Master cylinder "bench bleed"?

...some claim it is...but I have never had a problem getting the systems beled without it...I leave the lines a little loose and tighten them once I have fluid...in essence bench bleeding in place... of course YMMV

...Gareth

That's my system too! (Gareth prolly stole it...) You can buy a pressure bleeder, but I never found it necessary.

Speedy Jim

Hmmm...that reminds me. I built a pressure bleeder out of one of those 12 volt tire inflators once. I wonder where it went?

You can install the new master cylinder in place on the car and just leave the hard-line fittings connected but loose, fill the reservoir up with brake fluid, then get a coke cap and drill a hole in it and hot-glue a tube to the cap... Then screw the coke cap on top of the fluid reservoir (it fits perfectly...thank Jan for that bit of trivia) and blow in the other end of the tube to force pressure into the master cylinder. You'll see the fluid leak out of the fittings where the hard lines are loosely attached. Immediately tighten them down. You will have ROCK SOLID brake feel after that. Or at least I did. I'd point you to my website where it details the procedure but this stupid firewall thingie I just installed on that computer is blocking access to the page and I'm too stupid/lazy to look at it long enough to figure out how to let the web traffic back through just yet.

Yeah, that is sort of like what I did except I used the inflator to provide air pressure on demand. Made it easy to do the wheel cylinders without a second person in the car too.

Too much information dude!

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Later model master cylinders use what is in effect, a double-chambered piston. (See a drawing of the MC to see what I'm talking about). The purpose of 'bench bleeding' is to ensure the rear chamber -- the one not directly connected to the outlet pipe -- does not contain any air prior to installation. Otherwise, after doing the install you may get two distinct sets of bubbles when you bleed the system, a large stream of bubbles reflecting any air in the system, followed a small set of bubbles from air at the aft end of the piston.

This generally isn't a problem if you simply run the bleed long enough but if the system is fitted with residual-pressure check-valves there is a significant gap between the two streams of bubbles, causing some mechanics to believe the system is bled when there's still some air in the line.

Just charge the master cylinder with fluid and pump it up until you have a good flow at the outlet. So long as the inlet is maintained upwards, the oil in the rear chamber will not escape. Takes mebbe a minute, if that.

Another of those 'unimportant details' :-)

-Bob Hoover

PS -- the tricky bit here is that the trapped air (if any) ends up in the first line (attached to that section of the MC) to be bled. The result is one or two wheels that don't brake as well as their opposite number, leading to all sorts of crazy diagnosies. The fix is to simply bleed them again and take it for a test drive.

Totally unnecessary. It's a good way to turn a straightforward job into a miserable one. In 35 years I've never done it, nor felt the need to.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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