[mittleiderj]

I recently upgraded my sons 60 C10 with a dual bowl master cylinder. At the time I had a lot of trouble getting the system to bleed but it did eventually bleed (although the system never did work very well).

A couple days ago the right front brake cylinder blew due to the star adjuster backing off completely. I have replaced all the springs and star adjuster on the fronts. I also added speed bleeders to all four brakes. Once again I CANNOT seem to get the system to bleed or at least I can't seem to get any pedal after bleeding. I have pumped nearly a gallon of fluid though the master cylinder and it will not pump.

Does anyone have a clue as to what might be the problem? or at least where to look? Could the speed bleeders be the problem? I will be plugging the master cylinder ports tomorrow to make the master cylinder is OK. But other then that I'm at a loss. thanks for any help you can give

[56 Project]

Is the rod and brake pedal allowing the master cylinder pistons to return all the way out? If the adjuster backed off then the spring must not be rubbing on the adjuster to keep it from turning by itself. Make sure that when you adjust the brakes you feel the spring catching in the star wheel. The brakes should be adjusted up so you can hear a slight scuffing sound when you spin the wheel.

[scholman]

mittleiderj,

I have run into this problem before. It seems that an air bubble gets in the system and it will not leave the system for love or money as long as you pump the pedal to bleed the system. See if you can rent a power bleeder from AutoZone. Go to each wheel one at a time and open the bleeder. As soon as you have no more air coming out of the bleeder, move to the next wheel. When you get finished you should have good pedal again as long as the brake shoes adjusted up like "56 Project" recommended.

Scholman

[mittleiderj]

Thanks guys, yes the original spring was incorrect. It wasn't making contact with the adjuster at all. I replace it with the correct one. It will not loosen now. I used to be able to rent pressure bleeders but for some reason there are none to be found. I'm not sure where an AutoZone is around here but I will check.

[mittleiderj]

to update my problem, I blocked off the mc ports and got a very high and hard pedal with no leak down. So I assume a good master cylinder. Then I reattached the brake lines and pinched the rubber lines (rear brakes only has one feeding both brakes).This time the pedal was lower but firm and no leak down. Unclamped the rear hose and again slightly lower pedal but still firm and no leak down. (I took it for a drive like this and actually was able to stop although the rears obviously locked up) I assume the rears are good. Finally I released the driver side front and the pedal went to the floor. I reclamped and then released the other front with the same result (pedal to the floor).

I then rebled the fronts (pumped two bowls full through the mc)but still no pedal. Is it possible the front portion of the mc is bad? Any other thoughts would be appreciated.

[mittleiderj]

Well, I finally got a good (better then before) pedal!! Out of frustration I removed the speed bleeders from the fronts (my son has been nagging me to do that for a couple days) and replaced them with the originals. Without doing anything else the pedal pumped up and stayed up. One of two things happened here 1.) the speed bleeders were leaking somwhere even when tightened or 2.) the act of pulling and replacing the bleeders allowed the fluid to bleed due to gravity removing the large volume of air. I think #2 is more likely. My son thinks #1. I hate when he's right

A drive test still shows the front brakes aren't contributing much. Yes the shoes are on correctly.

[56 Project]

Are you using a proportioning valve on the system? I think I heard of some one making an adjustable valve so that you could get the proportioning adjusted to better suit your application.

[mittleiderj]

No I don't have a proportioning valve installed. As I understand it the valve will reduce the pressure to the rear brakes thus equalizing the braking force front to rear. Because the front brakes are so marginal I don't want to reduce the rears now. Does the proportioning valve somehow improve the front brakes? If I can improve the front brakes, I will probably add an adjustable proportioning valve to the system.

Thanks for the advice.

[Henry Highrise]

Anytime you run a dual master cylinder you need a proportioning valve.

[yesgo]

mittleiderj, you understand correctly. A proportioning valve only reduces rear pressure. Front drums will get full master pressure. A prop valve cant possibly improve the fronts. Full pressure is all you can get

GMs did not use any prop valve with 4drum vehicles that I've seen, just a switch. People will call it a prop valve, but there is none inside.

Glad you got it bled out. Let's try to deal with the other issue.
Did it stop well with the fruit jar master? How far down does the pedal go before you can feel any resistance? What exact master are you using now? How are the lines run? Do the rears actually lock before the fronts? How do the front shoes and drums look?

[mittleiderj]

yesgo,

It did seem to stop better with the fruit jar but even that was scary as it pulled. We replaced shoes and wheel cylinders when we upgraded to the dual mc. The mc is a "new" 68 1/2 ton version. The lines are stock configuration with the fronts (1/4in)running from the front mc port, rears (3/16) from rear mc port. I have about 1/8 to 1/4 in of "slop" in the pedal before it engages the mc. The pedal typically will be about 1" from the floor when the brakes are full on. The rears will alway lock before the front. The fronts never lock even when power braking the truck pushs through the brakes. I have the fronts adjusted up very tight (too tight I think).

Any other thoughts are appreciated.

[yesgo]

I guess the pull is gone?
If you haven't replaced the hoses yet, I would.

I guess all I can think of for now is to double check that the short shoe is to the front, long to back, on the front brakes. You might also check that the front replacement cylinders are the proper 1 1/8" bore. I've seen replacements be the wrong bore size, and a smaller w/c bore will decrease your braking.

Usually GM used 1/4" for the rear line and 3/16" for the front on tandem masters, but having it opposite shouldn't really effect your braking. Out of curiosity, what are you using to split the lines between front and rear?

[mittleiderj]

Yes the pull is gone. I have replaced the hoses in the front not the single hose in the back. I also checked to make sure the shoes were on correctly and they are. Good point about the wc diameters. I replaced the w/c on all four corners and then had to replace the guts of the right front after it blew due to the adjuster problem. I didn't actually measure the bore though, I assumed it was OK since everything fit.

The front/back brakes are split at the mc (front brakes are on the front mc port and rears on the rear mc port). The left and right are split with 'y' blocks on the xmember and rear axle.

I am going to pressure bleed the system again. I have to make my own adapter for the mc (aluminum plate and inner tube seal) since I can't seem to find anything resembling that around here.

Thanks again for your efforts.

[56 Project]

Are the emergency brake cables totally releasing the rear shoes? This could cause the rears to grab early and would also cause a low pedal.

[mittleiderj]

The emergency brakes have been cut. The guy that did the installation of the 350 removed the e-brake hardware. That's on the list of things to fix.