When bleeding brakes, why do we have to start from the farthest to the nearest ? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-28-2009, 08:57 AM
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When bleeding brakes, why do we have to start from the farthest to the nearest ?

Hi, I know that this rule of doing thing is common knowledge. I just wonder why. Also, If I replace my front calipers, do I have to bleed all four brakes too to remove the air?
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Old 11-28-2009, 09:40 AM
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Hi klingfilm— so you won't have to do it all over again after you do just the fronts!

The air is trapped in the system and we have to get it ALL out— so do the furthest from the MC first or some of the air will stay in the lines.

Think of it as an opportunity to bleed your brakes anyway. Our brake systems are really open systems— they're constantly, more or less, accepting air and moisture. That's why it's a good idea to flush the brake fluid regularly (who does that), because DOT 3 literally sucks water out of the atmosphere!

I just finished installing a hydroboost brake system on an old truck and had to bleed the system a dozen times just to get a firm pedal. Then I waited a week and did it all over again! I used DOT 5 silicone fluid because it's hydro-phobic, but I'm only mentioning it because silicone brake fluid (this one boils at 500 degrees), retains bubbles until they're percolated out by heat. You probably won't have to bleed your brakes more than once or twice— that's a good thing!

Last edited by marzini; 11-28-2009 at 09:46 AM. Reason: add'l info
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Old 11-28-2009, 10:12 AM
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Ok, I'll the the four brakes anyway. I am asking because I changed my front brakes yesterday and I bled them (only the front). But today, I feel like the pedal is a bit too soft... I'll bleed everything.
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Old 11-28-2009, 10:24 AM
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You don't really have to, but going to the furthest gets the most air out right away.......I usually go over the whole bleeding thing at least 2x just to make sure its all out.
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Old 11-28-2009, 11:12 AM
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X2 with what poncho said.

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Old 11-28-2009, 12:05 PM
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Also---it may be of interest to you that the right rear is NOT ALWAYS the farthest wheel.

I know that on X-frame Chevies----the farthest wheel is actually the left rear. This may apply to other GM cars of the 60s as well.

What makes the left wheel that farthest is the routing of the lines. On the X-frame the line goes from the master to the left front, to the right front, down the passenger side of the car to the rear end and a short line to the right and a long line to the left.
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Old 11-28-2009, 01:01 PM
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I have a large number of GM factory service manuals from the 1950s through the 1990s. Some of these manuals call out starting from the furthest, some from the closest. Frankly, with a modern dual circuit brake system, it shouldn't matter, since air in the front is unrelated to air in the back. I suspect this is an urban myth that may have been true when brakes were only single circuit. It's sort of like the "don't put a battery on a concrete floor" myth.
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Old 11-28-2009, 03:07 PM
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Quote:
Originally Posted by joe_padavano
I suspect this is an urban myth that may have been true when brakes were only single circuit.
Said that right IMHO. But there are a LOT of single pot master cylinders still out there- like older P-U's, mainly. Don't know if the prop valve isolates the fronts and rears or not.
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Old 11-29-2009, 10:02 AM
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Quote:
Originally Posted by cobalt327
Don't know if the prop valve isolates the fronts and rears or not.
All cars sold in the US since the 1967 model year have been required to have dual circuit brake systems that are completely isolated so that a leak in one half doesn't affect the other half, so yes, the factory prop valve isolates the front from the rear.

Of course, with ABS, all bets are off. Some ABS systems require the pump to be actuated using an external test tool while bleeding the brakes. Early Cadillac Allantes, for example, have a failure mode in the ABS where you can loose the entire braking system if the pump goes bad. Not a great design...
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Old 11-29-2009, 08:52 PM
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Quote:
Originally Posted by joe_padavano
All cars sold in the US since the 1967 model year have been required to have dual circuit brake systems that are completely isolated so that a leak in one half doesn't affect the other half, so yes, the factory prop valve isolates the front from the rear.
Was referring to a prop valve used on a single reservoir master cylinder brake system- does it prevent shared fluid front to rear.
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Old 11-29-2009, 11:38 PM
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Gravity bleeding will be a better option if the master cylinder will not be rebuilt or replaced, & has been installed for a while.
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Old 12-14-2009, 07:33 PM
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From what I've read in this thread, it sounds to me like the air in a new system installation will take a while to get the air out. How do the auto manufacturers blead brakes on cars coming off the assembly line? They don't have time to bleed 6 times to get them right! I suspect they power bleed. A power bleeder theoretically will not allow air into the fluid when it is injected into the lines and master. My question is which way should the fluid be pushed or pulled? Does the master valving prevent fluid flow in one direction? I have residual valves on front and rear lines. Seems to me those valves will prevent fluid from flowing back into the master AND the associated air bubbles would likely not pass thru the valves anyway. Am I making any sense?
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Old 12-14-2009, 09:58 PM
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I don't know how the manf does it, but at the dealship I worked at had a pressure bleeder that attached to the top of the reservoir. You would fill up the pressure bleeder with new fluid, then pressurize it with shop air.

Also, there's no check valves (one-way valves) in the brake system..Flows both ways.

The pressure bleed setup is just like the gravity bleed, but a lot faster. The reason I suggested the gravity bleed (over the pump up method) is because it prevents damage to the master cylinder, & most people don't have a pressure bleeder at home.

Last edited by Zimmo; 12-14-2009 at 10:10 PM.
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Old 12-15-2009, 06:14 PM
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When I first saw the title of this thread, I thought ho-hum. Then I wondered why there were so many responses.

Now that I have read them, I must say I learned something. Thanks guys. The last time I did brake bleeding was about three years ago and that was on my bike with separate master cylinders,

The other thing that impressed me was the author's title which did an excellent job of properly describing the subject as is I believe protocol and is unlike so many other threads.
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