Brakes go to floor

I am having a problem with my brakes on a 90 silverado. I have replaced the master cylinder, the booster, and blead the brakes 3 times. The problem is that while bleeding the brakes with the truck off I have a pedal. Once the truck is turned on the brakes go all the way to the floor. Does anyone have an idea of what is going on? Need help!!

Were you having these problems before you replaced the booster and the master cylinder ?? It is not uncommon to get a bad master cylinder. Are you sure there are no leaks ?? You may still have air in the system if you haven't blead enough fluid through it. Is the fluid level staying full or is it going down ?? Why did you change all the components to begin with ?? If you can shed a little more light on what lead you up to the point of replacing everthing such as were the brakes acting up. I am sure someone on this board can help you.

It all started while I was coming to a stop sign and the breaks went to the floor. When I got home I blead the brakes but then I noticed a leak at the master cylinder so I replaced that. Then I noticed a sucking noise coming from the booster so I replaced that since I read on a post that if you have a leaking master cylinder there is a good chance you could blow out the booster. Once that was done I blead the brakes again and the pedal felt fine until I started the truck. They once again went to the floor and I have no brakes. There are no leaks anywhere on the brake system. I have checked. If it was just are in the lines why would I have brakes until I started the truck? Help anyone

Are you sure that you've got all the air out? You start with the furthest line from the master cylinder: Right rear....Left rear.....right front...left front. Tell us what the final fix was so that we all can learn something.

yeas. Blead everything just as you said. No air left in this trucks lines. I will let you know when I find out.

Well this is a real puzzler? With the dual piston brake cylinder at least half the system should function: either the front or the back? Is it possible that both front half and back half failed at the same time? Or maybe half the master cylinder had gone bad and wasn't noticed followed by the other half which gave the impression that both halves failed? There is a flow divider under the chassis, but that thing never goes bad. Do you have ABS? I wonder if that could be the problem? Yeah, please let us know what you find. You have a rare problem indeed...something that most mechanics never experience!

Yeah really stumped. Brand new master cylinder and everything seems fine when the brakes are blead until I start the truck then straight down to the floor they go.

One thing I did notice when pumping the brakes was that the fluid was bubbling up inside the master cylinder. Not sure if this is suppose to happen.

Sure sounds like air in the lines. Okay, if it were me, in the absents of a pressure bleeder that clamps onto the top of the master cylinder, I'd go buy one of those $35 suction bleeders. The suction brake bleeder has a reservoir cup and a squeeze handle to apply suction. There is a tube that plugs on to the bleeder fitting at each brake station. If that didn't get it, the only other option is to give it to the dealer and pay whatever they want. I don't have a manual for your vehicle to check the trouble shooting list. I wonder if that manual addresses this situation? You might stop by the library and check the factory manual for your particular vehicle. Most librarys have such manuals available for you to research, but not to borrow. The copy machine is always nearby.

Doug, Do you think it would be worth removing the lines from the mastercylinder and connecting the 5 dollar bench bleed kit and bench bleeding the master alone and then bleeding the whole system again ?? If there are no leaks then it has to be air or a bad master cylinder I would think. It could be leaking out the back seal on the master cylinder, But I doubt it if the fluid stays full. I have gotten bad master cylinders many times. I have one for you, About fifteen years ago a guy brought me a truck and said he fixed a leak and replaced the master cylinder and gets no pedal. I tried bleeding the sytem, No luck, Pressure blead the system no luck. I raised it up on the hoist and started looking it over and here the guy spliced the line with a piece of fuel line and it was blowing up like a balloon when the brakes were applied.

I dont think I metioned this before but yes the truck had RWAL ABS on it. I actually have had two master cylinders on it. Took back the first one and got a second cause I thought it was bad. If it was the master cylinder why would I have brakes until I start the truck.

A fuel line for a brake line? Wow...that was a crazy thing for your "mechanic" to do. Hey, what I did only a few years ago for a suction bleeder, because I was out of town at a construction site and didn't have anyone available to pump the brakes for me....I made a jury-rig "suction bleeder." I stopped by a hardware store and bought 4 feet of clear tubing, actually three different small sizes of tubing because I knew one of the sizes would fit over the brake bleeder. I got a mayonaise jar out of a trash dumpster. After I found out which plastic line fit properly over the brake bleeder I drilled two small holes into the cap of that mayonaise jar. I slowly used larger drill bits until the tubes could be forced tightly into the holes. I pushed both tubes about 1" into the jar. With one tube in my mouth sucking, and the other tube on the brake bleeder, I was able to suck the air bubbles out of each brake line without too much trouble. Hell of a lot cheaper than spending $35 for the fancy squeeze handle suction pump. That's how I was able to replace my master cylinder without help from anyone else. With the one quart mayonaise jar you can start sucking until there is vacuum in the jar, and then open the bleeder valve and watch the fluid flow. Right before the flow stops completely, close the bleeder valve. Worked like a charm. Total cost for the homemade suction?: about 6 bucks.

Two master cylinders?.....Hummmmm. Well I have to admit, this one is out of my area of knowledge. I've never worked on an ABS system. I was long out of the car repairs racket before they were even invented. I still think that you should stop by the library and look into the shop manual....at least that's what I'd do next.

maybe its a broken brake pedal? :lol:

Yeah, probably will be something very simple, and simple to overlook. I'd be REAL curious as to what the final fix will end up being? This is guaranteed to be a "learning experience" for all.

funny but I would like some real help here if possible. This is getting to be frustrating.

Boy it was easier putting a LT1 into this truck than to figure out the brakes. The truck is fast but now I cant stop it. Funny how things work out. I will be sure to let everyone know what the problem is once I get it fixed.

