The Stovebolt Site Forums The Shop Area Engine Shop dual master cylinder brake trouble!

Today I went out and tested my brakes while pushing the truck...the result: the pedal took a little pressure to push down, but went all the way down without stopping the truck. I realized that if I pumped it a little it would finally get a little pressure, but I don't think that is very good. I went through and bled everything again. I got a little bit of air from 3 of them and alot of air from the left front. I started with the longest, left rear, second longest, right rear, third longest, right front, then the shortest, left front. The left front took several fill-ups of the reservoir to get the air out. There was alot of very small little bubbles, and the fluid came out fast and kind of frothy. I finally got it to come out with no air. I went back and put the cover on the master cylinder and the result was still the same. I now don't know what to think. I have thoroughly checked all connections and found no leaks.
Here are some things that I observed that might help someone tell me what is wrong, but I don't know what to make out of them. (keep in mind I have a dual master cylinder setup)
Here's a few preliminary shots:
[img]http://inlinethumb55.webshots.com/11446/2635929330100058208S425x425Q85.jpg[/img]
[img]http://inlinethumb64.webshots.com/12799/2966805610100058208S425x425Q85.jpg[/img]

-When I push the pedal down with the lid off, a powerful squirt of fluid comes up out of this hole: [img]http://inlinethumb11.webshots.com/12938/2276525640100058208S500x500Q85.jpg[/img]

-The rear line is actually above the master cylinder, but I don't have a check valve or anything installed in the line. Do I need one?
[img]http://inlinethumb58.webshots.com/11705/2594510630100058208S500x500Q85.jpg[/img]

A pair of residual pressure check valves may solve the problem.

I have used them on two of my dual circuit master, under the floor, conversions, and I have never had brake problems. I also has 4 wheel disc brakes, I assume you have drums?

The check valves may solve it.

But can anyone explain why the original master cylinder did not need one, and now they tell you if the master cylinder is under the floor you do need them? What changed between then and now?

Grigg

I don't know if that will work or not, and yes I have the drums on all 4. I did think of something, though. While reading some posts about spongy brakes I think I remember something where the port that is squirting fluid out on mine is supposed to be covered by the plunger initially. Mine is a few mm's from covering it to begin with. If I get the cab on and adjust the pedal so that it starts in this position, would it make a difference?

Hi Justin,
I converted my 51 half ton truck over to a dual master cylinder recently. It has worked flawlessly so far.
I have all drum brakes and fabricated all new hard lines. The original lines didn't work in my case.
I seem to remember that when I purchased my dual master cylinder there was one for disc brakes and one for drum brakes. that would be worth checking into.
Also, did you put in new flexible lines in the front? The old rubber lines tend to expand.

The main reason brakes get spngy is that there is air in the lines. If you are bleeding down by pushing the pedal you might not get all the air out. The way to go would be to pressure bleed up from the individual wheel cylinders (or calipers if you have discs). This is a simple and cheap way to go. I made a pressure bleeder from a bug sprayer bottle from Lowes and a length of fuel line. Simply put fluid in the bleeder tank and pump it up a little and attach it to the wheel cylinder. Be sure to remove the cover from the master cylinder and crack open the bleeder screw.
Careful though, if you use too much pressure you'll get a geyser and ruin that nice paint!
Also start from the furtherest wheel and work your way back.
Hope that helps,
Reso-Joe

If the master cyl. is made for drum brakes it should have check valves built in. All the old single master cyls. had a check valve built in the rear of the cyl. Make shure your shoes are adjusted up close to the drum. If that,s O.K. you still have air in the system. The master cyl. push rod must come all the way back so that port opens up. You can tell which part of the system is causing the problem by pumping up the brakes & releaseing them. Pick the lid up on the master cyl. & a stream of fluid will be shooting up on the side that is causing the problem.

Did you bench bleed the MC before starting?

Wrenchbender Ret., are you talking about a stream of fluid like the one I am describing? or are you talking about after the pedal is depressed and then let go again? Mine squirts up when I just start to press the pedal. When I pick the lid up after pumping the pedal, I don't see a stream of fluid. I did bench bleed the master cylinder, but I am thinking somewhere along the line while bleeding the wheel cylinders, I might have let it go dry once, so I will try again.

I went out and bled the master cylinder again
[img]http://inlinethumb20.webshots.com/11539/2959362960100058208S425x425Q85.jpg[/img]
A few bubbles were present, but I think that was probably just the air in the tubes I hooked up. I reattached the lines and it feels about the same, maybe slightly better. One thing I did notice, though, was a hissing sound when I depressed the pedal quickly. It kinda sounded like it was coming from the front of the m/c by where the plunger goes in. I took off the rubber boot and the rod isn't going in exactly straight, possibly pushing the piston to one side a little more than the other. I will try to rework the rod and move it about 3/4" to the right. I don't know if this will make a difference. The "hiss-like" sound could have also been the fluid squirting up into the m/c lid. I'm just not sure. I think Since there is a good possibility of air in the lines somewhere I will do this:
-Straighten out the rod and reattach
-Push all the fluid out of the lines and start over using a different method
-First I will bench bleed the m/c and reattach lines
-Put fluid in the reservoirs
-Have a helper open one of the bleeder screws
-Depress pedal and then have helper close bleeder screw
-Let pedal go back
-Open bleeder screw and repeat until fluid starts to come out
-Repeat on all four wheels

*I just though of this, but does it sound logical to open the bleeder screws on both the rear wheel cylinders because they are both off the same line? I could also do this on the front.

