Re: Another reason to avoid DOT5 brake fluid?
Posted: Mon Nov 25, 2013 9:22 am
Recently, I've been involved in reproducing a rubber brake seal part that is now discontinued by Honda (GL1000 rear caliper halves seal - Honda part #43217-371-006). I've re-confirmed some information I've long-long known.
The rubber engineer I work with had a long career in Detroit as a brake component specialist and shared the following with me as he was helping me engineer this part.
Specifically -
Glycol vs. silicone (mineral) brake fluid seals
Glycol-type brake fluid variants (DOT 3/4/5.1) must use EPDM rubber (ethylene propylene diene monomer (M-class) rubber) for seal construction. Mineral-based brake fluid (i.e. DOT 5.0 silicone brake fluid) must use an altogether different synthetic rubber material for seal construction. The main limiter is that the rubber types cannot be interchanged as the incompatible fluid will cause the seals to swell ...initially causing sticking, and as the rubber weakens, then they fail altogether.
Unfortunately, there is no synthetic rubber material to make seal components from that is 100% compatible with both of the 2 main classes of brake fluids.
There are 2 failure modes:
1. Introduction and “mixing” of brake fluid type into a system designed for the other category of brake fluid. This results in relatively quick and obvious seal failure.
2. Introduction of the “wrong” brake fluid type into a 100% clean hydraulic system with virgin, new seals. In this case, the failure is more insidious and slower, but the failure will reduce the effective life of the seals until they inevitably and eventually swell and fail. Example: introducing Silicon DOT 5.0 brake fluid into a vehicle with seals designed for DOT 3/4 brake fluid. Even with a pristine clean system and with brand new seals, the seals will eventually fail as the EPDM rubber is actually attacked by mineral oils See: http://www.jjshort.com/Rubber-Properties.php
The rubber engineer I work with had a long career in Detroit as a brake component specialist and shared the following with me as he was helping me engineer this part.
Specifically -
Glycol vs. silicone (mineral) brake fluid seals
Glycol-type brake fluid variants (DOT 3/4/5.1) must use EPDM rubber (ethylene propylene diene monomer (M-class) rubber) for seal construction. Mineral-based brake fluid (i.e. DOT 5.0 silicone brake fluid) must use an altogether different synthetic rubber material for seal construction. The main limiter is that the rubber types cannot be interchanged as the incompatible fluid will cause the seals to swell ...initially causing sticking, and as the rubber weakens, then they fail altogether.
Unfortunately, there is no synthetic rubber material to make seal components from that is 100% compatible with both of the 2 main classes of brake fluids.
There are 2 failure modes:
1. Introduction and “mixing” of brake fluid type into a system designed for the other category of brake fluid. This results in relatively quick and obvious seal failure.
2. Introduction of the “wrong” brake fluid type into a 100% clean hydraulic system with virgin, new seals. In this case, the failure is more insidious and slower, but the failure will reduce the effective life of the seals until they inevitably and eventually swell and fail. Example: introducing Silicon DOT 5.0 brake fluid into a vehicle with seals designed for DOT 3/4 brake fluid. Even with a pristine clean system and with brand new seals, the seals will eventually fail as the EPDM rubber is actually attacked by mineral oils See: http://www.jjshort.com/Rubber-Properties.php