why do you have to bench bleed? isn;t the same if you bolt it up and use the pedal with the lines off. I guess i am missing the point. Is it mess factor? or will the brake rod do damage to the cylinder using the pedal. I think not. any one?
It's just a lot easier, and you are able to readily observe the air being removed. And it's not time consuming. 4TTRUK
If you have somebody to pump the pedal for you or a pressure bleeder, bench bleeding saves nothing. Larry
Bench bleeding a master takes less time that it does to explain it. If you're that poorly versed in brake service, turn it over to a professional. Bob
Bench bleeding offers one large advantage over the install and bleed it on the car technique. You purge the air from the master cylinder before it has a chance to enter the rest of the system. This can save you hours of torture trying to get an air bubble out of a proportioning/metering valve or some other nook/cranny in the system. ... air that you sent there from the master cylinder. Do it once, do it right.
When I was first doing brake jobs, I was teamed up with this guy who knew it all. I asked what happens if you don't bleed the master first, he said, let's try it. A 5-minute bleed took about 20 minutes to accomplish. Bob
Mount the MC, put a container under it to catch the fluid, fill the reservoir then place your fingertips over the ports and have somebody slowly depress the pedal, the air and fluid will push by your finger's and will close off the ports when the pedal is released, do this until there is no air. install the lines and bleed the system. I have been a professional mechanic for 30+ years and this system works. Larry
10 bucks and it fits most applications. Buy one now and keep it in your tool box. You won't find a good brake shop that doesn't have one. There is no real mess or wasted brake fluid with this kit. You can actually see when there is no more air.
"Bench bleed" as the term goes is as literal as it gets. If your master is all by itself on the bench being bled it will be bled and isolated as a single component. You could bleed the thing anywhere including on the car as long as its isolated and a single component. It wouldn't be the first time a master cylinder got changed where there wasnt s bench for miles. Without bleeding it separately, you'll be moving a bunch of air out and back in to the master because the master contains air but no bleeder. Its similar to leaving a bleeder open and pumping the brakes but instead of drawing atmosphere air in, the master draws the air it just pushed into the system.
Also, the lower the car and the kinkier the line routing, the harder it's going to be to push all the bubbles out if not bench bleeding. Easiest way in my opinion...at NAPA, ask the old guy who actually knows stuff to bring out the little kit of MC adapters...get one for each port in the MC. Buy a short piece of pre-terminated brake line, cut it in half, bend the pieces into neat curves from ports up into fluid reservoir. Keep this little kit as a permanent tool. Just much nicer than the nasty push-in or orange plastic kits. Bleed away until bubbles stop...no significant waste of fluid this way.
I know everyone knows Engineers like myself are stupid and complete morons. Although my speciality is Racing Engines, I do talk to many other moronic Engineers, some of them have dedicated there lives to BRAKES. For what will never be the last time, when bleeding a M/C, Open the master cylinder resevoir. Pump SLOWLY and watch the bubbles, not how far or not so far the pedal and the piston that can't be seen without a x-ray vision travels. On a dual chamber M/C, you need to slowly pump the pedal far enough for the piston to go past the port for the chamber farthest from the push rod. As the piston travels past the opening or orifice for the farther most chamber, air will rise in the form of bubbles that you can see rising to the top of the resevoir. This is how far you pump- period. Bubbles stop rising and release. Repeat this until the bubbles no longer form and rise to the top of the chamber. Remember to pump and release the pedal SLOWLY.
Risk contaminants?? I don't believe I've ever bench bled a cylinder that wasn't new or rebuilt and clean inside. The fluid being recirculated into the MC is new and clean, and if something did turn up it would be hard not to notice it peering into a clean reservoir full of clean fluid to watch the bubbles. Once done with that, all fluids pumped downstream from MC would likewise be clean and new. Amount of fluid pumped beyond MC in the system bleed can then be increased beyond that necessary to clear air from tubing and slave cylinders for more cleaning, but generally at this stage of work wheel cylinders would likely also have been at least checked and cleaned. Any sludge out thataway is going to be at the bottom and unlikely to exit via the final bleeding process, so disassembling cylinders to see what's happening is a good idea.
