Page 18 - WHITE BOOK

fluid. TheclosertolerancesofthevalvesusedinABSsystems,alongwiththeexposureofthe fluid to increased system surface area via the return lines and potential increased flow rates presents the possibility for a higher rate of component failure.
Sampling the fluid in an automotive brake system presents a special set of difficulties. Brake systems – even ABS-type systems – are usually static as apposed to flowing and constantly mixingsystemslikelubricatingsystemsorhydrauliccontrolsystems. Thedataobtainedfromthe sample is highly dependent on the location from which the fluid is removed. It is also highly dependent on how the vehicle has been operated. Sampling the master cylinder doesn’t necessarily inform us of the condition of a brake cylinder and vice-versa.
NotallABSsystemsarealike. Somevehicleshaverearwheelanti-lockbrakesandsomehave fourwheelsystems. Mostvehiclestodayhaveseparatebrakehydraulicsforthefrontandrear brakesforsafetyreasons. Somecompletelyisolatethefluidreservoirintotwosectionsandsome mix the two supplies. To further complicate matters, it is recommended by the manufacturer, SAE and DOT that flushing of the system through the wheel cylinders be avoided, to prevent flushinganddepositingabrasivematerialsintotheclosetolerancecomponents. (Thelinesshould be disconnected from the cylinders prior to any flushing other than simply venting air out of the system.) ThiswouldruleoutamethodinvolvingrecirculatingthefluidthroughtheABSreturn lines in order to evenly mix it. This leaves us with developing the best method around the specific, most likely causes of brake fluid problems.
The most frequent causes of brake hydraulic system component failure are abrasion and/or decomposition of the seals and close tolerance valves. Reference A divides the factors related to brake fluid chemistry that influence this abrasion and decomposition into three areas: intrinsic factors (chemical and thermal stability of the fluid, system design and materials, absorption and permeation rates of seals), extrinsic factors (dependent on the vehicle operating conditions, maintenance conditions, environmental conditions) and abnormal events (accidents involving loss of system integrity and subsequent introduction of contaminants).
The intrinsic factors can all be covered by simply establishing an appropriately conservative frequency of overhaul of the brake system. The extrinsic and abnormal factors are considerably more challenging. These comprise the vast majority of reasons for variability in system performancefromonevehicletothenextofthesamedesign. Eachdifferentdrivingstylecan shift the most likely location for first failure, and thus shift the most appropriate place to sample andexaminethesystem. Developingageneraltestforallsystemsmeanstestingtheonelocation that is most likely to be the source of the problem (the master cylinder), or testing all the likely sources(themastercylinderplusallfourwheels)individually. Thesecondalternativewouldbe considerably more costly, but also more reliable than the first. A cost/risk-benefit analysis is in order.
Of all the extrinsic factors, the human error-related factors are likely the greatest. These error factors are more prevalent at the master cylinder. The frequency of opportunity for error at the master cylinder is far more than at the wheels. The possibility of introducing dirt, water or petroleum based products into the system at the master cylinder is very apparent, and these cause abrasion,corrosionanddeteriorationofpolymersandrubbermorethananyotherfactors. Itisfar more likely to find the first indication of such an occurrence at the master cylinder, where it is likelytostayforalongerperiodoftime,giventhestaticnatureofthesystem. Testingthemaster cylinder fluid doesn’ t necessarily tell us anything about the condition of the fluid properties such as anti-oxidant or anti-corrosive levels, water, viscosity or amount or wear metal at the location wherethesepropertiesconcernusthemost. Furthermore,testingforcertainofthesepropertiesin
101 Corporate Place - 2 - Vallejo, CA 94590
800-645-5227 www.Herguth.com