Brake bleeding rules

[Don Coatney]

I read a few threads on brake bleeding and it got me to thinking.

This has bothered me for a number of years and I have not been able to wrap my brain around it. Rules for bleeding brake systems suggest that you start at the point in the system most distant from the source.

However. In any closed loop system moving liquid or gas, the medium (liquid or gas) tends to take the path of least resistance. So when this medium comes to a "T" in the line it will tend to go straight through the T (path of least resistance) as opposed to going down the third leg of the T. When this happens in a closed loop system there is a pressure reduction in the third leg (venturi effect) thus allowing anything sitting stagnent in the third leg to be sucked towards the path of least resistance or into the portion of the straight line.

So when bleeding a closed loop brake system if you start at the (right rear) most distant point from the source (master cylinder) at the first T in the line any available air will be sucked into the fluid taking the path of least resistance and thus will take longer to bleed all air from the system. Seems to me that starting at the point most close to the source (usually left front) would make for a more effecient way to bleed brakes.

Any and all comments and criticism are welcome. Please share your thoughts and correct me if I am wrong.

[Merle Coggins]

I'm with you Don. I'd start closest to the master cylinder and work my way out. I don't know how much venturi effect there really would be, but my thinking is to start with the lines closest and keep purging the air away from the start point.

Merle

[Don Coatney]

I'm with you Don. I'd start closest to the master cylinder and work my way out. I don't know how much venturi effect there really would be, but my thinking is to start with the lines closest and keep purging the air away from the start point.

Merle

Thanks Merle;

Glad I am not the only one who understands.

[55 Fargo]

I'm with you Don. I'd start closest to the master cylinder and work my way out. I don't know how much venturi effect there really would be, but my thinking is to start with the lines closest and keep purging the air away from the start point.

Merle

I have thought of this too, what your saying makes sense, but is it actually the case, when air and liquid shares the same space.

When bleeding brakes, we pump the pedal up, then bleeder screws are opened to allow fluid and air to purge through, does not this action of pumping the pedal cause vacuum, and allow the air bubbles to move around in the lines or in the cyls.

So really when bleeding brakes from the farthest line from the master cylinder, would that not allow a high percentage of air to be expelled 1st throughout the system,then by working backwards toward the neareast wheel cyl, would that not purge the system more efficiently and effectively.

Woul the use of a pressure bleeder eliminate this and work more efectively in bleeding the closest wheel cyls or calipres first.

Why have automotive engineers state to bleed the farthest bleeders first, are they wrong. I am just saying, but really what is the correct way to bleed a system, or does it really matter, as long as all the air is purged out.

[Rusty O'Toole]

I always start with the closest wheel. It is very hard to get the fluid moving when there is no pressure and you can't build up any pressure when the system is full of air. I find it takes forever to get the first wheel bled when you start at the farthest one.

If you start with the nearest one it goes a lot quicker and once you get the 2 front wheels bled, you start to get a little pressure when you pump the pedal and the back brakes don't take so long.

This was before I built my electric brake bleeder.

Basically, I took the pump out of a small junk refrigerator. Wired it up to act as a suction pump. Took a pickle bottle and soldered a couple of tubes in the top. Got 10 feet of clear plastic tubing and hooked it up. O ya put a switch on the cord so I could control it standing by the car.

Loosen the bleeder, hook up the hose, fill up the master and flip the switch. Keep one eye on the master and the other on the hose, when brake fluid comes through shut it off.

I did a complete brake job on a 66 Coronet, new brake lines, all wheel cylinders, master cylinder the works. The whole system was bone dry. With this device I bled it out, solo, in half an hour without getting a drop of fluid on the floor.

[RobertKB]

What Don says seems to make sense. However, I can't help thinking that all those engineers at all those car manufacturers, and all the mechanics who worked on those cars must have a reason for doing it the way it is suggested in the shop manuals.............furthest one first.

