Flushing Brake Fluid
Senior User
Joined: Nov 2012
Posts: 315
Likes: 0
From: Pacific Northwest
Like I posted earlier...
Sure it would be ideal. But it is not NECESSARY like you posted.
only necessary if you want to completely flush the system.
THAT is the point I have a problem with.
You have to remember, the ABS HCU is a sealed system, unlike the rest of the brake system, which is exposed to atmosphere. So it doesn't absorb water from the air like the rest of the system.
it is only sealed until you have an "abs event" then you introduce old wet fluid into the hcu, where it now stays until the next "abs event". how often does your truck experience an abs event? all the more reason for a complete flush on a regular schedule.
Also, just FYI, it isn't "hydroscopic" its "hygroscopic."
thanks, I always get the 2 confused. the principal is still the same.
I made the same mistake for years until one of the members here let me know.
Stewart
Sure it would be ideal. But it is not NECESSARY like you posted.
only necessary if you want to completely flush the system.
THAT is the point I have a problem with.
You have to remember, the ABS HCU is a sealed system, unlike the rest of the brake system, which is exposed to atmosphere. So it doesn't absorb water from the air like the rest of the system.
it is only sealed until you have an "abs event" then you introduce old wet fluid into the hcu, where it now stays until the next "abs event". how often does your truck experience an abs event? all the more reason for a complete flush on a regular schedule.
Also, just FYI, it isn't "hydroscopic" its "hygroscopic."
thanks, I always get the 2 confused. the principal is still the same.
I made the same mistake for years until one of the members here let me know.
Stewart
DOE
Elder User
Joined: Oct 2001
Posts: 544
Likes: 2
From: Jersey Shore Not Seaside!
Actually the ABS module runs through a self-check at every start-up, cycling the valving, pump and accumulator with some if not all fluid exchange within it's cavities. As the fluid is exchanged the fluid is so hygroscopic that the moisture is disbursed within the fluid lines and M/C reservoir to reach equilibrium rapidly. Which is why the ABS and vehicle manufacturers do not require the manual forced ABS function during normal bleeding. It is required for air entrapment as the valves and other cavities allow air pockets to rise to the tops of these areas.
Super Moderator
Joined: May 2004
Posts: 29,380
Likes: 118
From: Central Coast of CA
Actually the ABS module runs through a self-check at every start-up, cycling the valving, pump and accumulator with some if not all fluid exchange within it's cavities. As the fluid is exchanged the fluid is so hygroscopic that the moisture is disbursed within the fluid lines and M/C reservoir to reach equilibrium rapidly.
Which is why the ABS and vehicle manufacturers do not require the manual forced ABS function during normal bleeding. It is required for air entrapment as the valves and other cavities allow air pockets to rise to the tops of these areas.
Thanks for the corroboration.
Stewart
Fleet Mechanic
Joined: Sep 2002
Posts: 1,388
Likes: 2
From: Greensboro, NC
Good points - I'll add one more: If you bleed your brakes the normal way as described you may still have a softer pedal than you would like. If this is the case, then the ABS bleed may be the last step needed.
Elder User
Joined: Oct 2001
Posts: 544
Likes: 2
From: Jersey Shore Not Seaside!
If you are really hyper about the fluid in the ABS controller, or you have some air, the fluid can be moved around to a higher degree then the initial start-up by taking the vehicle to a slippery surface (dirt, grass, etc) and do a 25-30 stop or stops.
We would get prototype vehicles and not have the equipment to electronically activate the controllers to fully bleed the system. We would take the vehicle out to a dirt road and fire off the ABS, then bring it back in the shop and do a full bleed. Doing it this way often would take 1/2 day and multiple bleeds, but if you can't get the equipment to do the work ... you can backyard it.
We would get prototype vehicles and not have the equipment to electronically activate the controllers to fully bleed the system. We would take the vehicle out to a dirt road and fire off the ABS, then bring it back in the shop and do a full bleed. Doing it this way often would take 1/2 day and multiple bleeds, but if you can't get the equipment to do the work ... you can backyard it.
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