Experience Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.
Organizations Associate member SAE
EAA member
Publications Import Car magazine
Education/Credentials ASE Master Auto with L-1 certification up to 2000
Question 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.