I would try the method suggested by resojoe, but I think my master cylinder has a valve built in that will prevent the fluid from coming back into the cylinder from the wheel cylinders and brake lines.

Do one cyl. at a time. This could be a good candidate for speedy bleeders. You might tighten up the brakes till you get a heavy drag & then when you get a good pedal back them off a little.

I went to NAPA today and the guy told me that if I pump the brake pedal about ten times and hold it down, then have someone crack the bleeder screw and then close it, that should get the air out. I went out and tried it, and I think I got it solved. I just don't know how the pedal is supposed to feel on this old truck. All I've ever driven is modern vehicles. Right now, I can pump it lightly once, then press it down again and get good pressure. The pedal stops before it bottoms out in the master cylinder. I am going to try bleeding it that way one or two more times still. Does it matter how long you leave the bleeder screw open, as long as the pedal stays down, or do you have to quickly crack it and close it before the pedal drops all the way down?

That initial spurt of fluid when you 1st press the pedal is normal. The M/C piston returns past the inlet port to allow excess pressue to backflow into the resivior (or with your high lines backflow the fluid, possibly dragging air past your rear piston seals) and fresh fluid to enter the lines as needed.
When you 1st depress the pedal that is the fluid in front of the piston coming out of the bore before the piston passes & closes the inlet port. You do not want to adjust this out as it is an important part of the operation.

When bleeding, pump it up till it's soid, then crack the bleeder & then close the bleeder wehn the fluid flow almost stops. No real time limit there, just dont want to draw air back in is all.
Definately wanna adjust all the brake shoes out to the proper clearance or you'll never get a firm pedal. Also, if you have done it once, rechecking it after you have pumped the brakes up a couple times may be in order as the if offcentered a bit your shoes can shift a bit when they contact the drums, widening the clearance again.
Otherwise, if you're not getting any air outta your bleeders and they aren't built into the M/C, a pair of residual press. valves may make the difference you need.

I'm sure the residuals are built into the master cylinder. I'll try the bleeding again and see how it turns out. Can anyone describe how these pedals felt on an original?

It should be like any other brake operation, press it one time and have a solid pedal, usually about 1/2 way down. If you have to press on it more than one time you still have problems.
Fred

I went out and bled the brakes again. I still got more air out of all four wheels. 3 of them appeared to get all the air out with two cracks of the bleeder. The front left, I did about 6 or 7 times because there was alot of air in it. It kept coming out frothy with alot of tiny bubbles. I stopped when it looked like it was solid fluid. I went around and did all four again....I got a little more out of all of them and stopped when it looked as if there was no more air. The pedal is much firmer, but still not as firm as I'd like it to be. It is pretty firm without pumping it, but with one pump, it is a little firmer, so I'll try the bleeding again tomorrow and I bet I'll still get some more air out.

I think I have finally finished. I performed the bleeding operation all the way around, and about 6 or 7 more times on the front left...I really don't know why it has so much air in it. After I had bled it several times, I opened up the m/c cover and very slowly pushed the pedal in. Several bubbles of air would come out of the front brake reservoir. I did this until I didn't see anymore bubbles. Then I repeated the bleeding until I didn't see any more air. And finally (this might have been part of the problem to begin with) I adjusted all the brakes tighter with the adjuster screws. I think the initial adjustment I did was misleading. I think the shoes weren't "seated" yet so I heard a scraping sound and left it there. Well this time, I adjusted them alot tighter until I heard the scraping then backed it off maybe one turn. You can still hear a faint scrape, but I think that is how it is supposed to be. Now the pedal is hard a little more than halfway down (the first time too!) I think this sounds like a finished job doesn't it?

take a clear plastic bottle put a hose on your bleeder and put fluid it the bottle when you pump then you can tell when it stops bubbling all the air is out works better then pumping and turning the bleeder it works the air out better

I had a hose connected to a bottle and That's how I saw the bubbles of air. I quit when I didn't see any more bubbles.

The shoe adjustment will change once you drive a bit. It will need to seat the shoes to the drum. Let it go for now, it sounds like you are close. After a few miles of driving and and a few heavy stops, climb under and readjust all four wheels.

I find a lot of the air you see in the clear tubes is from the bleeder threads and not acutally in the line. If you open a bleeder and let it run with a cleat tube, wiggle the bleeder and watch, you will see air bubbles! Joe