brakes go to floor

[K10-Kansas]

EDIT: SOlution found. I had my calipers on with the bleeder screw pointing down instead of up.

EDIT: Update: I replaced the master cylinder brake lines because I felt that they weren't getting a good seal. They were 51 years old. Bench testing the master cylinder shows it working well. I adjusted the back drums and they are so far out now that it is really tough to get the drums on and off. I put the bleeder bolt in the proportioning valve. Bled all of the lines again. I added fluid to the master cylinder everytime it got down halfway. That happened about times. And the brakes still go to the floor. I can build up a little bit of pressure if I pump the brakes several times but even doing that, it doesn't stop the wheels from spinning. I've got it up on stands with the tires off.

Any suggestions?


Hello all. I hope you can point me towards things to check. My truck is a 71 c10 cab and what I am guessing is a 77 or 78 K10 chassis. I've owned it for a year working on it had have driven it for a whopping 2 minutes in since I bought it -onto a trailer and then off a trailer.

I had it up on blocks working out some confusion with the transmission. I started the truck and put it in drive and pushed on the brakes and saw that the wheels were still spinning so I push them to the floor and then the right front brake line burst. I replaced the brake line, bled the brakes, and they still went to the floor. I pulled off the drums, adjusted the brakes, bled them all again and still going to the floor and not stopping the rotors and drums from spinning. The booster is brand new put on there 5 months ago and seemed to work fine for the whole 2 minutes I pulled it off the trailer, parked it, moved it a few times 20 feet.

If I push the pedal in and out 3 times it builds pressure and then doesn't go to the floor but if I wait a few seconds and push it in again it goes to the floor.

[K10-Kansas]

I read that I should test manifold pressure and brake booster vacuum with a gauge. Running to the auto part store now to buy a gauge.

[mongocanfly]

thats not a booster issue.. youve either got air in the lines,a bad leak ,or a bad master cyl..
if the pedal is going to the floor , how did you bleed the brakes

[K10-Kansas]

Originally Posted by mongocanfly

thats not a booster issue.. youve either got air in the lines,a bad leak ,or a bad master cyl..
if the pedal is going to the floor , how did you bleed the brakes

I put a break bleeder hose and bottle on the bleeder nut and had someone push the brake in and out several times until there was only fluid coming from the lines starting with the nut closest to the master cylinder and working my way around.

[K10-Kansas]

Every time on the third press it would build up pressure and not go to the floor while bleeding the brakes. However if you wait a few seconds and then press again it does go to the floor and then you have to pump it a few times before it no longer goes to the floor.

[K10-Kansas]

I put a vacuum gauge on the manifold and got 18psi. When I put it on the booster and pump the brakes the needle bounces around going up and down a halv psi barely moving off zero.

[Chevy nutcase]

Originally Posted by K10-Kansas

I put a break bleeder hose and bottle on the bleeder nut and had someone push the brake in and out several times until there was only fluid coming from the lines starting with the nut closest to the master cylinder and working my way around.

Just to clarify, you opened the bleeder and then had someone pump the brakes? If you do this, every time the person working the brake pedal leaves off, the bleeder will suck air back into the line. Pump the brakes until resistance is felt then have them hold the pedal down, you open the bleeder, let the air or bubbly fluid escape and close it. THEN resume pumping the pedal, holding, opening and closing the bleeder however many times it takes until no air comes out. As stated earlier, start with the rear brakes. Also make sure someone didn't install a caliper upside down with the bleeders on the bottom

[K10-Kansas]

Originally Posted by Chevy nutcase

Just to clarify, you opened the bleeder and then had someone pump the brakes? If you do this, every time the person working the brake pedal leaves off, the bleeder will suck air back into the line. Pump the brakes until resistance is felt then have them hold the pedal down, you open the bleeder, let the air or bubbly fluid escape and close it. THEN resume pumping the pedal, holding, opening and closing the bleeder however many times it takes until no air comes out. As stated earlier, start with the rear brakes. Also make sure someone didn't install a caliper upside down with the bleeders on the bottom

I set the bottle a foot higher that the bleeder nut so that air would go up well and if it did suck anything back it would only suck back brake fluid. Yesterday I put a bleeder vacuum pump on it and it was sucking a lot of air bubbles for like a full 5 minutes with no sign of stopping. It really seems like the lines are not solid at the Master cylinder. The lines look original and I think have been moved, removed too many times. I pulled them off yesterday and today I'm stopping at LMC on the way home to get new lines. Hopefully this fixes the issue.

[72c20customcamper]

Originally Posted by K10-Kansas

I set the bottle a foot higher that the bleeder nut so that air would go up well and if it did suck anything back it would only suck back brake fluid. Yesterday I put a bleeder vacuum pump on it and it was sucking a lot of air bubbles for like a full 5 minutes with no sign of stopping. It really seems like the lines are not solid at the Master cylinder. The lines look original and I think have been moved, removed too many times. I pulled them off yesterday and today I'm stopping at LMC on the way home to get new lines. Hopefully this fixes the issue.

I have found if you don’t put some sort of sealant on the bleeder threads the vacuum pulls bubbles from the threads . Now I’m not talking a lot just a little Teflon paste on the thread not near the blender seat . A vacuum will suck air before viscous brake fluid .