[Tom Skinner]

I think Don has a good point, because I have always had to "Go Around"

several times (2-3) Bleeding the Brakes to get a firm/high Pedal.

I think I will try it backwards - nearest to furthest next time I Bleed the Brakes.

Tom

[Jim Shepard]

I've got an old friend who is a mechanic and was trained by his Dad back in the 40's (who was also a mechanic). He says his Dad trained him to start with the closest wheel and work toward the farthest. That's the way I did it the last time and, frankly, I don't know if I can tell the difference...

[wallytoo]

i don't know what the rationale is for the "correct" method. perhaps by having a set method, there are fewer opportunities for errors in performing the flush/purge (such as forgetting to bleed the right front, or the left rear, etc.).

fwiw, i've always performed the bleed by starting with the cylinder farthest from the master and worked my way closer. i've never had to "go around twice" to get it done properly.

Edited May 11, 2012 by wallytoo

[flatheadtim]

I gravity bleed the system cracking bleeders on all cylinders that have been worked on then I bleed singular from the farthest. The reason to bleed from the farthest is to minimize the amount of cross-contamination between the new and old fluid.

[Don Coatney]

I gravity bleed the system cracking bleeders on all cylinders that have been worked on then I bleed singular from the farthest. The reason to bleed from the farthest is to minimize the amount of cross-contamination between the new and old fluid.

How can you gravity bleed a system when the master cylinder is lower than the wheel cylinders?

[Merle Coggins]

Would the use of a pressure bleeder eliminate this and work more efectively in bleeding the closest wheel cyls or calipres first.

That's what I do with a new system with lots of air. But I still work from closest to farthest.

Merle

[55 Fargo]

On my truck brakes, which have fairly worn rear shoes, front brakes stock, but with lots of lining, and some scoring, the back drum brakes modern, but worn thin.

I have bled these brakes, pedal is high, and hard, but when you hit the brakes, the hard pedal has to be pushed hard, and brake response is poor.

I have adjusted both fron and rears,, any ideas on this one, I do paln to replace lining, and drums to be machined or replaced.

The brakes work, but are very poor, no brake fluid leaks or loss. My 47 Chrysler stock brakes are 100 times better, adn stop you real quick.

[JBNeal]

I reckon the lowest point on these brake systems is the hydraulic line at the rear axle, so starting from the most distant point from the source is done to help flush contaminants in the hydraulic system, be it air or contaminated fluid, as from this hydraulic line the bleeding can be done by gravity then by pressurized fluid. With the brake pedal partially engaged to allow fluid to travel from the master cylinder to the circuit, the rear axle cylinders can be purged by gravity, especially if the vehicle is angled so that the master cylinder is slightly higher than the branch to the rear axle. Once air is purged from the rear axle cylinder circuit, the fluid acts as a barrier to prevent air from each front branch from travelling towards the reservoir. The front wheel cylinders have to be bled by pressure since they are higher than the reservoir.

If bleeding is done by pressure alone, then you would want to start at the closest branch and work away from the reservoir. Of course, the reservoir cannot be allowed to go empty with either method.

[40P10touring sedan]

I think Don has a good point, because I have always had to "Go Around"

several times (2-3) Bleeding the Brakes to get a firm/high Pedal.

I think I will try it backwards - nearest to furthest next time I Bleed the Brakes.

Tom

That's the way I've always done it, 2-3 passes till nothing but fluid comes out...maybe it's me but on the 2nd pass I always seemed to get one lil air bubble somehwere and that makes me go around again.

[Rusty O'Toole]

If it is a complete brake job and the system was dry I like to let the car sit overnight and bleed it again. Air can get trapped or dissolved in the fluid and takes time to percolate out. You usually get a few more bubbles of air that you could never get out if the car did not set.

[TodFitch]

If it is a complete brake job and the system was dry I like to let the car sit overnight and bleed it again. Air can get trapped or dissolved in the fluid and takes time to percolate out. You usually get a few more bubbles of air that you could never get out if the car did not set.