A PS on that: On the MC you are bleeding the thing mostly downhill, as downward as you can and still get air out, so a dirty MC could easily download a lot of its crud collection into the system. The lines really should be downhill mostly and so failrly flushable, but we screw that up through heavy lowering on cars with low MC's, and Detroit sometimes screws it up too, like all the Chevelle type cars with a severe down-loop under the MC. At the wheels, the slave cylinders ans calipers are nearly perfect dirt and water traps providing little motivation for dirt and water to exit uphill through a tiny bleed port, so if they are not disassembled the crud is going to live right there til the cylinder fails from pitting.
On used masters, if you push the plunger past the range of normal travel, the cups get pushed into that ring of crud at the bottom of the bore... and all of a sudden you have a leaky master. It has nothing to do with wheel cylinders or hoses. On any brake system I bleed, new or used master, I never bottom it out to the stop. The first bleeds are a bit slow, but it picks up once the bulk of the air is out of the lines. Bob
There is no reason not to fully stroke the master out of the box or while bleeding. With a custom (non-stock) system, it is critical to make sure the master can be fully stroked before the pedal bottoms out. It will also bleed faster with full pedal strokes.
I always bench bleed 'em. When I bled the brakes on the Diamond T last week, all I had to do was open the bleeders, one at a time, untill fluid ran out with no bubbles. Took no time at all, no pumping, no vacume pump, no muss, no fuss. Easiest time I've ever had bleeding a new brake system. Brian
MA Hey Bob, i have seen your post on other threads about this stroking issue. I do respect your opinion and its with never a waiver, you stick to it. I admire that.. Now down to brass tacks of the matter. I did a master on a 80's chevy truck last week and a 74 VW this week. Two different re manufacturers yet both had relatively the same paperwork. It goes like this.... ATTENTION Slowly press in and release the master cylinder using short strokes 3/4 to 1" NEVER STROKE PISTON MORE THAN 1" And its formatted just like that. Large or bold. Later on there's the standard boiler plate disclaimer that not following the instructions exactly can cause serious injury and death as well as void the warranty. Now if I was buying my master cylinders from you, and you were providing the warranty, I certainly would listen to your instructions that came with the kit. Yours would say " Be sure to stroke the piston fully to the bottom" . You would probably have a fit if someone suggested otherwise about your product. OK now I'm not saying your wrong on your opinion today, but you arent providing the warranty. I can't remember any manufacturer ever stating to be sure to fully stroke the piston. But I remember a whole bunch that explicitly say don't. I was taught " never" Do you have anything to back up your opinion, I'm willing to change mine if your info is better.
I never liked to bottom out a used master, the crap in the bottom of the bore is for real. New ones, I've never seen the warning/disclaimer but I've probably thrown out or never read what comes with a new master. My guess for the note is that the bottom of a deep piston bore may not be as perfect as the rest of it. Totally a manufacturing issue. In all my years of doing brakes, I've probably seen maybe 5 new masters that were bad out of the box, maybe quality control or lack of it has gotten worse in time? My true opinion, anyone can do whatever they want with installing a new master or bleeding brakes. Have fun with it. I did brakes day in & out for 10 years straight, so my numbers are not in the hundreds, they are in the thousands. I still do a few a year for friends that lack the skills. I may have a conservative approach to bleeding, but it has worked for me and you don't see ME posting for brake help here. One thing that I have to do, is stop giving out brake advice on internet forums, it seems to be ill-received by those who are probably not qualified to pick it apart (your posts are not among those). Bob
My "opinion" is based on many years of working with and testing vehicle hydraulic brake systems. Why some master remans state stroke limits is beyond me. Master cylinder piston stops (some early models) or springs limit piston(s) full travel, not some number pulled out of thin air. I would contact the reman company and ask them why they print the warnings.
The entire bore has to be smooth and free of defects. If not, junk/replace the master or wheel cylinder, period.
I didn't see this mentioned, but It has always been my understanding that the reason you bleed a master cylinder on the bench first is that when installed in a car, many sit at an angle. Allowing the possibility of air to be trapped at the highest point. By bleeding on the bench beforehand you can set the master cylinder level and purge all the air out of it.
I recall reading in a Ford shop manual, circa 2000, to place a 2" block under the brake pedal, before manual bleeding the brakes. It didn't say "why",...but we figured it was to prevent the cups from being damaged, if they hit the bottom of the m/cyl bore. 4TTRUK
ok this is the most logical answer so far. i just didn't see why it mattered if it was in a vice on a bench or bolted to the car with no lines attached. i guess there is no reason not to bench bleed it .
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