truck brakes, what are we missing

74 C65 truck brakes werent working right and it acted like the booster, pulled that off and the diaphram was tore and cylnder was leaking, put on a rebuilt one. Adjusted up the brake pads so they were close to the drum, front pads at 10% rears at 30%, still had to pump to get brakes and they are still weak. We swithced out the master cynlder and put on a rebuilt and you still have to give a second pump to get a bite and they are still a little weak. Would new pads make the difference, just seems like we are not getting enough fluid moving, enough travel on the brakes or are we missing something else.

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Its been 25 yr's but it seems like you need to bleed the brakes at the hydravac (booster) and at the wheels in order to get a solid peddle. If it were mine and the pads as you say are at 10 and 30 I would sure change them.
Angle Iron

Check the vacumn hose going from intake to booster
Had one colapse as brakes were applied. Took a
while to figure that one out.

The right way to adjust brakes on something like that is out tight,back off,hit the pedal a few times,adjust tight again.In other words you might not have been able to adjust them out good enough yet which would mean they have to travel farther than one pump will take them to work.Just adjusting them one time is most always not enough because once you apply the brakes a couple of times they move a little and will be loose again.Chevy can be hard to get all the air out of also.I have cracked a bleeder loose and let fluid drip out slow overnight,fill back up and tighten bleeder the next day and fixed it.Might try and bleed everything again starting from the furthest away line,then to the booster,which has a bleeder on it somewhere,then the front axle.Also if everything is worn,and pads are thin,you might be using all the travel you have before the brakes apply.If drums have been turned or are wore bad that could be the problem.See if there is a ledge where the pads ride that is wore into the drums.You might have to replace the pads to take up enough slack to get good brakes.

The right way to adjust brakes on something like that is out tight,back off,hit the pedal a few times,adjust tight again.In other words you might not have been able to adjust them out good enough yet which would mean they have to travel farther than one pump will take them to work.Just adjusting them one time is most always not enough because once you apply the brakes a couple of times they move a little and will be loose again.Chevy can be hard to get all the air out of also.I have cracked a bleeder loose and let fluid drip out slow overnight,fill back up and tighten bleeder the next day and fixed it.Might try and bleed everything again starting from the furthest away line,then to the booster,which has a bleeder on it somewhere,then the front axle.Also if everything is worn,and pads are thin,you might be using all the travel you have before the brakes apply.If drums have been turned or are wore bad that could be the problem.See if there is a ledge where the pads ride that is wore into the drums.You might have to replace the pads to take up enough slack to get good brakes.

The air on those size trucks can be a real pain to get out of the system.A portable bleeder tank connected to the master cylinder sometimes is the only way it can be done.Check around for a shop or a buddy that has one.One little pocket of air will require that extra pump.

Definitely time for new linings, drums rotors... whatever is on it.
Probably lines and who knows what else...
Personally I hate the sight of those trucks and those brakes. Not so much the truck... just the hydraulic brakes.
Never worked on them and never will.
Can't help with the particulars of bleeding the brakes, but at 10% you need new linings.

Better yet. Parkit and get something with air brakes before you or someone else dies in the damn thing.

Rod

OK, Master cylinder should have been bench bleeded before installed, if it was, just gravity bleed the system by cracking the bleeders starting with the farthest. Or vacuum bleed them but don't pump the brake pedal, that just makes things worse unless done very slowly.
Just sounds like too much air in the system.

i think old ford mech. has it right,..we had c65 chevys' and the only way to bleed was with power bleeder,...they can be a pistol...

I have worked on many JUICE brakes on med. duty truck , First off if YOU think you have them adjust up and ya have to PUMP the brake to get pedal then they still need adjusted . ^To Bleed them ya need a power bleeder as the master Cylinder does not have enough stroke to push the air out .

You need a bleeder ball similar to this. We had a Wagner at work. You needed to bleed the bleeder ball too before it was used after filling. Hal

Thanks for all the replies. We checked the vaccum hoses before putting on the new booster, their seemed to be good suction down by the booster. We primed up the master before hooking up the line and primed up the booster too, but we didnt use a power bleeder for bleeding, might have to borrow or steal one of those and see if that helps. . We might switch the front pads, on our 67 we ran them until they were gone, but we tried adjusting them up tighter today and there is no more adjustment on the front ones, so maybe we'll have to break down and put new ones on. Mostly just a silage truck and yes something newer and nicer would be nice, maybe next year.

