I've read lots of posts/web articles/service manuals about how you should bench bleed your MC if you rebuild it or buy a new one, but they don't mention that special case where you happen to be as stupid as me and let the fluid completely drain out of the rear resevoir while bleeding the rear calipers.
I have a feeling that I should pull the MC out and do the bench bleed anyway, because I want to flush the entire system.
I'm just curious now, because the only reason given is usually just "It will speed up the rest of the bleeding process". but I haven't found an explanation why, and what exactly happens if I don't? Why wouldn't the air just come out during bleeding at the calipers?

Also, we don't have to bleed our proportioning valve, right?
Thanks.

 

You can bleed the MC on the car. If you have bleeder screws on it, just bleed as if it were a caliper using the 2 person method. If it does not have bleeders, remove the lines and run lines up over the top back into the fluid and pump until you have no more bubbles.Some auto parts stores sell lines made for this purpose or you can make your own. BE CAREFUL as brake fluid will eat the paint on the car or in the engine compartment. :cheers:

 

Thanks... didn't know I could do it on the car.
Anyone have any comment about the proportioning valve or why it takes longer if I don't bench bleed?

 

Proportioning valve does not have to be bled. If you don't bleed the MC when it runs dry as in the case you described, I don't think the system will ever bleed completely. I am not sure about the exact reason why but I assume air is trapped inside the cylinder and with that, there is insufficient pressure to force it through the lines. (I am sure one of the engineers will give you a better (correct) answer. Bottom line you have to bleed the MC. Its really no big deal. I suggest sucking the old, murky fluid out with a turkey baster rather than forcing it through the lines. :cheers:

 

Sounds good. I'm glad you said that bit about the turkey baster, since I was planning on pushing it all out. I can see how that would be safer.

Thanks.

 

Yes- I had the same problem with old fluid in the system. I sort of scrapped the gunk up from the bottom of the MC and sucked it out, added fresh fluid, scrapped out some more etc. I got it pretty clean this way without having toremove the MC. Did that over a year ago when I changed calipers, pads, lines and have had no problems since. Change the fluid yearly from here on in.

 

You bench bleed your master cylinder to remove air. If you don't remove the air, then when you try to bleed your brakes you will just be adding air to the system. You will never get a firm pedel.

 

The turkey baster is excellent advice...never use a rag to wipe the inside. Also, be sure you have a good explanation ready for when November comes around so you don't blurt, "Honey, it's in the shop from when I did the brakes...I'll go get it."

I always recommend visiting your local state bureau of automotive repair office and buying/reading their brake inspection and repair manual. Each automotive shop which has a state inspection station sign must by law have the manual in the shop at all times...walking into a shop around closing time with a couple of beers sometimes gets you a peak at the book and some "free" advice.

Here's what I've learned:

Bench Bleeding the M/C is ONLY to test it and make sure fluid exits both outlets before installing it and trying to figure out why fluid isn't coming out of a bleeder. Trying to install an M/C that is full of fluid is just plain dangerous to your paint and doesn't do anything worthwhile for the brake system.

When you have the brake system back together then you will want to run brake fluid through the entire lines...this will push all the air out through the bleeders...that's what the bleeders are for. Bleeders are not designed for letting out a few bubbles at a time...it's for when you want to get the air out and install fresh fluid, like in the event that you have to repair the system as in the case of replacing the m/c. Always run fresh fluid all the way through so that the new fluid is coming out the bleeder.

You do not need to bleed the proportioning valve.

I'm serious about having a story for the turkey baster, unless you want to be banned from the kitchen while you are working on your car.

 

Wow! That's very interesting! But now I'm REALLY confused. :crazy:
One manual I have says that failure to bench bleed will inevitably lead to air in the system. Another manual says that it only slows down the bleeding process. And now you say that the inspection manual says that its only to check for proper operation. In that case, since I know it was working before I sucked the fluid out, it should still work after putting fluid back in. Hmmm.

The main reason I started this thread was because I just couldn't understand why normal bleeding wouldn't suck out any air trapped in the MC. If there's a bubble in there and you pour fresh brake fluid in the resevoir, the bubble is now trapped between fluid on top and on bottom. So, pumping would just push the air bubble along until its out the bleeder.
Unless there's something about the mechanics of the MC that I'm not understanding where an air bubble can 'hide out' and not get pushed out, I think what you said is probably true.

I'd be curious to hear what others have to say about their experiences with the MC.

By the way, I have the Mityvac pump, so I can just pump the old fluid out the MC, and save the wife's turkey baster for Thanksgiving! ;)


[Modified by Marc78, 3:26 PM 5/3/2002]

 

Yes sir. Just because a brake system runs dry is no reason to dismantle it to bench bleed. Now, a local garage might try to convince their customers that they need that done to get the extra 30 minutes@$60/hour (not to mention several calipers and a rotor or two).

Yep..that's what happens.

I hope she appreciates your effort! :jester

 

Working on other old Chevy master cylinders, there would more often than not be a bubble that stayed in the master cylinder if the MC was not bench bled. I don't know if it is the angle that the MC sits at when mounted or what that made it virtually impossible to get that bubble out. Seems like bench bleeding a level MC removed those bubbles for me. I hope someone like Tom454 would ring in and give you the right scoop.

 

:eek: :crazy: :cry :( :jester :conehead :hat
Now I'm really confused! :D

To bleed or not to bleed! That is the question!

(By the way, just to be safe, I'm going to do it while its still on the car, but that doesn't change the fact that we haven't resolved this issue!)

 

i did a complete overhaul of my brakes and suspension last winter.after replacing the MC i did not benchbleed it.i filled the reservoir and started bleeding the way it is described in the manual while leaving the cover off of the MC.it worked fine for me.the pedal is hard and the brakes very responsive.just my experience.

 

Marc,
I probably bleed my brakes more than anyone on the forum as I rebuild my calipers regularly, and bleed some "old stuff" out (usually) daily at the race track. I wouldn`t do it if I thought it wasn`t necessary as the fluid I use is around $55 a litre (at a deal). There is no reason to bench bleed unless you have pumped the pedal with an empty master. I have run out of fluid while bleeding and you must simply start the "bleed process" over. A new or rebuilt master MUST be bench bleed to remove air before it is installed and it is more than a test it`s a MUST. The last MC I put on came with a couple of plastic plugs that you screw in and they take the place of the brake line "kit" while you are bench bleeding, it`s also easier to use the plugs when mounting a full MC on the car. ...redvetracr

 

Thanks to all for all the responses. It looks like there is only one correct answer to the question:
There is not only one answer to the question. :jester

Seriously though, just the fact that there is a possibility that an air bubble will lodge itself somewhere in some mysterious crevice in the MC is reason enough to bench bleed.