soft pedal, brake lights stay on

Looking for some help trouble-shooting, or maybe confirmation of my current thinking/approach.

Completed a brake system overhaul,replacing the following components: master cylinder, booster, front and rear hoses, front calipers and rear wheel cylinders. Did not replace any hard lines, prop valve or hold-off valve. Everything was replaced as original, no upgrades or reconfiguring.

I have bled the system twice now (once with helper, once gravity). Both times I thought I had good pedal until I fired the car up and the vacuum booster kicked in. The pedal has gotten firmer with each bleeding, but it still pushes too close the floor. I can stop the car in the driveway, but it doesn't feel right. Additionally, the brake lights do not go off (lights on the outside, that is - dash light will go off and on).

Here's what I think is going on - if I'm missing something, please chime in: Basically, I think I've still got air in the line somewhere and need to bleed the brakes again. I had the whole brake system opened up, after all. I'm a bit uncertain about the brake lights staying on. I've been assuming that that's a function of the hold-off valve not resetting, because there's air in the line somewhere. Does that sound right? I didn't know to do it the first time, but the second time I bled the brakes I used shims to depress the button on the hold-off valve while I was bleeding.

Thanks for reading.

 

Brake light staying on is because there is a pressure difference between the front and rear brakes.

There was just a exchange of info about this in the last month or so.

Did you properly bench bleed the master cylinder ? I have the feeling you have introduced more air in to the system.

Simple answer is you have air in the system and if the master cylinder is not bench bleed properly bleed you will keep putting air into the system.

 

this was basically the exchange that I was talking about. Good reading and it seems to apply to your situation .

http://www.v8buick.com/showthread.php?304936-Brake-problem

 

Thanks for the reply. Yes, I bench bled the MC.

Thanks also for the link to the other discussion. I had read that thread as well (and noticed the similarities to my situation), but I didn't want to hijack the thread by posting there. If it's better to keep the discussion all in the same place though, I'm happy to move this discussion that way...

 

After re-reading the discussion you mentioned and giving it 30 minutes of careful thought, I've decided to pull the MC and re-bleed it.

Sometimes it takes a little encouragement to get excited about re-doing something...

I'll report back after another try. Thanks again.

 

After you bench bleed it try gravity bleeding it. You will use like 2 quarts of brake fluid. Sometimes its best to work your way to the rear .

Like just crack the lines at the Distribution block and let them drip a little . Then at the rubber hose connections and let them drip a little. And lastly at the wheel cylinders themselves .

Keep a watchful eye on the fluid level.

 

I'd check the push rod between the master and booster. Sounds like it's too short. To check it, separate the master from the booster. Use a marker and mark how deep the rod goes into the booster. Then, measure the depth of the rod into the back of the master. It should be about 1/8th inch shorter than the total area when assembled. If it's too short, then you've got too much travel before the rod even engages the back of the master to start applying pressure to the brakes.

- Josh

 

Thanks, Josh. I'll check that as well.

 

A disheartening day. Funny how brake work (in my experience) seems to lend itself to that conclusion (until it works, that is. then there is glee.) Bottom line: pedal feel is way worse now than when I started this morning. Here's what I did:

I pulled the MC and bench bled it again. I used the Cardone method, with plugs in both ports, pushing with a wooden dowel. Got some bubbles from the rear port at first. Let it sit a while, then pushed again. No bubbles. Travel was around 1/8 to 3/16 of an inch.

While I had the MC out, I compared it to the old unit. They are the same. As I mentioned before, the booster was rebuilt. I don't believe push rod length is an issue.

After reinstalling the MC, I gravity bled the brakes, one wheel at a time. I had shims between the hold-off valve button and the booster to keep the button depressed. Result: no pedal.

I got out the Mity Vac and re-bled all four wheels (order: DF, PF, DR, PR). Shimmed the hold-off valve, same as before. I've never had success with the Mity Vac method, and today was no different. Result: no pedal.

My wife helped out with the third attempt. She pushed the pedal while I held the hold-off valve button by hand. Bled each wheel, same order as before. Result: no pedal.

I'm a little flummoxed in addition to being disheartened. Not sure where the error is. This is not my first rodeo - I have done this before on other vehicles. That said, I am definitely feeling like a rank novice. :shock:

Questions:

Should I consider a bad MC? There was a bit of fluid between the MC and the booster when I first pulled it this morning. That seems like it ought not to be the case.

Should the button on the hold-off valve be depressed when bleeding the rear drums, or only the front discs?

Lastly. I had the negative lead disconnected while I was bleeding the brakes. I reconnected it each time I completed bleeding in order to start the car and check the pedal feel with the engine running. In each instance, the rear brake lights came on as soon as the negative lead was reconnected. Any reason to suppose that the hold-off valve is operating incorrectly?

 

If you've removed all air bubbles, and there are no puddles under the car (i.e. holes in brake lines or hoses), I'd say your master cylinder is bypassing internally.

-Josh

 

I would never bleed a master cylinder the way Cardone says. How will it bleed with those plugs in place of lines feeding back into the resavoir ?. You CAN NOT get a full stroke on the Master that way.

If there was fluid between the master and the booster it is a bad master leaking . Is this a Rebuild ? I would NEVER use a rebuid. NEW ONLY.

The Master must be bleed using lines fitted from the outlet ports right back into the resavoir . Then starting with short strokes watching for bubbles not only with the stroke but also the return stroke. Then gradually move the stroke to a full stroke.

 

I always bleed in this order......RR,LR,RF and then LF. Or as you say.....PR,DR.PF and then DF.

 

You cannot bleed a master cylinder with a stroke of 1/8 to 3/16 of a inch. There is air in the master and no matter how much you bleed you will never get it out .

 

Thanks to all for the replies. I will pull the MC in the morning and re-bleed as instructed. :laugh:

Any instruction with respect to the button on the hold-off valve?

 

Thank you Jason . 110% right.

 

I have never had to mess with it either.

 

I pulled the MC and bench bled using the tubes from the ports back into the reservoir. Pushed in the piston until it bottomed out - slow and steady. Did this until there were absolutely no bubbles for several strokes. Reinstalled and bled all 4 wheels. Regrettably, the result is no different from yesterday: brake lights come on immediately, and pedal goes to the floor (when engine is running).

Next Saturday is probably my next opportunity to work the problem. MC was level in the bench vise when I bled it. Should this be otherwise?

I'm gonna dig back into the service manual and relevant threads and see if I can figure out what's wrong (operator error or defective part or what).

Thanks for sticking with me.

 

Is that a rebuilt master cylinder or a new one ??? Return it for a NEW master cylinder.