Brake bleeding - do it backwards?

[AlanBrown]

I have been reading multiple threads on this subject (mainly as an incentive to get up and do it myself), and it occurred to me that there are two problems common to virtually all of the various procedures on offer, which enthusiastic amateurs like myself usually use as an excuse to not even attempt the job: air in the system, and spills.

 

All the systems reviewed, from the commercially available to the hand-built, rely on a gravity feed to try to ensure that the fluid level is kept as high as possible. First, this method is not infallible: over-use of the brake lever in moving the new fluid into the system may cause the level to drop uncontrollably fast. Second, maintaining a "safe" high level involves constant checking (necessitating being in two places at the same time), and increases the likelihood of spillage.

 

Now, far be it for me to try and overrule what is probably the result of many long hours deliberating over the ins and outs of the subject, and far be it for me to claim to be a professional mechanical engineer (oilfield engineer, yes, mechanical engineer no), but it seemed to me that if the problem were to be attacked BACKWARDS, both of those problems could be averted.

 

To put it simply, by pumping the fresh fluid INTO the caliper bleed point, air can be automatically prevented from entering the system. The fresh fluid then displaces the old fluid back into the main reservoir. So what about the rising level in the main reservoir? Well, a system such as a simple double acting pump that links the pumping (into the system) of one volume of fresh fluid to the drawing of one volume of old fluid out of the reservoir and diverts it into a waste container might work.

 

I have attached a (simple) diagram that illustrates the system. On one stroke of the actuator, a set of suitably arranged check valves would allow one volume of new fluid to be pumped from the left chamber into the system at the same time as one volume of old fluid is drawn into the right chamber of the pump. The second stroke of the actuator fills the left chanmber with fresh fluid on the left, and dumps the contents of the right chamber to the waste container.

 

End result - a static level in the main reservoir. No fluctuation, no spill, no air.

 

Or am I being too simplistic?

[Stan Walker]

I for one would be reluctant to push all the crud from the calipers back up into my ABS and master cylinder.

 

Stan

[Art..]

1) Most of the contamination is at the caliper end anyway.

2) Spilling brake fluid on paint destroys paint.

3) The current method is easy.

 

 

No thanks.

[Ken H.]

Interesting thought, but I can't imagine the ramifications of trying to push brake fluid backward through an ABS system. I think I'll stick with the tried and true method.

[Pilgrim]

I did it that way for years, until I discovered speed bleeders.

 

It sure does keep the air out of the system better than the suck method.

 

I used a new pump-style oil can with a piece of clear hose slipped over the nozzle, then to the nipple.

 

You just gotta remember to get the excess fluid out of the reservoir or it will overflow. That's less nuisance (to me) than trying to get air out of the system.

 

The point about crud in the calipers might be a good one, but I never encountered a problem with it.

 

Pilgrim

[oreana123]

This is the correct technique for many aircraft when changing components like the brake assembly. It really lessens the need for bleeding brakes and the system is designed to allow the free flow of fluid towards the master cylinder. I saw a turkey baster at the dollar store the other day that would suck out the rising level in the master cylinder. However, for flushing the system, one with an ABS control module like our bikes have, I don't know about the possibility of contamination from the brake assy's. The actual movement of fluid in a closed system like this during normal operation is quite small. How much mixing of the fluid that takes place, from the master cylinder to the brake assy is anybody's guess. If you kept up on the system fluid change from the time it was new, maybe it'd be OK. Disassembling the ABS master control to clean crap off of the valve seats may be more of a job than you want if it goes that way. Speedbleeders sound like a good investment to me. John S Boise

[Jeff from Massachusetts]

I think speed bleeders are the way to go.

I'm suprised nobody noticed that your check valve system will not work as shown. You need to change the "fresh fluid" out and "old fluid" return checks to flow out. But we know what you mean.

[BMT]

Or am I being too simplistic?

Hmm, quite the reverse, I fear!

 

The brake bleed is really very straightforward. None of the parts specified in the various documented procedures are absolutely critical, with the exception of the reservoir caps -- just a necessary expense, but less than getting the dealer to do the job.

 

I understand your approach, but there comes a time when you just have to stop reading up on it and get on with doing it!

[jellero]

you can just do it in the time it takes to read this thread. wrap a towel around your master cylinder to protect the paint.

i've bled the brakes on my harley and sidecar backwards for years because both the master and slaves were so old and worn. i took an old style pump oil can and put the tube between oiler and nipple using brake fluid of course.

i recently bought a tube with a check valve that fits over the bleeder nipple for under $10 at napa and use it on all vehicles after finding out first hand what can happen with old fluid. i lost the brakes in high heat in my car driving in the city. j

[Wooster]

I have attached a (simple) diagram ...

 

Or am I being too simplistic?

 

Alan,

 

Not for 1985 RT; it's service manual (two microfiche) depicts brake bleed system similar to yours; mechanism attaches to reservoir w/bottle of fresh fluid by caliper.

 

ABS & servos in modern beemers, well, not so simple.

 

Wooster w/gravity supported bleeding preference