I worked alone in my repair shop. I used a pressure bleeder hooked up one at a time to the wheel cylinders to force the air up to the master cylinder.

I have the same problem with my 1975 C65. In the last few years the whole brake system has been replaced, master cylinder, booster, lines, cylinders, shoes, adjusters, drums. I still have to pump the pedal once to get a firm pedal. I know there is no air in the system because the pedal is firm after the second pump. If it had air in it the pedal would be spongy. I adjusted the shoes by the book. Tighten to they drag and back off 3 clicks, pump the pedal hard and repeat until they adjust no more. The brakes work great, I just have to give them that second pump. I'am at a loss at what could be causing the problem as my other truck works perfectly.

After you did all of that and they still take 2 pumps Im thinking the rod that pushes the master cylinder is wrong,not pushing far enough,or the master cylinder is the wrong size,not putting enough fluid out.Like it needs a bigger piston to push more fluid.Ill also say that I worked on a truck exactly like the one in the picture,even the same color,and the brakes were not right on it either.I also replaced the booster,bled with a power bleeder,and brakes would work a day or 2 then back to pumping them.I got them to work a lot better,but never did get them right.

I read all the replies. You have air trapped in the rear wheel cylinders. I don't want any of them in my shop anytime soon! The rear wheel cylinders aren't at the top and bottom of the rear backing plates therefore the bleeders aren't at the highest point on the wheel cylinders. A power bleeder is the only way to bleed them and it still takes a lot of fluid and time to get it right. I'll never understand why the cylinders weren't mounted properly. I've already repositioned the backing plates to set the cylinders properly and made the bleeding job a cinch. HTH.

(quoted from post at 17:35:10 10/02/09) Thanks for all the replies. We checked the vaccum hoses before putting on the new booster, their seemed to be good suction down by the booster. We primed up the master before hooking up the line and primed up the booster too, but we didnt use a power bleeder for bleeding, might have to borrow or steal one of those and see if that helps. . We might switch the front pads, on our 67 we ran them until they were gone, but we tried adjusting them up tighter today and there is no more adjustment on the front ones, so maybe we'll have to break down and put new ones on. Mostly just a silage truck and yes something newer and nicer would be nice, maybe next year.

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The type of brake system on this truck is not as dificult to bleed if you are patient and follow the flow of fluid from the mastercylinder to the wheel cylinders. Pressure bleeders are nice but should not be needed on this system with mastercylinder seperate from the booster . you can get a hard pedal on this system and still have air in the first part of the booster cylinder. the brakes will apply but not have much effect. I run into this all the time and this is how I bleed the brake with two people.
1st remember not to pump the brake pedal to build pressure when trying to remove large amounts of air.( this only churns up the fluid). Start at the mastercylinder lines, open the loosen the line and have someone push the pedal down( close line ) let pedal return up and wait until the mastercylinder refills the plunger area of the cylinder.( this cant hapenn if you are pumping the pedal) continue until no more air bubbles come from lines. move to the next component ( should be the booster cylinder) at the point where the line enters the cylinder there may also be a bleeder screw.( some do not have this at this point where the line enters only at the line exiting the cylinder) this is the area that the air gets trapped and is most likely causeing you to pump a second time to get the booster to start applying. so the next step would be to crak this line entering the booster and push ( not pump) brake pedal down ( close line) let pedal up and let it recover for a few seconds before repeting. ( this is key when bleeding brakes, don't get in a hurry) continue to the outlet end of the booster cylinder in the same manner then to the wheel cylinders. This should remove all the air from the brake lines, booster and mastercylinder. I know this sounds like a lot of work but if you do it this way you can force the air out. How do I know? I just finished this procedure on two Internationals cabover trucks and a chevy C 65 . all three trucks were brought to me after they had boosters replaced. they all macthed the condition you described ( two pumps to get a good pedal, but not much brake aplication) If you try to push this air through the system ( even with a pressure bleeder) you will go through alot of fluid and still have trapped air in the booster. Start at the mastercylinder, then booster at entry point and exit point then at wheel cylinders. Why? the entry point of the booster is what actuates the booster and if it has even a small amount of air in it, you will have to give it a second pump before the booster will apply. If the brake shoes have any linning left the brakes should hold and stop the truck. But if you put new master cylinder and booster on Installing new brake shoes would be next and checking the wheel cylinders. they can suck air in and not leak fluid out. You can tell if you only get air out a certain wheel cylinder all the time. I know this is long but i hope it helps. Hal