Double down on that if you've used DOT-5, it entrains bubbles very easily.

[55 Fargo]

It kinda sounds like those front shoes might be contaminated or glazed over. My dad had a '96 Ram with glazed over rear shoes and that truck took planning to whoa-down, even with new front pads...replacing those thick shoes with new ones was night & day difference. Also, my neighbor's hay trailer had a bearing shell out that he replaced but kept the brake shoes after "cleaning" them with brake cleaner...he couldn't even lock the brakes up on an empty load on that side, so the shoes were replaced and problem solved.

Pulled front drums left side scored, shoe lining scored, but lots left, right side, drum not bad, shoe lining almost nothing and that fell off shoe, nice huh, and me driving at 60 mph sometimes.

Had drums machined, they measured 10.040, within spec for here.

Getting shoes relined next week, place does bonded lining $15.00 each.

Back shoes 80s Mopar 10 inch bendix type, shoes need replacing, drums need a clean-up,maybe replacing.

MC, wheel cyls, hoses and lines all okay no leaking.

Back brakes would lock up if pushed hard, but not fronts

ThAnx for your reply

[Alshere59]

Interesting thread. FWIW I would think that starting with the farthest wheel would have the brake return springs forcing the old fluid (and any air) midpoint into the line as soon as the bleeder was opened.

Like Rusty and others a pressure bleeder is the way to go if possible. It is not only faster but I think it cuts down on agitating (for lack of a better word) the fluid and any air.

Thinking back, I bled my fronts first as my lines are connected at the master cylinder with that line at the top of the plug to get rid of air in my master cylinder.

[flatheadtim]

How can you gravity bleed a system when the master cylinder is lower than the wheel cylinders?

You either invert a large can of brake fluid in the master opening (like a chicken watering station) this also insures you wont run out of fluid during gravity bleed. Or make a master cyl cap adapter with remote res.

[meadowbrook]

Are the brakes on these old mopars (1950 Dodge Meadowbrook) supposed to be able to lock up the rear wheels? Mine frankly will not lock them up on dry pavement even in a panic stop and though adequate are not all that strong (my 1965 VW, also with drums all around is MUCH better, but again, it is an 1800 lb car.

I assumed this as normal, since it is not power assisted, and old technology, but never thought to ask the group here.

Everything but the drums is new in my car, shoes, wheel cylinders, metal and rubber lines and master cylinder. No leaks and the pedal is firm.

One wrinkle is that I did replace the original master cylinder with a kit that replaces it special brackets that allow the fitment of a 1967 Mustang (dual circuit) master cylinder, still under the car, but further back.

[Dodgeb4ya]

Stock brakes with linings worn into 100% contact on all four drums should lock up no problem.

Modifications to the original equipment- all bets are off unless you know what you are doing to make the brake system work properly.

Bob

[TodFitch]

One wrinkle is that I did replace the original master cylinder with a kit that replaces it special brackets that allow the fitment of a 1967 Mustang (dual circuit) master cylinder, still under the car, but further back.

Does that dual master provide equal pressure to both front and rear? If not then you've explained your symptom...

[flatheadtim]

Does that dual master provide equal pressure to both front and rear? If not then you've explained your symptom...

If you understand braking--doppler effect, gravity and centrifugal forces then you know that 80% of braking force is applied to the front brakes and 20% to the rear brakes. When you apply brakes the car puts the bulk of the weight on the front wheels and da da da da OK.

[TodFitch]

If you understand braking--doppler effect, gravity and centrifugal forces then you know that 80% of braking force is applied to the front brakes and 20% to the rear brakes. When you apply brakes the car puts the bulk of the weight on the front wheels and da da da da OK.

True that most braking is in front. But with the stock MC the pressure in the lines to the rear is equal to the pressure in the front and the wheel cylinder diameters and brake lining area, front and rear, were designed with that in mind.