The booster would not affect the mechanical functioning of the brakes. If it wasn't functioning, you'd only feel a hard pedal requiring more pedal pressure to apply the brakes. The pedal going to the floor usually means air in the lines or a leak. Did you check the actual metal lines going into the master? Sometimes the line splits or cracks from vibration or rusts and although you changed the master, it still leaks. Check especially around the flared end of the line. Did you bench bleed the master before you bolted it to the truck? If not, you forced the air in the master into the lines. I would recheck all the lines and calipers/wheel cylinders(if applicable) for leaks. I doubt two masters in a row were bad. You said there was a leak at the master. Was the leak near one of the lines, or at the back at the booster. Was the internals of the old booster wet when you took the master off? If the line(s) to the master was leaking, it should be wet below it also. This may give you a hint to where the leak is. Good luck!

I wonder if you could actually have a bad booster.I have never seen anti locks cause this , But I suppose maybe it is possible. Usually antilocks either get super touchy or they don't work. I wish we could help you more, But it has us stumped too.

I have norrowed it down to the back brakes aren't working. If I look in the resivior the fluid is only bubbling in the back section. Blead the brakes again and still nothing. I jacked the truck up and the back brakes are not grabbing at all. I can still spin the tires. There is no air left in the lines. I get a steady flow but maybe just not enough pressure for the back brakes to work. Anyone with ideas now?

Are your wheel cylinders in good shape and not leaking? I would say that they are after all the other checking you have done. I have a Chilton's for this truck....I will check it and post back in a few minutes!

Yup. No leaks anywhere.

OKAY HERE IT IS I AM GOING TO POST IT ALL
Alot of this is common sense,but read it all there may be something that you and your helper have missed...PAY CLOSE ATTENTION TO STEP NUMBER 11. IF YOU HAVE A FAX AND YOU NEED ME TO I CAN FAX YOU THE PAGE THAT COVERS THIS. This is out of Chilton GM Fullsize trucks 1988-1998
Bleeding the ABS system

1. Clean the master cylinder of excess dirt and remove the cylinder cover and diaphragm.

2. Fill the master cylinder to the proper level. Check the fluid level periodically during the beeding process, and replenish it as necessary. Don't allow the master cylinder to run dry or you will have to start over.

3. Before opening the bleeder screws you may want to give them each a shot of penetrating solvent. This will reduce the possibility of breakage when they are unscrewed.

4. Attach a length of vinyl hose to the bleeder screw of the brake to be bled. Insert the other end of the hose into a clear jar half full of clean brake fluid, so that the end of the hose is beneath the fluid. The correct sequence for bleeding is to work from the brake farthest to the master to the one closest. Right rear, left rear, right front, left front.

5. The combination valve must be held open during the bleeding process. A clip, tape, or other similar tool (or an assistant) will hold the metering pin in.

6. Depress and release the barke pedal three to four times to exhaust any residual vacuum.

7. Have an assistant push down on the brake pedal and hold it down. Open the bleeder valve slightly. As the pedal reaches the end of it's travel, close the bleeder screw and release the brake pedal. Repeat until there is no more air in the lines.

8. Repeat this procedure at each of the brakes Remember to check the master cylider level occasionally. Use only fresh fluid to refill the master cylinder, not the stuff bled from the system. (like i said common sense)

9. When the bleeding process is complete refill the master cylinder install the cover and diaphragm and discard the fluid beld from the brake system.

10. Perform 3 function tests with the TECH 1 scan tool. The brake pedal must be firmly applied.

11. ON MODELS WITH REAR WHEEL ABS
a. Refill the jar with clean brake fluid and attach the bleed hose to the bleed valve on the Isolation/Dump valve.

b. Have your assistant slowly depress the brake pedal and hold it. Loosen the bleed valve and expel the air. Tighten the valve and slowly release the pedal.

c. Wait 15 seconds and repeat this procedure. Repeat bleeding the Isolation/Dump valve until all the air is expelled.

12. On Models with 4 wheel ABS repeat step 1-9

I hope this helps

Followed those exact steps when I blead the brakes and nothing. When I press the peddle there is very little travel in it in order to releave the air. You know how when you depress it and then someone cracks the bleeder screw loose it should go down further. Well when I start it goes to within an inch of the floor. Thanks for looking that up though.

OK! This is getting to be annoying!! Decided to check something out just for the hell of it. Tried the brakes while the truck wasn't running and had a decent brake pedal. Started the truck and they went to the floor. Turned off the truck again and removed the check valve and had a decent brake pedal. Started the truck without the check valve in place and still had a decent brake pedal until I put the check valve back in, then right to the floor they went. The check valve is working cause I checked it according to troubleshooting. Blew in one side and couldn't blow in the other. Check valve is inserted the right way also since I have checked this in the book and several vehicles. No chance it is in backwards. Any ideas on this one.

Is it possible the ABS motor is staying on? I have seen this a few times. Do you hear anything under the hood that sounds different...asif it is on or running? Is your ABS light on? If it is on, pull the fuse for the ABS and see how the brakes react. Maybe you should have the car scanned . Maybe the whole problem was the ABS pump and not the master originally. Just a thought......

The power (brake) assist apparently works with vacume from the engine on my 86 Wrangler. I had the same problem but it seemed to be mostly solved by replacing a few vacume hoses. Then again it may be a little different on newer models... but my pedal doesn't have nearly as much travel as it used to. By the way, my brake pedal was fine when the ignition was off, as soon as the truck was started the pedal would sink right to the floor.

SBHONEY,
This is a long-shot, but a logical deduction: Pull off both rear brake drums to inspect the shoes. If the lower spring holding the adjuster in position is broken and the adjuster has fallen out of position you'll never get pressure to the rear brakes and the pedal will go to the floor. Those springs are cheap to buy....about $5 a pair from a brake shop.