[gwaan83]

I have a 1979 Chevy Caprice.My brake pedal go to the floor but my car don't stop at all..I bled several times and still nothing..I bought a new master cylinder and brake booster put them on bled the system still nothing so I took the master cylinder back swapped it three times still no result.I don't have a clue what the problem could be.All the parts was brand new.

 

[kingnutin]

Did you bench bleed the master cylinder to get the air out of it? If you don't you will have those problems. Have you checked the hoses, calipers, and drums and wheel cylinders? If you have a leak anywhere you won't get any pressure.

 

[gwaan83]

Yes i have bench bled it but I didn't check all the other things you listed.I'll check them and let you know the outcome.Thanks

 

[Beetlebill]

Check your vacum. If you have a cam with performance duration or leaks in a hose, you can lose your assist causing braking probs.

 

[kingnutin]

Which should result in a hard pedal and poor brake performance, a sinking pedal means fluid is leaking past something somewhere, or not there to begin with n

 

[gwaan83]

Ok..Now I've check everything and I didn't see any leaks but I tried to bleed them once more.When I got to the rear left side,while bleeding it,it just stop shooting out fluid.It barely leak but it didn't shoot out like it usually do..The master cylinder was full with fluid but fluid dripped out of the bleeder screw.the rear right side shoot out clean fluid with no air bubbles.I'm confused..What could be the problem?Thanks in advance

 

[kingnutin]

Probably crap in the line clogging it, but that doesn't explain the pedal going to the floor either. Without there being a leak, or any air in the system, I'm at a loss as to why it would be doing that.

 

[77Impala]

OK, lets back up a bit and find out why the master cylinder and booster were both replaced. Gwaan83 would you mind letting us know please.

Now on these cars the brake system is very simple. Starting with the master cylinder (will skip the booster and brake pedal) the fluid goes through two lines down to the brake modulator valve (This senses if one side looses braking power). From there there are two brakes lines that, one goes to the front calipers, and the second goes to the back of the car just above the rear axle, then a rubber hoses, one on each front wheel that leads to each caliper, and one at the top of the rear axle which turns back to steel lines that take it to the rear wheel cylinders.

If no fluid is being lost (other than the brake bleeding) and no air is in the system then the master cylinder is bypassing inside itself. Or you could have a bake hose that is swelling up but most times they will burst if that weak. At this moment I am still suspicious of the master cylinder being your problem. I know you stated that you have bench bled it, but for most people that is not an easy task to do. What I do is put the new unit on the brake booster and have a helper push the pedal for me as I keep both my thumbs over the brake line ports. Have the helper push slowly and retract slowly so that all of the air is pushed out of the unit. This will make some mess but is very effective in removing all the air from it. Many times I then hook up the brake lines and really don't need to do any more bleeding of the system.

 

[gwaan83]

OK, lets back up a bit and find out why the master cylinder and booster were both replaced. Gwaan83 would you mind letting us know please.

Now on these cars the brake system is very simple. Starting with the master cylinder (will skip the booster and brake pedal) the fluid goes through two lines down to the brake modulator valve (This senses if one side looses braking power). From there there are two brakes lines that, one goes to the front calipers, and the second goes to the back of the car just above the rear axle, then a rubber hoses, one on each front wheel that leads to each caliper, and one at the top of the rear axle which turns back to steel lines that take it to the rear wheel cylinders.

If no fluid is being lost (other than the brake bleeding) and no air is in the system then the master cylinder is bypassing inside itself. Or you could have a bake hose that is swelling up but most times they will burst if that weak. At this moment I am still suspicious of the master cylinder being your problem. I know you stated that you have bench bled it, but for most people that is not an easy task to do. What I do is put the new unit on the brake booster and have a helper push the pedal for me as I keep both my thumbs over the brake line ports. Have the helper push slowly and retract slowly so that all of the air is pushed out of the unit. This will make some mess but is very effective in removing all the air from it. Many times I then hook up the brake lines and really don't need to do any more bleeding of the system.

Well i was driving the car like 3 days out of a week and the brakes worked fine and I stop driving it for about 4 months.It was parked in my garage so..one day i was pulling it out of the garage and tried to stop and the brake pedal when straight to the floor and the car didn't slow down at all so I pressed the emergency brake and it stopped.I was told it was the Master cylinder so I changed it,bench bled it and went to each wheel and bled it.Still no good.I took it back got another one,same results.Someone else said it could be the brake booster so I got one and still the same problem.I tried one more Master cylinder and it still won't stop.I heard today the it could be the proportioning valve.A guy checked it today and he had the back tires off the car while on jacks and put the car in Drive and told me to press the brake pedal and the back wheel kept spinning but it stopped once.The brake shoes wasn't pushing out but it did once.I'm so lost..

 

[kingnutin]

I would say off hand that you didn't get all the air out of the master cylinder, like Kenny said above it can be a PITA. His way works, I usually lock it into a vise and use a large screwdriver to push the plunger, make sure you keep your fingers covering the holes, let air and fluid escape, but let nothing in. Make sure you have pure fluid with not even the tiniest of bubbles coming out.

 

[77Impala]

Another trick I learned for brake bleeding is to have your helper ready to push the pedal when you tell them to. Then go to each wheel and instead of having them pump the pedal several times have them push the pedal as you open the bleed valve and tell you when they have the pedal on the floor, then close the valve and then let them release the brake pedal slowly. Repeat this as many times as needed to get clear fluid out. Do this for each wheel. This way helps push the air through the line easier than pumping up the pedal.

 

[kingnutin]

I gravity bleed, but I don't recommend it to other people often.

 

[gwaan83]

Okay..I did everything that you guys said to do..I even changed the rear wheel cylinders.Still no stop!! The only thing I have left to do is change the lines because it's driving me crazy!

 

[kingnutin]

It is possible in theory that the lines are swelling and causing this issue, but odds are greatly against it. Nothing will be hurt by replacing them obviously, so if you don't mind the expense and the labor its certainly good peace of mind. But, something here is not kosher...

 

[77Impala]

...But, something here is not kosher...

I agree, this fellow should have strong brakes by now. I am leaning that the problem is still in the master cylinder. I know the OP has changed it several times now but I ran into an issue just over 5 years ago with a rebuilt carburetor, I put three on from one auto parts store and had nothing but trouble. Got my money back and bought a rebuilt from another parts dealer and this one has been trouble free this whole time.

gwaan83: I recommend taking the master cylinder back to the place you bought it and going to another parts store for a new one. I would not doubt that you will find that fixes your problem.

 

[kingnutin]

Makes sense, I concur.

 

[gwaan83]

Ok..I've checked the lines and there's no swelling..I just called Autozone and they told me the Master Cylinder that I've been getting is a remanufactured one.I've changed it 3 times and that didn't work.So today I will go and get the brand new one.Hopefully that will work

 

[kingnutin]

Hope so, good luck!

 

[77Impala]

Keep us posted.

 

[Gregg-O]

Are you using the rod (for lack of proper term) supplied with the master....if there is one. I seem to remember that there are several different length's ...and they make a big difference in function.