Brake information/help

[Hx16457]

I have installed and bled new brakes. I have an old booster from a pick and pull with a new 1976 disc/drum master and proportion valve. I am running a 1976 front end (swap), w/1972 rear drum setup.
I bench bled the master.
Old school pump the brakes and bled the lines.
My issue;
When I start the engine (touching nothing else), my pedal sinks almost to the floor, i can just watch it sink!
WHY?
Once it sinks, and I drive the (1965 C10), I have a "hard pedal", as the pedal continues to stay sunk....(brake lights stay on due to pedal not coming up high enough to touch the switch)
Help me, what is wrong with my system?
I have never seen this before!
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[Captainfab]

So the brake pedal sinks all by itself without pushing on it? Are the brakes applied when it does this? How does the pedal feel with the engine not running?

[Hx16457]

Yes it sinks down right after you start it, you can see it sinking with your eyes. I am not pushing it, I goes down on its own.
Before you start it the pedal feels normal, like it should. A little Resistance then brake.
I have also adjusted the rear drum brakes to where they are barely touching th drum, trying to take the slack out.
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[Captainfab]

I have never seen or heard of something like this. My best guess is there is something wrong with the booster.

[luvbowties]

Originally Posted by Hx16457

Yes it sinks down right after you start it, you can see it sinking with your eyes. I am not pushing it, I goes down on its own.
Before you start it the pedal feels normal, like it should. A little Resistance then brake.
I have also adjusted the rear drum brakes to where they are barely touching th drum, trying to take the slack out.
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Do you have any braking action at all before it goes to floor? How about after? Adjust rear brakes tighter, actually too tight--temporarily--& see what happens to pedal. Are bleeders on front calipers at the top or bottom?--supposed to be at the top or you'll never get them bled. Check that all linkages have been matched/adjusted to reach: (1) pedal-rod to booster, & pedal should stay up there w/out return spring holding it(2)booster pushrod to m-cylinder(if m-cyl has a recessed hole in back of piston, its booster must have a pushrod extending beyond its flange to fit into recess with no pressure when m-cyl is pushed into position w/booster; if back of piston is only slightly beveled, its booster should have pushrod that is about flush with booster flange--again w/no pressure when the 2 are held into proper position. Look at Autozone's site & compare pictures of a 1971 pickup booster to some later pickup boosters, & you'll see difference in these pushrod lengths. Do same w/pics of different m-cyls and see the beveled pistons and recessed pistons. You should have a matching pair.)

More suggestions, depending on what you find. That pedal-rod-to-booster length is often found to be too short when troubleshooting someone's pick-a-part changeover--not to downgrade such a procedure at all: else, how do you think I have all these suggestions???

[Hx16457]

Cap; your answer is where I was leaning too (booster).
Luv; I will get bake in there and check all of your above mentioned items.

I, like ya'll have never seen such action from a brake system, but on here I was hoping some one else has.

Thank you both, now at least I have some thing to check, besides just running fluid into a bowl, re-bleeding the system over and over...
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[tincan1966]

Originally Posted by Hx16457

Yes it sinks down right after you start it, you can see it sinking with your eyes. I am not pushing it, I goes down on its own.
Before you start it the pedal feels normal, like it should. A little Resistance then brake.
I have also adjusted the rear drum brakes to where they are barely touching th drum, trying to take the slack out.
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Sounds like the booster has an internal leak and the engine vacuum is pulling the diaphram.
Also the rear brakes should have a slight amount of drag. Just barely touching the drums will cause a low pedal and spongy brakes.

[Hx16457]

Okay wo what I hear you gusy saying is:

#1, pull the drums and adjsut the read brake out until it is almost keeping the drum from going back on. (no rear sight adjustment holes in my rear end). Make sure they are out as far as I can get them and still fit the drum. "Draging" in other words.
#2 seperate the M/C from the Booster and "Check that all linkages have been matched/adjusted to reach", (as mentioned by LUV).
#3, re-bled entire system (old school style).

I had the same thought, Tincan. Leaking behind the dia., and I am not against changing the booster at all. But I will do the above first and let you gusy know is any thing changes.

Also to answer a few of the questions you guys asked above.

Yes the brake doesn't actually touch the floor so there is some space, even though the pedal is rock hard at this point, I can press it and make the truck stop. But it would never stop on a dime, it is more of a slowly crawl to a stop kind of thing. The pedal is really hard and you can't get much on it, in terms of movement downward.

New cal's on front, and yes the bleeders are on top.

Rolling to a stop is not any good for my Blood pressure, with all the work I have in Her!


thanks everyone!

[Hx16457]

Well, I checked the matting of the booster/MC and those match up. So next step will replacing the booster. I'll let you know if I gain on it, w/ the new booster.
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[Hx16457]

Okay new booster came in yesterday. I will get it put it in the morning. I had to go to Lowes and buy a coupler and a 3inch piece of all thread. These items are to get the push rod longer, from the booster to the pedal. The new booster is 3/8 X16NC, where the old one was 7/16 X20NF, so I had to build a new extender for the push rod.
Oh, also the new Booster has its own mounting bracket made to it (I was not happy, as the model Booster (NAPA) shown that I ordered was to have bolts not a mount made on), so that means I can use my old mount! Now I'll chop up the mount that is made to the booster so I can make it work.
Trim here, drill there, notch out that, etc.... You know, CUSTOM stuff..
I'll let you know what happens!

[Captainfab]

What was the application for the booster you ordered? If it wasn't what you wanted, I would have had them order the correct one.

[Hx16457]

I ordered a 1976 C10 booster, around here it seems none of the parts place have boosters in stock.
I went ahead and "modified" the mounting bracket to work on the firewall. It turned out okay. I had my wife help me pump and hold "bleed" last night.
Afterwards I started the truck to see if I had gained anything.
Remember; my problem was the pedal was sinking to just shy of the floor when I start the engine.
Result; it doesn't sink anymore, so it would seem I have solved that problem!
Today I will bring her out of the garage an see how the braking is. But at the moment I am just glad a have gained some ground on it.
Super stressful working with brake fluid around all this paint!
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[Captainfab]

Yep, the '73-'87 truck brake boosters have the mounting brackets riveted to the booster. Glad to hear you may have solved the problem.

[Hx16457]

yep got'r done!

[Captainfab]

Glad to hear that the new booster fixed the problem