it sounds like you have covered everything well.
let me know,
Howard
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