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Discussion Starter #1
Okay, this may be a really stupid question, but I havent found the answer in a couple hours of researching, so I am going to ask and maybe someone else may some day need this answer. After all, I am new to working on cars, so bare with me...

How do I bench bleed my master cylinder?

I did a front disc brake conversion from a manual drum setup to a power disc front brakes and got the proportioning valve and everything is installed. I am having problems with the pedal going to the floor. After reading here, like I said for hours, I am thinking maybe I need to bench bleed my master cylinder. It is on an angle and I did not bench bleed it so maybe that is my problem. I am getting a pretty good stream of fluid out the front, but the rear (drum) brakes are not getting the same amount coming out. I have been messing with this for 3 days now. My wife sits in the car reading a book while I am trying to bleed these brakes, that is how much I have been working on this problem. There does not seem to be air coming out of the lines, the front ones seem to really flow great, but the rear just kind of squirt a bit and then flow straight down. I am doing this with the car not started. Sometimes it seems like the pedal is hard and then other times it goes all the way to the floor. The brake light will come on when the pedal hits the floor if I have it running. Does this sound like a bench bleed is needed?
 

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Bench bleeding is nothing more than getting all the air out of the MC before it feeds the brake lines. The best way I can explain it is you want to create a loop so the fluid is flowing through the MC and out the out put fitting on the side and back to the bowl. You do this on the bench and level so there is no place in the fluids path to trap air. Once mounted on the firewall the MC is angled and creates a pocket air can become trapped in. You've read of guys jacking up the car to level the MC out. It should work but you need to be able to push the rod all the way into the MC which the brake pedal may not do.

It's easy to fashion some old brake line to hook into the bowls and connect to the output fittings. If you don't have a tubing bender or flare tool they are rent-able and you can but short sections of tubing already flared if you don't have scrap for a couple bucks.

As for power brakes, I have never bled them without the engine running, I'm not sure you would get a good push on the rod without vacuum to the booster. Like I said, I've never tried though...
 

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Make sure the ends of the tubes are in the fluid when you bench bleed. How are you bleeding them? I had a similar problem when I was trying to vacuum bleed my converted brakes. I finally borrowed a pressure bleed setup and that fixed it.
 

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I had to jack up the rear of the car to get the master cylinder level and then hook up the fittings and loop the hoses back into the master to bleed out the new master on my old 68 that was converted 4 wheel discs. You don't need the car running to bleed power disc brakes. Sounds like you have an air bubble stuck in the rear portion so try getting the rear raised up to level the master and then get the bleeding kit if it didn't come with one.
 

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The best way is to use the kit that usually comes with MC's, or they are available at most parts stores.

I use a big Phillips screwdriver to push piston.

It's best to work slow, and watch until all the bubbles are gone.

Then, after installed on the car, have someone push pedal down and crack the lines one more time.

 

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Discussion Starter #6
Okay, I got this setup and plunging it, but there are still some tiny bubbles in the lines that just dont seem to come out...I have been plunging it for about 15 minutes maybe 50 times, but the bubbles only go so far and dont completely go away...when I stop plunging, the bubbles just kind of float upwards...i am talking tiny tiny bubbles...is this okay like this or am I doing something wrong?
 

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Are you sure your fittings are tight? You might be sucking a little air back in around them when you release the piston. Another thing to do is hold the piston compressed until the bubbles gather at the highest point in the tube, then slowly release it without sucking the bubbles back into the cylinder. You need to be slow and deliberate or you'll froth the bubbles and make them hard to get out. You also might just let the whole thing set for 10 minutes so all the bubbles will rise to the top.
 

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Discussion Starter #8
I left it sit for about a half hour and there are no more bubbles...

Now, once I have this bench bled...how do I install it without getting air into the lines? I can probably hook the master cylinder back up to the booster, with the plastic lines installed, but when I remove the lines, wont air get back into the master cylinder? is there a trick to this step?
 

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Mount the MC full of fluid with the cover sealed on it and the outlets plugged. Then carefully remove a plug and install the hard lines one at a time. You'll get air in the lines but the MC should be fine. With the cover sealed you shouldn't get much mess from the MC. Remember the trick when you were a kid where you stick the straw in your beer and put your thumb over the end and it doesn't leak when you pull it out of the bottle? ;)
 

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I would hold the brake pedal until all the tiny air bubbles worked their way up and out. I spent a couple of hours doing it while it was in the car. It worked fine and the car was the best braking car I've ever driven.
 

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Discussion Starter #11
This did not seem to help with my ongoing brake problem...I notice now that the brakes seem to pump up pretty good when the car is off, but when the car is started they go all the way to the floor...any idea of what would cause this to happen?
 

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Discussion Starter #14
In reading other posts and web sites...I am wondering if my calipers are installed correctly...I read somewhere that the bleed screw needs to be at the top of the caliper...mine is located on the side. could this cause air to remain in the caliper? maybe I should switch sides? here is a picture...

 

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You most definitely want the bleeders at the top.
 

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Discussion Starter #16
so these look like they are installed on the wrong side? should the bleeders be moretowards the front?
 

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Discussion Starter #18
that is the exact picture i was looking at when i decided to see where my bleeder is. i am wondering if the caliper is on the wrond side or maybe should be on the front side of the wheel instead of the back...here is another pic showing the side view...



 

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Discussion Starter #19
In looking at David Pozzi's web site, it shows the pictures below. The bleeder valve is pointing downward...almost in the same position mine is...does anyone know which way is correct?



 

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You should swap the calipers from side-to-side, which would put the bleeders on top. They might already be marked L and R.
 
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