[K10-Kansas]

Originally Posted by mongocanfly

thats not a booster issue.. youve either got air in the lines,a bad leak ,or a bad master cyl..
if the pedal is going to the floor , how did you bleed the brakes

I look everywhere and there are no leaks. The master cylinder is brand new. I'll look up instructions on how t test it. I'll go ahead and bleed the brakes again to see if it changes anything.

I've got the C10 show in 6 days in Independence Missouri. This was going t be the first time I entered a show. Man, I'd really hate to miss it.

[3drburb]

Start over. Bleed from the passenger rear to driver rear to passenger front to driver front.
Farthest first.

[K10-Kansas]

Originally Posted by 3drburb

Start over. Bleed from the passenger rear to driver rear to passenger front to driver front.
Farthest first.

Okay I'll try that. But also, putting a vacuum gauge on the booster and pumping the brakes should show an increase in pressure on the gauge right? I'm not showing any pressure increase above 0.5 opsi when pumping the brakes. Or maybe that's not how you test a booster. Ok, here I go, 3rd time bleeding the brakes today.

[K10-Kansas]

Could a bad proportioning valve cause this behavior?

[Dead Parrot]

You mentioned a busted brake hose. There is a piston thing in the prop valve that can get stuck to one side or the other during a partial brake failure like a burst hose. There is a re-centering procedure. Search the forum. (So far, I haven't had to do that from any of my brake issues)

[K10-Kansas]

Originally Posted by Dead Parrot

You mentioned a busted brake hose. There is a piston thing in the prop valve that can get stuck to one side or the other during a partial brake failure like a burst hose. There is a re-centering procedure. Search the forum. (So far, I haven't had to do that from any of my brake issues)

If that proportioning valve is stuck to one side, is it the case that when you bleed the brakes, you wouldn't get any fluid from 2 wheels?

[K10-Kansas]

I bled the breaks again and they all bled really well no issues.

The booster makes a Ssss noise and air exchange hiss burst for for 1/3rd of a second each time you pump the brakes.

[72c20customcamper]

Did you replace the master cylinder ? Sounds like a bad one to me

[K10-Kansas]

Originally Posted by 72c20customcamper

Did you replace the master cylinder ? Sounds like a bad one to me

I just got back from buying one. I'll try it tomorrow.

I wonder if i damage the other one when the brake line burst and my foot suddenly slammed to the floor.

[Dead Parrot]

Originally Posted by K10-Kansas

I bled the breaks again and they all bled really well no issues.

The booster makes a Ssss noise and air exchange hiss burst for for 1/3rd of a second each time you pump the brakes.

The hissing noise is normal. A quick check for a booster leak is run the motor for a couple of minutes, turn off, wait for a couple of minutes, then press on the brake pedal. If you get a hiss for two or three presses, booster isn't leaking or any leak is slow enough not to matter.

[K10-Kansas]

Originally Posted by Dead Parrot

The hissing noise is normal. A quick check for a booster leak is run the motor for a couple of minutes, turn off, wait for a couple of minutes, then press on the brake pedal. If you get a hiss for two or three presses, booster isn't leaking or any leak is slow enough not to matter.

Yeah if i do this, I'll get a half second hiss everytime 1 second after pushing the pedal.

[mongocanfly]

The proportioning valve can trip to one side..make sure its centered..they make a tool to hold it centered while bleeding brakes.. but I dont think that's causing your pedal to the floor issue..
Did you bench bleed the master cylinder?

[K10-Kansas]

Originally Posted by mongocanfly

The proportioning valve can trip to one side..make sure its centered..they make a tool to hold it centered while bleeding brakes.. but I dont think that's causing your pedal to the floor issue..
Did you bench bleed the master cylinder?

I didn't but I can. Thanks for suggesting that. I'll try it tomorrow.

[geezer#99]

Originally Posted by K10-Kansas

I didn't but I can. Thanks for suggesting that. I'll try it tomorrow.

Before you bench bleed that new master you should check it’s the right one.
Read post #8 in this thread. You need to check the depth of the pushrod hole in the master.
Possibly it’s the source of all your symptoms.


https://67-72chevytrucks.com/vboard/...d.php?t=844976

[truckin 79]

I would check the other rubber brake lines, they are probably deteriorated and squishy like the one that burst. You made sure there is no leaking from seals in all four corners right? That being said you should be able to get brakes even with old hoses. You just sucked up a lot of air when that hose burst . Not trying to be a jerk but when you had someone in the cab pumping brakes. That person held brake pedal down and then you cracked open bleeder correct? If so just keep bleeding brakes and pedal should come around

[K10-Kansas]

Originally Posted by truckin 79

I would check the other rubber brake lines, they are probably deteriorated and squishy like the one that burst. You made sure there is no leaking from seals in all four corners right? That being said you should be able to get brakes even with old hoses. You just sucked up a lot of air when that hose burst . Not trying to be a jerk but when you had someone in the cab pumping brakes. That person held brake pedal down and then you cracked open bleeder correct? If so just keep bleeding brakes and pedal should come around

Yeah, there are no leaks anywhere and I'm not loosing any fluid from the master cylinder. Yeah, we bled them by the book.